the satilla river estuarine system

THE SATILLA RIVER ESTUARINE SYSTEM:
THE CURRENT STATE OF KNOWLEDGE
A report submitted to
The Georgia Sea Grant College Program
and
The South Carolina Sea Grant Consortium
8 September 2003
Alber, M.1, Alexander, C.2, Blanton, J.2, Chalmers, A.1 and Gates, K.3
1 - Department of Marine Sciences, University of Georgia, Athens, Georgia 30602
2 - Skidaway Institute of Oceanography, 10 Ocean Science Circle, Savannah, GA 31411
3 - Marine Extension Service, 715 Bay Street, Brunswick, GA 31520
TABLE OF CONTENTS
Introduction................................................................................................................................................... 1
Physical Setting............................................................................................................................................. 3
Land Use in the Satilla River Watershed ......................................................................................... 3
Framework Geology ........................................................................................................................ 5
Bottom Sediment Characterization .................................................................................................. 5
Geology and Groundwater Input ..................................................................................................... 7
Physical Processes ...................................................................................................................................... 12
Discharge ....................................................................................................................................... 12
Salinity Regime.............................................................................................................................. 13
Flushing Time ................................................................................................................................ 14
Tidal Currents ................................................................................................................................ 16
Residual Circulation....................................................................................................................... 18
Non-uniform Distribution of Bottom Stress .................................................................................. 19
Sediment Transport, Mobilization and Resuspension.................................................................... 20
Numerical Modeling ...................................................................................................................... 21
Water Quality.............................................................................................................................................. 23
Components Entering the Estuary ................................................................................................. 23
Dissolved Inorganic Nutrients .......................................................................................... 23
Dissolved Organic Nutrients............................................................................................. 25
Total Suspended Solids..................................................................................................... 25
Components Within the Estuary .................................................................................................... 25
Dissolved and Particulate Major Elements and Trace Metals .......................................... 25
Dissolved Inorganic Nutrients .......................................................................................... 28
Dissolved Inorganic Carbon ............................................................................................. 30
Dissolved and Particulate Organic Material ..................................................................... 30
Suspended Sediment Characterization........................................................................................... 32
Distribution and Concentration......................................................................................... 32
The Estuarine Turbidity Maximum .................................................................................. 34
Suspended Sediment Quality ............................................................................................ 35
Biological Components............................................................................................................................... 36
Primary Producers.......................................................................................................................... 36
Marsh Vegetation.............................................................................................................. 36
Phytoplankton ................................................................................................................... 38
Animals.......................................................................................................................................... 39
Summary and Recommendations for Future Studies.................................................................................. 39
Literature Cited .......................................................................................................................................... 41
Appendix 1.................................................................................................................................................. 47
Appendix 2.................................................................................................................................................. 48
Appendix 3.................................................................................................................................................. 51
INTRODUCTION
The Satilla River drains a watershed of 9,140 km2 in the coastal plain of Georgia. The
river is 420 km long and tidal influence extends 106 km upstream. The main channel of the river
is 8-10 m deep and follows a sinuous path. Freshwater inflow to the estuary is small, averaging
70 m3 s-1, although it rises in excess of 300 m3 s-1 during rainy periods and falls to nearly 0 m3 s-1
during severe droughts. Salinity averages 29 PSU at the mouth and reaches 0 PSU at a
maximum of 50 km upstream. In the lower 25 km of the estuary, the width of the main trunk
from upland to upland averages 2.4 km with an average depth of 3.6 m. The volume from the
mouth to the head of tide is 379 x 106 m3 whereas the freshwater volume averages 194 x 106 m3.
An extensive salt marsh fringes the estuarine system and the intertidal region accounts for more
that 70% of the total area.
The semidiurnal tidal range near the ocean varies from 2 to 3 meters at neap and spring
tide, and typical tidal excursions are between 12 and 15 km. Depth-averaged currents can reach
velocities of 1 m s-1. Density variations are usually dominated by salinity changes and vertical
stratification varies in strength between spring and neap tide. Top-to-bottom density differences
average about 2 kg m-3 at spring tide and 5 kg m-3 at neap. Tidal mixing, particularly at spring
tide, provides enough turbulent energy to break down stratification. Overall, the estuary ranges
from partially mixed to well mixed.
The channel of the estuary can be conceptually divided into compartments bounded by
large sand shoals at the Bulkhead, Bailey Cut and at the east end of Crow Harbor Reach (Figure
1). These compartments appear to have their own internal sediment sources that are remobilized
and redistributed by tidal currents. Suspended sediment concentrations range from 10 mg/L to
greater than 5000 mg/L with great spatial and temporal variation.
The Satilla River estuary has a heterogeneous distribution of bed sediments throughout its
course. Sandy sediments are intercalated axially and laterally with muddy sediments. Large
sand shoals are found along the edge of, and in some case across, a channel that may be floored
by sandy or muddy sediments. There appear to be important sources of fine-grained material
from the river as well as from recycling of salt marsh and tidal flat sediments by erosion. Each
of these sources may be locally important and contributes to the spatial heterogeneity of the
estuarine system.
The vegetation along the estuary exhibits a zonation pattern that correlates with the
overall salinity regime of the estuary. The extensive salt marshes surrounding the Satilla are
generally dominated by salt marsh cord grass, Spartina alterniflora, at lower elevations, with
black needle rush, Juncus roemerianus in areas that are infrequently flooded; brackish marshes
are dominated by S. cynosuroides and S. alterniflora along levees, with monospecific stands of J.
roemerianus throughout the mid-marsh; freshwater marshes characteristically contain a greater
diversity of species, including wild rices, Zizania aquatica and Zizaniopsis miliacae.
1
2
Figure 1. The Satilla River estuary. Distances upriver (in kilometers) shown by white circles.
The purpose of this report is to define the current state of knowledge of the Satilla River
estuarine system. This mesotidal estuary is in a relatively pristine state compared to most
estuaries in the Southeastern United States and is a useful reference point against which the
effects of development in other coastal areas within the region can be assessed.
PHYSICAL SETTING
Land Use in the Satilla River Watershed
The Satilla River watershed lies entirely within the Coastal Plain. The topography of the
watershed is relatively flat and is characterized by sandy, porous soils. Particularly along the
coast and adjacent to the river there are extensive wetlands in the Satilla watershed. Prior to
European settlement, the Coastal Plain was covered by extensive longleaf pine forests, which
were gradually converted to agriculture in the years prior to the twentieth century. Over the past
100 years much of the land of the Satilla watershed has been converted from agriculture to
forest, although much of the forest is cultivated pine. Modern mechanized silvicultural and
agricultural practices have disturbed natural hydrologic regimes and soil drainage through
plowing and extensive ditching, used to convert seasonally flooded wetlands to pine plantations,
and through increased use of irrigation. The hydrological impacts of this changing land use and
silviculture have not been adequately determined.
Population growth in the Satilla watershed has exceeded 20% per decade in recent years,
although the watershed is still primarily rural. There has been an increase in urban and
residential areas over the past 10 years, with about 100 km2 classified as developed (urban,
commercial and industrial) in 1974 increasing to about 800 km2 in 1994. This figure probably
underestimates low density residential land use. Figures 2 and 3 show land use in the watershed
for 1974 and 1998, respectively.
The impact of increasing population and changing agricultural and silvicultural practices
on water use and water availability in the Satilla watershed, and indeed in coastal Georgia, is one
of the most important issues facing Georgians. Alber and Smith (2001) compiled information on
water use in the Satilla River watershed from the Georgia Water Use Program, which regularly
surveys both water sources (groundwater and surface water) and water uses (domestic,
commercial, industrial, mining, irrigation, livestock, thermoelectric, and hydroelectric) as part of
the USGS National Water Use Synthesis. In 1995, total water withdrawals in the Satilla
watershed amounted to about 40 mgd (million gallons per day), of which 78% was withdrawal of
groundwater. The largest withdrawals in the Satilla watershed were for public use (27%) and
irrigation (49%). Domestic use accounted for 11% of the withdrawals, while mining, industrial,
livestock and commercial accounted for the remaining 13% (Alber and Smith, 2001). Of the 40
mgd withdrawn, an estimated 63% (26 mgd) was consumed, mostly via irrigation. Water use
patterns have remained stable over the past 20 years, although there were probably large changes
prior to the time that adequate records have been kept. Future changes in land use and
population growth will likely change these patterns and depletion of the aquifers presently used
for most of the water consumption in the Satilla watershed could have far-reaching impacts.
3
Figure 2. Land use in the Satilla River watershed in 1974.
Figure 3. Land use in the Satilla River watershed in 1998.
4
Framework Geology
Along its course in coastal Georgia, the Satilla River is incised into the Satilla Formation
and the underlying Cypresshead Formation, which in turn overlies the Charlton Member of the
Coosawhatchie Formation. The descriptions of these units are taken from Huddleston (1988).
Beneath a surficial veneer of modern fluvial and nearshore sands (0-3 m thick), the Satilla
formation is the youngest unit, being Middle Pleistocene to Holocene in age ( ~1 My - Modern).
Within the Satilla Formation are contained the components of the Pleistocene Pamlico, Princess
Anne and Silver Bluff emergent shoreline complexes and the Holocene barrier island complex.
Therefore it is a lithologically heterogeneous unit that consists of variably fossiliferous, shelly
sands and clays of offshore, inner continental shelf origin; predominantly bedded to non-bedded
barrier island deposits and marsh deposits. Sands are typically well sorted and are fine to
medium grained. Humate-cemented sandstone is common locally and large boulders of humate
sandstone can be observed. Erosion of some of the Pleistocene, emergent barrier island
sequences apparently provide a large portion of the sand being transported down the Satilla River
today (J. Crawford, 2001, personal communication). The Cypresshead Formation underlies the
Satilla Formation and is Late Pliocene to early Pleistocene in age (3.6 My – 1 My). It too
represents coastal, beach to back barrier environments and displays great lithologic variability
over short spatial scales. It can be distinguished from the Satilla Formation based on coarser
grain sizes and reddish, iron-stained sands with thin clay partings. The Charleton member of the
Coosawatchie Formation is mid to late Miocene in age (16-5 My). The typical Charleton
member consists of clay, dolostone and limestone, with clay the major lithic component. All
lithologies are sand and phosphate poor. The lithology of riverbed outcrops of limestone and
calcareous clays in the Satilla are consistent with descriptions of the Charlton member. In
summary, the units being cut into by the Satilla River provide a wide range of materials for
transport, ranging from fine to coarse sands and pebbles, stiff to unconsolidated clays and
calcareous clay outcrops.
Bottom Sediment Characterization
Estuaries tend to be coarse-grained at their heads from accumulation of sandy, fluvially
derived material, and at their mouths, where energetic marine-dominated waves and currents
winnow out finer-grained material (Nichols et al., 1991). In the middle reaches of an estuary
lower energy conditions and the enhanced trapping associated with a turbidity maximum zone
allow fine-grained sediments to accumulate. In contrast to this generalization, the Satilla River
estuary has a heterogeneous distribution of bed sediments throughout because it is a physically
energetic estuary with complex bathymetry. The river channel itself is in a sense divided into
compartments by large sand shoals at the Bulkhead, Bailey Cut and at the east end of Crow
Harbor Reach. These compartments may each have their own internal sediment sources that are
remobilized and redistributed by strong tidal energy. (Howard and Frey, 1975; Blanton et al.,
1999; Jahnke et al., 2003; Alexander, unpublished data). Bottom samples show that the
dominant bed sediment type along the main stem of the river changes repeatedly within the
estuarine portion of the Satilla (Figure 4). As a generalization, sandy sediments are found
associated with the sound mouth and in reaches delineated by river kms 0-4, 6-16, and 28-42,
whereas muddy sediments dominate river kms 4-6, 16-18 and 24-28. Cross-channel texture is
variable as well, with large sand shoals along the edge of, and in some case
5
Figure 4. Bottom sediment type with distance up river. The zero km position corresponds to the sound
limits; negative values are seaward from the inlet. Note that there are two general areas where
sediments are dominantly fine-grained: kilometer 4-6 and 24-28, possibly reflecting the seaward and
landward excursion of the suspended sediment load.
across, a channel that may be floored by sandy or muddy sediments. Based on a set of box cores,
Howard and Frey (1975) suggested that the Satilla – St. Andrews Sound system is relatively
unique among Georgia estuaries, as it has abundant quantities of both sand and mud, yielding a
variety of bedding types, and has a strong fluvial influence, given the preserved dominance of
physical over biogenic sedimentary structures in the channel bed sediments. They suggested that
fluvially-derived sands and clays imported from the ocean are of equal importance to the
estuarine sediment budget. The other potentially important sources of sediment to the estuary
include: fine-grained material transported down the river, erosion of the Satilla and Cypresshead
Formations, and recycling of salt-marsh and tidal flat sediments by erosion. Each of these
sources may be locally important and contribute to the spatial heterogeneity of the estuarine
system. Windom et al. (1971) used clay mineralogical signatures to demonstrate that oceanic
import may be the dominant source of fine-grained sediment up to the limit of salt intrusion,
approximately 50 km upstream. In conjunction with examination of the clay fraction, Windom
et al. (1971) also examined the heavy mineral fraction of the sediments. They determined that
both the heavy mineral fraction, which is enriched in the more resistant minerals (zircon,
tourmaline, and rutile) and depleted in the less stable minerals (epidotes and amphiboles), and
the clay mineralogy, which is dominated by montmorillonite, reflect the heavily weathered and
reworked coastal plain and continental shelf sources for river and estuarine material. In
summary, although many sources have been identified and suggested as dominant, no consensus
exists as to the source for the bulk of the sediment within the estuary.
6
Sidescan sonar surveys of the lower 45 km5 of the river’s course document the strong
heterogeneity of the environment and show that a variety of sediment types dominate the
riverbed (Jahnke et al., 2003; Alexander, unpublished data). These continuous sonar images of
the river bottom provide visual interpretive data that can be used to extend the representative
spatial coverage of physical samples once adequate ground-truthing of the imagery is obtained.
At present, adequate imagery of the bottom in the main stem of the river exists, but a coordinated
groundtruthing of the imagery has not occurred, limiting our ability to make environmental
interpretations from the sidescan data. No sonar surveys of White Oak Creek have been
conducted.
Based on the sonar imagery, the following observations can be made. In the upper
reaches of the Satilla, seaward-oriented, migrating sandy bedforms cover the channel bottom
from Woodbine (the upper limit of surveying) as far seaward as Ceylon. Below the I-95 bridge,
extensive mudflats and a sandy, bedform-rich channel co-occur. Between Ceylon and White
Oak Creek, portions of the bottom show old, semi-consolidated, exposed muds over which
isolated, starved sediment waves migrate. One section of the channel bottom, just below Ceylon,
has no modern sediment, with relict, calcareous clays, possibly of the Charlton member, exposed
at the riverbed. At the confluence of White Oak Creek and the main channel of the Satilla River,
a scour hole cuts down into the Charlton member, producing a coarse lag of pebbles, limestone
fragments, shell molds and other fossils. From White Oak Creek to the seaward end of Crow
Harbor Reach, bed sediments are extremely underconsolidated muds and exhibit rapid
accumulation, as documented by Pb-210 accumulation rates (Figure 5). However, large sand
banks flank the southern side of Crow Harbor Reach. Between Crow Harbor Reach and Bailey
Cut, sediment type is variable, exhibiting a range from outcropping calcareous clays, to
furrowed, gas pockmarked muds (Figure 6a). Similar furrows have been observed in other
systems, where they are thought to be initiated and maintained by helical flows created by swift
tidal currents (see Dellapenna et al., 2001). The pock marks may represent gas blowout or fluid
escape structures, which have been observed in many settings from the deep sea to shallow
estuaries (see Yun et al., 1999). Below Bailey Cut, sand content in bed sediments increases
seaward, although mud content is still significant. Cores of the bottom here reveal interbedded
sand and mud with beds ~10 cm thick (Figure 6b). Sands dominate the system from the
Bulkhead to about river km 6, with large shoals crossing the river and paralleling its axis.
Between river kms 4-6, muddy sediments at least 50 cm thick and appearing to accumulate at
rates of ~0.3 cm y-1 are present on the bottom (Alexander, unpublished data). From river km 4
seaward, the system is again characterized by sands and sandy bedforms because of the
dominance of wave and tidal energy.
Geology and Groundwater Input
The geologic heterogeneity in the system may play an important role in the geochemistry
of the sediments and waters of the Satilla. The river banks (i.e., the saltmarsh/channel interface)
are characterized either by broad, gently dipping ramps where depositional processes prevail or
by steep scarp faces where erosional processes prevail. In most cases, numerous subaqueous,
bank-parallel lineations are observed. These lineations provide evidence of layering within the
saltmarsh deposits. On sidescan images, sandy bed materials show up as lighter-colored areas
and the steep banks of the channel, cut into the saltmarsh deposits, exhibit a black character,
7
indicative of a strong return of acoustic energy (Figure 6b). The steep banks of the river
channels exhibit white and black banding, which represents a ledge-like structure to the banks
themselves and indicates either sandy-muddy interbeds within the marsh or partings between
depositional units in the muddy salt marsh deposits. As portrayed on this image from Crow
Harbor Reach, these subaqueous ledges are ubiquitous throughout the study area. Assuming that
this layered structure extends from the river banks back into/under the marsh, the sidescan sonar
imagery provides evidence of a conduit for submarine recycled groundwater discharge (Jahnke et
al., 2003). Based on a series of benthic chamber deployments, these recycled waters, which are
pumped through the marsh by hydrostatic forcing, have been documented to exhibit up to 6 times
higher salinity and several orders of magnitude higher nutrient concentrations (i.e., silicate,
nitrate, ammonia, phosphate) than the overlying river water. Measured microbial mineralization
is insufficient to sustain measured benthic fluxes. A simple advection-diffusion model for
estuarine nitrogen suggests that recycled water influx would need to be occurring in only 0.05%
of the estuary’s area to account for the observed water column distribution of nitrate.
Figure 5. Radiochemical and porosity profiles for core SL+024, in Crow Harbor Reach, which
document the dynamic and rapid nature of sediment accumulation in this region. Porosity in this
core (right panel) is extremely high, revealing the rapidly deposited, underconsolidated nature of
the muds in this region. The distribution of excess Pb-210 activity throughout the core (left panel)
reveals that all the sediments are younger than 100 years old and that two depositional units (0-20
cm and 20-55 cm) are present, separated by an erosional hiatus. This hiatus encompasses ~50
years, given the difference in Pb-210 activities in the two layers. The Cs-137 distribution (middle
panel) supports this result, as it is present in the upper layer and absent below. Cs-137 was first
introduced into the environment in 1954 (~46 y ago at core collection).
8
A
B
Figure 6. Sidescan images of the Satilla River Estuary. Strong energy returns (i.e., channel banks,
coarser, sandy material) are black; weak or no energy returns (i.e., muds, gently sloping ramps)
are white. A) Section of the estuary exhibiting erosional furrows and pock marks, potentially from
biogenic gas release or groundwater flow. B) Section of the estuary exhibiting subaqueous ledges
that may provide evidence of and pathways for groundwater flow into the estuary from the marsh.
Note also the ebb-oriented, sandy bedforms moving along the channel bottom.
9
Dissolved, long-lived radium isotopes provide more evidence for significant input of submarine
groundwater discharge into the estuary. These isotopes enter the estuary from riverine and
oceanic input and submarine groundwater discharge (SGD) (Moore, 1996). The riverine input is
composed of radium dissolved in river water and radium desorbed from particles transported by
the river. The dissolved input may be measured directly and the desorbed input can be
calculated. The dissolved input has been found to be in the range 1-2 dpm/gram for both 226Ra
and 228Ra (e.g. Elsinger and Moore, 1984; Krest et al., 1999). The inputs from suspended
sediments and SGD cause a nonconservative increase of radium in the intermediate salinity zone
of the estuary. In other words, measured activities are greater than the linear mixing line
connecting the riverine and oceanic endmembers on a plot of dissolved radium vs salinity.
The activity of 226Ra in the Satilla River increases almost linearly from 6 to 40 dpm/100L
in the salinity range 0 to 17 PSU and then decreases by a factor of about 2 between 17 and 35
PSU. 228Ra follows a similar pattern but increases by a factor of 17, then decreases by a factor of
about 2 (Figure 7). These patterns reflect strong nonconservative inputs of radium to the estuary.
The input from radium desorption from suspended sediments only adds an additional
0.04 dpm/L to the dissolved concentration. Thus the nonconservative input from suspended
sediments is negligible. The remaining input must be due to submarine groundwater discharge.
To calculate the volume of groundwater required to support the measured nonconservative
increase of radium in the estuary requires knowledge of the residence time of water in the estuary
and the activity of radium in the groundwater. Efforts are currently underway to quantify these
two unknowns (B. Moore, 2003, personal communication).
Other forms of additional data are presently being collected to identify areas of SGD in
the Satilla River Estuary as well. Thermal infrared aerial photography of the region and
porewater chemical data provide evidence of SGD at Dover Bluff and in the area of White Oak
Creek (M. Joye, 2003, personal communication). Detailed information on sediment porewater
characteristics (inorganic nutrients, dissolved organic material, sulfate, hydrogen sulfide,
chloride, methane, and reduced iron) over a range of seasons can be found in Westan et al.
(submitted).
10
Figure 7.
228
Ra and 226Ra along the Satilla estuarine gradient.
11
PHYSICAL PROCESSES
There has been substantial progress in our understanding of physical processes in the
Satilla River, particularly since the advent of the Georgia Department of Natural Resources'
Coastal Incentive grant-funded research for observations and Georgia Sea Grant funded research
for numerical simulations. We now have a much better description of the salinity and discharge
regime and their relationship to flushing. We have made advances in our understanding of the
transport of momentum and salt and related studies of sediment transport. These advances have
primarily resulted from direct observations of the competing effects of density-driven versus
tidally driven circulation in a single 2-km reach of the estuary (Seim et al., 2002; Blanton et al.,
2003). We are also developing a better understanding of the effects of lateral (i.e. secondary)
circulation on momentum, but the required data are insufficient to allow extrapolation to
secondary circulation effects on the distribution of salinity. We are also able to document that
bottom stress (or drag forces) at the bottom are variable in time, and probably, in space as well.
There are also recently developed hydrodynamic and sediment transport models in the Satilla
River estuary, providing high-resolution simulations of currents, salinity and suspended sediment
concentration.
Discharge
3
-1
Discharge (m s )
There is a USGS gage at Atkinson
450
(gage #02228000), which is located
approximately 133 km upstream of the
300
mouth of the estuary. This gage has a
period of record dating back to March 21,
150
1930. From 1930 to 1999, median flow
3 -1
was 25 m s , although maximum and
minimum recorded flows were 1903 m3s-1
0
and 0.4 m3 s-1, respectively. The median
O N D J F M A M J J A S
monthly discharge is shown in Figure 8.
M o nt h
Although there is a great deal of variability
in the record, there is a seasonal mininum
Figure 8. Median monthly discharge to the Satilla
in discharge during the fall and a
River estuary over 30 years (1968-1997). Values shown
maximum in February, with a secondary
are medians, 10th and 90th percentiles. From Alber and
maximum in August. The August peak in
Sheldon 1999a.
discharge coincides with a peak in rainfall
in the southeastern part of the state (Plummer, 1983). There has been a slight trend upward in
annual mean flow (p= 0.1) (Elkins, 2001). Seasonal discharge patterns have altered as well, with
increased flows in the winter and decreased flows in the summer. Shifts in the seasonal pattern
of rainfall could partially account for the changes in discharge, with a trend toward lower rainfall
in the summer months (Elkins, 2001).
Analysis of the historic flows on the Satilla River measured at the USGS station at
Atkinson was performed using two different methods that used higher resolution datasets to
identify more subtle changes in the hydrograph (Elkins, 2001). The first analysis was performed
using the Indicators of Hydrologic Alteration (IHA) trend-analysis method, developed by Richter
12
et al. (1996), for The Nature Conservancy, which generates 34 metrics of alteration using dailyflow data. That analysis indicated statistically significant increases in winter maximum and
minimum flows and in measures of both the slope of the hydrograph and high-pulse behavior.
However, aggregate seasonal precipitation measured in the coastal plain region for the period
1895-1989 was unchanged in the fall, winter, and spring, though it declined slightly during
summer months. A second analysis of Satilla River flow was performed using a hydrologic yield
calculation modeled after the method of Chagnon et al. (1996), and Moglen and Beighley
(submitted), which can help to assess the impacts of urbanization on runoff characteristics for a
basin. The hydrographic yield (a ratio of runoff to precipitation) after typical storm events was
calculated for storms between 1948 and 1998. The ordered set of these values was then analyzed
on a seasonal basis and, again, the most striking results were observed for winter storms. While
the range of yield values for storms in spring, summer, and fall was reasonably consistent, there
was a marked increase in the variability of yield values for winter storms. As hydrographic yield
is strongly influenced by land use, this pattern suggests that seasonally changing land uses (or
land uses in which the land cover changes on a seasonal basis) may significantly be affecting
runoff patterns in the Satilla basin.
Salinity Regime
The salinity regime varies greatly along the axis of the estuary of the Satilla River (Figure
9). The generally well-defined decrease in salinity with distance from the ocean is highly
Figure 9. Satilla River estuary average mid-tide water column salinity vs. distance from the
ocean. Individual observations (symbols) are from paired high and low tide transects, with the
exceptions of 11/94, 7/96, and 3/97, which had single transects at mid-tide. Curves are 4parameter logistic fits of salinity vs. distance, with 2 parameters (freshwater and ocean salinity)
fixed. Compiled from Georgia Rivers LMER data by J. Sheldon.
13
variable at a fixed point in time and is related to freshwater discharge and tidal phase. From
January 1999 to July 2000, freshwater discharge varied 8-fold, with consequent effects on the
salinity observed along the estuary. For example, the location of 15 PSU changed from
approximately 10 km from the mouth during high discharge to further than 35 km during low
discharge.
During the period of 1999-2001 there was a severe drought, as documented in the
discharge record (Figure 10a). Two experiments, sponsored by Georgia's Coastal Management
Program, were conducted during two different discharge regimes. The response of the axial
salinity distribution to extremely low discharge is evident when one compares the progressive
increase in observed salinities (Figure 10b). Thus, the estuary and its habitats can experience
large changes in salinity exposure over a period of a few months.
Figure 10. (a) Satilla River discharge into the estuary, 1/1/99-8/31/00. Dates of salinity transects are
circled; dates of instrument deployments are shaded. (b) Salinity distributions in the Satilla River
estuary at mid-tide. All transects represent surface salinities except 7/21/00, which is average water
column salinity. From Blanton et al. (2001).
Flushing Time
Transport times furnish the context for many of the biological and chemical processes
that occur within an estuary, as they establish time scales for conservative physical transport of
river-borne material, such as nutrients, organic matter, and suspended sediment. They can thus
be compared against the time scales of biogeochemical and other non-conservative processes that
might affect these constituents to determine whether transformations are occurring in estuaries.
Flushing or transit time is one of several transport time scales. Other scales can be defined, such
as residence time and age of water, but they require the application of hydrodynamic models
requiring considerable effort (Oliviera and Baptista, 1997; Sheldon and Alber, 2002). Flushing
times are usually calculated by either fraction of freshwater methods or tidal prism methods
(Dyer, 1997) and are based on the ratio of the total mass of a tracer in a defined volume to the
input rate of that tracer. Tidal prism methods can be applied to systems with a negligible input
14
of freshwater, but the fraction of freshwater method is appropriate for riverine estuaries such as
the Satilla River estuary.
FlushingTime (days)
Alber and Sheldon (1999a, 1999b) have performed an extensive analysis of flushing
times in the Satilla River and other Georgia coastal estuaries using the fraction of freshwater
method (Dyer, 1997). Over a 30-year period (1968-1997), they estimated that median flushing
time in the Satilla River
estuary was 67 d. More
200
importantly, their analysis
suggested that small changes
150
in discharge could result in
very large changes in flushing
100
time. The seasonal pattern of
flushing times for the Satilla
50
River are shown in Figure 11.
Note that peaks and troughs in
0
flushing times (Figure 11)
correspond to troughs and
O N D J F M A M J J A S
peaks in discharge (Figure 8).
Month
For example, the annual peak
in discharge in February and
Figure 11. Pattern of monthly flushing time in the Satilla River Estuary
March corresponds to a trough
over 9 water years (1974-1982). Values shown are medians, 10th and
in flushing time, whereas the
90th percentiles. From Alber and Sheldon 1999a.
long flushing times in
December and January reflect
observed troughs in discharge
in November and December.
There is also considerable variability in flushing times in the Satilla at both inter-and
intra-annual scales. At an inter-annual scale, median flushing times ranged from 22 d (1991) to
144 d (1981). Within a single year, differences in flushing times exhibited an even larger range:
in 1980 the Satilla had a minimum flushing time of 2.6 d and a maximum of 118 d. Over the
entire period (132 observations), the minimum and maximum observed flushing times ranged by
a factor of 104 (2.6 - 270 d). This underscores the point that there is not just one flushing time
that can be used for a given estuary, as there can be large variability both among and within
years.
Discharge during the last half of 1999 and through 2000 was among the lowest on record.
Salinity increased throughout the estuary over this period and by July 2000 values were near
record levels. The freshwater volume of the estuary also varied in response to changes in
discharge. Between February 1999 and July 2000, freshwater volume decreased from 230 x 106
m3 to 78 x 106 m3, 60% below the average volume of 194 x 106 m3 estimated for this system, and
the corresponding flushing times ranged from 31 to 119 d (Table 1). Note that the lowest
flushing-associated discharge in July 2000 did not correspond to the longest flushing time, a
result of two competing effects. Although a decrease in river flow can generally result in an
increase in flushing time (slower flushing), it will also cause a decrease in freshwater volume.
15
Since the flushing time is dependent on both factors (it is the quotient of freshwater volume and
discharge), a reduction in freshwater volume acts to moderate the effect of decreased discharge
on flushing time. As the drought continued through summer 2000, river flows were consistently
low long enough that the freshwater volume dropped to the point where it had an ameliorating
effect on flushing time (Blanton et al. 1999).
Table 1. Estimated flushing times and related calculations for the Satilla River estuary.
Date
Freshwater
Volume
(106 m3)
Flushing
Time
(days)
Average Prior
Discharge
(m3 s-1)
02/23/99
03/09/99
05/12/99
06/24/99
07/21/00
230
216
125
107
78
31
35
82
119
96
86
71
18
10
9
These estimates implicitly include flushing by seawater (Sheldon and Alber, 2003), but
because the calculation is done on a tidally-averaged basis, it cannot provide information on
processes occurring at subtidal time scales. In order to address mixing processes at finer
temporal and spatial scales, such as the flooding of intertidal marshes, a 2-D hydrodynamic
model would be required. The Satilla should be particularly well suited to this because of the
large data sets presently available that were obtained with support from the Coastal Zone
Management Program. An approach could be followed that solves for conservation of
momentum and salt analytically, thereby avoiding the issues of model stability and computation
times associated with highly scaled numerical models.
Tidal Currents
Moorings and anchor station data indicate tidal currents can vary between ±100 cm s-1
during spring tide and ±60 cm s-1 during neap tide (Figure 12). Note subsurface flood maxima
between 4 and 6 m above bottom and surface maxima in ebb. These maxima can occur through
straining of the cross-channel density field by lateral velocity shear (Turrell, Brown and
Simpson, 1996). Flood and ebb currents achieved maxima within 1.5 to 2 hours after slack,
indicating a relatively high degree of asymmetry. Asymmetry is strong throughout many
shallow mesotidal estuaries like the Satilla (Blanton et al., 1999; Blanton, Lin and Elston, 2002).
16
Figure 12. Vertical profiles of salinity (top), axial current in cm s-1 (middle) and suspended sediment
concentration in g l-1 (bottom) for spring and neap tide. Measurements were made at a 25-hr anchor
station during LMER4 (April 1997) in the curving channel 2 km seaward of Bailey Cut. Note the different
scales for spring and neap for the suspended sediment profiles. From Blanton et al., 2003.
A comparison of axial and cross-axial currents at the west and east ends of the curving
channel seaward of Bailey Cut (Seim, Blanton and Gross, 2002; Blanton et al., 2003) showed
significant differences. During spring tide, flood currents were consistently stronger at the
landward end than at the seaward end. On the other hand, ebb currents were consistently
stronger at the seaward end. This indicates that the currents increase in strength along the flow
direction, an effect that is particularly strong during spring tides. These findings were attributed
to the addition of momentum to bottom currents by the cross-axis secondary circulation acting
throughout the channel bend (Seim and Gregg, 1997).
17
Tidal currents have a significant effect on salinity profiles and suspended sediment
profiles (Figure 12). There is more vertical shear near the bed during flood than during ebb that
will affect the bottom stress. As discussed later, the flood stage has higher velocity shear and
higher stress at the bottom than the ebb. Note also the difference in salinity profiles. Vertical
salinity gradients are stronger during neap than during spring tide. The gradient is also stronger
during ebb due to tidal straining of the salinity field (Simpson et al., 1990). This also affects the
bottom stress regime.
Note the strongly layered suspended sediment distribution at the bottom. The effect is
strongest during spring tide. The mobilization of these layers has been described (Blanton et al.,
1999) and is the subject of continuing research. These will be discussed in more detail below.
Residual Circulation
Circulation patterns alter the longitudinal and lateral density (salinity) gradients of an
estuary. These gradients impose forces that affect the degree of mixing between freshwater
discharge and ocean water. The classical view of estuarine circulation states that there is outflow
of low salinity water at the surface and landward flow of higher salinity water at the bottom.
There is increasing evidence that the classical view of estuarine circulation must be modified in
partially mixed estuaries like the Satilla. During spring tide, the axial current along the bottom
may, at times, export material while in the intertidal zone above mean low water (MLW), the
current may import material (Seim, Blanton and Gross, 2002; Blanton et al., 2003). The change
to landward flow above MLW is consistent with strong tidal fluxes (tidal pumping). As spring
tide approaches, increased tidal pumping drives mass landward on the shallow flanks which
requires seaward flow in the deep channel to balance the mass (Li, 1998). Recent data indicate
that the increased tidal flux at spring tide may completely shut down the gravitational mode at
the bottom in some portions of the Satilla (Figure 13).
Figure 13. Subtidal
fluctuation of currents in
a bend in the Satilla
estuary. West, mid and
east locations are 1.0,
1.6 and 2.5 km seaward
of Bailey Cut. Distance
above bottom is given in
meters (mab). "N" and
"S" show the times of
neap and spring tides.
From Blanton et al.,
2003.
The implication for salt flux is important. While neap18
tide
profiles show import at depth (consistent with gravitational circulation) and export from the
middle of the water column, transport in the presence of strong spring tides can reverse the
classical residual transport model. Yet landward flux of salt is apparently maintained in the
intertidal zone, even when the subtidal seaward flow along the bottom becomes downstream.
This zone would include the extensive shallow areas at the sides and would indicate that
numerical simulations must accurately simulate fluxes in all intertidal areas in order to calculate
the salt balance of the estuary.
The presence of channel bends causes strong cross-channel currents (secondary
circulation). Their strength also follows a neap-spring tide pattern. During neap, currents are
weak. As spring tide conditions evolve, the cross-channel currents near the bottom begin to flow
from the outside to the inside of the curving channels (Seim, Blanton and Gross, 2002),
indicating the presence of secondary circulation in the curving channel. The strength of this flow
is not spatially uniform along the channel. There always appears to be strong outward flow in
the intertidal zone (above MLW), and the data suggest the presence of a single cell (outward
flow near surface, inward flow near bottom) during spring tide. During neap, on the other hand,
there is a more complicated structure, suggesting the presence of (perhaps) two cells, stacked one
on top of the other. The evolving structure of the secondary flow during the neap-spring cycle is
the subject of a PhD thesis presently underway.
Current fluctuations at frequencies below the semidiurnal tide (subtidal frequencies)
(Figure 13) correlate well with similar fluctuations in salinity (not shown). The fluctuations are
caused by subtidal changes in water level which can be driven by wind-generated transport on
the continental shelf (Klinck, O'Brien and Svensen, 1981) or by remotely forced shelf waves
(Schwing, Oey and Blanton, 1988). These cause depressions in water level that are accompanied
by depressions in salinity and represent a seaward shift of the salinity field. The opposite is true
for episodes when water level rises. Thus sub-tidal water level fluctuations can be driven by
forces remote from the estuary and can cause axial translations of the salinity gradient up and
down the estuary.
Non-uniform Distribution of Bottom Stress
Estimates of the bottom drag coefficient (Cd) from the instrumentation that produced the
data shown in Figure 12 were based on direct measurement of the bottom Reynolds stress (Seim,
Blanton and Gross, 2002). Reynolds stress increased by a factor of four over the interval from
neap to spring tide. Estimates of Cd gave an average value of 0.0017, but there was considerable
scatter in the relationship. Moreover, many of the highest stress estimates during spring tides are
not well represented by this regression, a value of Cd = 0.0025 being more appropriate. The
higher drag coefficient and the higher velocities at spring tide generate significantly higher
bottom stress at this time.
The presence of vertical density stratification near the bottom inhibits turbulent
penetration of turbulence to the bottom thereby decreasing the bottom stress. Many partially
mixed estuaries exhibit more stratification at ebb than at flood due to tidal straining of the
salinity field (Simpson et al., 1990). Studies indicate that near-bottom stratification in some
reaches of the Satilla is present at ebb but not during flood (Seim, Blanton and Gross, 2002).
19
This can raise the gradient Richardson number well above 0.25 over most of the ebb phase and
thereby lower the shear stress at the bottom. There is more vertical shear at the bed during flood
than during ebb. As a result, the flood stage has higher velocity shear and higher stress at the
bottom than the ebb. This provides a regime that preferentially drives sediment transport
landward.
Sediment Transport, Mobilization and Resuspension
Recent ADCP (Acoustic Doppler Current Profiler) studies have provided an opportunity
to use the echo intensity data as an uncalibrated source of information on suspended sediment
mobilization and resuspension. The strengths of currents and echo-intensity are significantly
higher during spring tide, as expected. Echo-intensity drops to minimum levels at high water
during both spring and neap indicating a relative clearing of the water column, but intensity
remains relatively higher at low water. Sediment transport is directly affected by the strength
and timing of tidal currents. In most narrow estuaries like the Satilla, wave-induced currents are
usually of secondary importance (Mason and Garg, 2001).
Sediment profiles during neap and spring reveal pronounced vertical gradients during the
low water part of the tidal cycle (Figure 12). The highly turbid layers near-bottom occurred
around LWS and during the early stages of flood and were found within the lowest 2 m,
coincident with the zone of maximum shear. Maximum concentrations near bottom were greater
by an order of magnitude during spring tide.
Large billows of mud resuspended from the bottom are observed at the surface during the
tidal cycle (Blanton et al., 1999). These billows have a horizontal scale of about 1 m. Based on a
single study, we found that near-bottom concentrations exceeded 1 g l-1 and reached as high as 8
g l-1. The most dramatic SSC profiles occurred within the first 2 hours of flood when mud layers
about 1-2 m thick over the bottom had concentrations between 2-4 g l-1 (see also Figure 12.)
Sediment fall velocities spanned three orders of magnitude from 0.001 mm s-1 to 1 mm s-1 in the
main channel. The fine particles with the slowest settling velocity would be continuously
resuspended before reaching the bed. However, the larger faster-settling particles would reach
the bed in a couple of hours when tidal currents were below the threshold value for resuspension.
The slowly settling particles could only reach the bed in areas that are very shallow and where
tidal currents remain small throughout the tidal cycle. This environment is found within the
small shallow tidal creeks feeding large expanses of salt marsh and probably accounts for the
presence of very fine-grained sediments on the beds of these environments (Postma, 1961).
Based on levels of organic carbon (Blanton et al., 1999), we speculated that much of the
suspended sediment load is lifted off fringing intertidal salt-marshes and mud aprons at the
marsh edge where it becomes entrained in the flow in the deeper part of the channel. Once in the
main channel, it is moved back and forth by tidal currents.
These results, developed since 1995, provide increasing evidence that suspended
sediments are preferentially transported upstream (Blanton et al., 1999; Seim, Blanton and Gross,
2002). Bottom stress is significantly greater during flood compared to ebb and significantly
higher during spring tide. However, the curving channels in the Satilla complicate this picture
due to the effect of secondary (cross-axis) flow. Recent studies indicate that flood flow in a
20
curving channel can carry more suspended sediments landward at the upstream end, compared to
the downstream end of the channel (Figure 14). During spring tide, OBS values were noticeably
higher at both sites. Near-bottom values were 50% higher in the west. High bottom and midwater values occurred near the time of LW and into the early stages of flood. The most
significant difference occurred in the higher OBS values in the west throughout the rising tide.
Mid-water values in the east were as high, if not higher, than near-bottom values during flood at
the late stage of ebb. Recall that at neap, secondary circulation is too weak to penetrate the
stratification near the channel bottom. Therefore, during spring-tide flood currents, when we
expect bottom stress to be highest, the stress also appears to increase landward along the channel.
This would account for significantly higher OBS values in the west observed during spring tide
(Figure 14 (upper right)).
One of the major questions concerns whether or not secondary flow has the ability to pick
up increasing amounts of suspended sediments along the sides during flood and add them to the
axial flow in the thalweg. Since the landward flow along the bottom of the thalweg weakens and
even reverses during spring tide, there appears to be a complex re-circulation system for resuspended sediments in curving channels that complicates the picture of a net transport of
sediments landward.
Numerical modeling
ECOM-si, a 3-dimensional hydrodynamic model of the Satilla River estuary, has been
used in several studies of the Satilla system (Zheng et al., 2003a,b). ECOM-si, a modified
version of the Blumberg and Mellor (1987) Princeton Ocean Model, incorporates a particletracking algorithm and the effect of wetting and drying of the salt marshes (Chen and Beardsley,
1998). It can incorporate a sediment transport module (Zheng et al., 2003b) that includes
sinking, sedimentation and resuspension. Nutrient and other field data collected by the Marine
Extension Service during their one-year monitoring of the lower reaches of the Satilla River are
presently being used to verify ECOM-si (K. Gates, 2003, personal communication).
Numerical simulations of the Satilla demonstrate that the flooding and drying process
plays a key role in accurately calculating tidal transport (Zheng et al., 2003a). Ignoring this
process led to a 50% underestimation of tidal current amplitude when compared to observations.
The model is able to compare the Lagrangian velocity of particles to the Eulerian velocity
measured at a fixed point in space. The difference in these two is a measure of tidal pumping.
The observations described above have indicated the importance of tide-induced transport in the
intertidal zone as well as near the bottom where, during spring tide, the normally landward flow
can be halted and even reversed.
Simple passive tracers in the sediment transport module were used to predict spatial and
temporal distributions of total suspended sediment concentrations (Zheng et al., 2003b). The
important outcome of these simulations suggested that maxima in the total suspended sediment
21
Figure 14. A comparison of OBS data at bottom and mid-water levels with bottom pressure at west and
east ends of a curving channel in the Satilla River estuary. Values of OBS have been multiplied by -1 for
flood currents, so that values during flood are less than zero. West end: Neap (upper left), Spring (upper
right); East end: Neap (lower left), Spring (lower right).
22
concentrations are controlled by (1) divergent and convergent patterns in the residual flow field,
(2) a non-uniform distribution of bottom stress and (3) the inertial effects induced by curved
channels (i.e secondary circulation). Many of the field observations discussed previously
indicate the importance of all three of these processes in the Satilla River.
ECOM-si is not able to adequately resolve small-scale tidal creeks and salt-marsh
topography. A finite-volume model (FVCOM) has recently been developed to study the Satilla
River Estuary (Chen, unpublished data). FVCOM is an unstructured grid, finite-volume primitive
equation model. A detailed description can be viewed at http://codfish.smast.umassd.edu/
research_projects/SATILLA/home.html. This new model has shown improvement in the
simulation of tidal elevation/currents, and freshwater-induced buoyancy currents/salinity and
water quality variables.
The demonstrated ability of numerical models to simulate many aspects of circulation
and transport of salt and sediment that are seen in the observations provides motivation to
validate details of the simulations against observations. The models have the ability to simulate
conditions throughout the estuary at scales not easily duplicated by observations. They can
predict changes in the circulation and salinity regime under a wide range of conditions and
assumptions. To have confidence in predictions such as these requires careful and methodical
efforts to validate specific features of the model. These include features such as spatial changes
in the amplitude and phase of the major lunar, solar and shallow-water tidal constituents
(velocity and water level) and spatial changes in the tidally averaged salinity gradient.
In any modeling program, there should be an examination of the costs and relative merits
of the elegantly detailed models discussed above and simpler analytical models such as that of
Thatcher and Harleman (1972). The former, when properly validated, offer detailed insight on
the entire system yet they can usually only be run by the originators of the model. While not
providing the spatial detail, the latter can be structured so that it can become an operational tool
for managers.
WATER QUALITY
Components Entering the Estuary
Dissolved Inorganic Nutrients
The USGS site at Atkinson (#02228000) was a water quality surface sampling station
until 1992. We combined the water quality data (EarthInfo, 1997) and daily discharge data
(http://water.usgs.gov) to get information on the concentrations of nutrients being loaded into the
estuary. Measurements of dissolved inorganic nitrogen (NH4, NO3+NO2) were taken between
10/79 and 9/92 and measurements of dissolved orthophosphate (PO4) ranged from 7/80 to 9/92.
Samples were usually collected on a quarterly basis.
Average concentrations of ammonium, nitrate plus nitrite, and orthophosphate are shown
in Figure 15. These estimates do not show significant trends over time, but there is clear
evidence for interannual variability. There were peaks in the concentrations of all three
23
0.45
0.4
NH4 Conc
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
Sep- Sep- Sep- Sep- Sep- Sep- Sep- Sep- Sep- Sep- Sep- Sep- Sep- Sep79
80
81
82
83
84
85
86
87
88
89
90
91
92
Year
0.7
NO3+NO2 Conc
0.6
0.5
0.4
0.3
0.2
0.1
0
Sep- Sep- Sep79
80
81
Sep- Sep- Sep- Sep- Sep82
83
84
85
86
Sep- Sep- Sep- Sep- Sep87
88
89
90
91
Sep92
Year
0.35
0.3
PO4 Conc
0.25
0.2
0.15
0.1
0.05
0
Sep- Sep- Sep- Sep- Sep- Sep- Sep- Sep- Sep- Sep- Sep- Sep- Sep- Sep79
80
81
82
83
84
85
86
87
88
89
90
91
92
Year
Figure 15. Concentrations of NH4, NO2, NO3 and PO4 (in mg N or P/l) over time at the
Atkinson USGS gauge.
24
constitutents in 1983, with a peak in both nitrate plus nitrite and orthophosphate in 1988. Over
the period of record, NH4 averaged 0.065 ± 0.060 mg N/l (s.d., n = 73), NO3 + NO2 averaged
0.19 ± 0.14 mg N/l (n = 74), and PO4 averaged 0.05 ± 0.05 mg P/l (n = 51). When examined on
a quarterly basis, there is no clear evidence for a seasonal pattern in nutrient concentrations.
However, there is some evidence for lower concentrations during the winter quarter (January to
March) and higher concentrations during the spring (April to June).
Average annual nutrient concentrations (Figure 15) were multiplied by average annual
discharge to obtain an estimate of the loads of dissolved inorganic nutrients to the estuary
(Figure 16). On an annual basis, an average of 143 mtons (± 170) of N as NH4, 417 mtons (±
245) of N as NO3 + NO2, and 120 mtons ± 79 of P as PO4 were delivered downstream. The load
estimates show no linear trends over time, but they do show consistent peaks (1983, 1987, and
1991), and troughs (1981 and 1989). It should be noted that these loads reflect what enters the
upper end of the estuary, and not what is loaded to the coastal ocean. The Satilla river estuary
has extensive intertidal marshes that process material (see discussion of tidal marshes, below),
and additional nutrient sources may be located downstream of the riverine USGS sampling
station (including groundwater input and tidal creeks). Moreover, the amount of time river water
spends in the estuary before being discharged to the coastal ocean will influence the amount of
material processing that occurs within the estuary (see flushing time discussion, above).
Dissolved Organic Nutrients
Alberts and Takacs (1999) did a similar compilation of USGS water quality data to
estimate DOC and DON loads into southeastern U.S. estuaries. Average DOC concentrations in
the Satilla (at Atkinson) were 19.1 ± 8.1 mg C l-1 and DON averaged 0.75 ± 0.33 mg N l-1.
These concentrations rank among the highest in the southeast because the Satilla is a blackwater
river. Alberts et al. (1990) also did a quarterly survey of DOC and POC in the riverine part of
the Satilla River Estuary, and the concentrations they measured fall within the above range
reported by USGS.
Total Suspended Solids
Total suspended sediment (TSS) was also measured as part of the water quality
observations between 1/74 and 9/92. Overall, TSS averaged 16.6 mg/l (± 16.7, n = 156) during
this period (Figure 17). There is no long-term trend in TSS, but there are two sharp peaks when
concentrations exceeded 100 mg/l (in 1975 and 1981). Overall, an average of 23 thousand
metric tons was loaded into the estuary annually (Figure 18).
Components Within the Estuary
Dissolved and Particulate Major Elements and Trace Metals
The chemistry of dissolved metals in the Satilla River has been studied for more than 30
years as a representative example of coastal plain, black water rivers. One focus of study has
been major elements and trace metals in the dissolved and particulate state (Windom et al., 1971;
Beck et al., 1974; Windom 1975; Windom et al., 1991). Windom et al. (1971, 1975) and Beck et
25
NH 4 Lo ad
NH4 (m tons y-1)
1 000
800
600
400
200
0
197 9
1 981
198 3
1 985
198 7
19 89
199 1
198 7
198 9
19 91
Yea r
NO 3 + NO 2
NO 3 + NO 2 (m tons y-1)
14 00
12 00
10 00
8 00
6 00
4 00
2 00
0
1 979
198 1
198 3
19 85
Yea r
PO 4
60 0
PO4 (m tons/y)
50 0
40 0
30 0
20 0
10 0
0
19 81
198 3
198 5
1 987
19 89
199 1
Yea r
Figure 16. Annual loads (metric tons/yr) of dissolved nutrients at USGS Atkinson Station. Error bars
represent standard deviations.
26
80
70
TSS Conc
60
50
40
30
20
10
0
Sep- Sep- Sep- Sep- Sep- Sep- Sep- Sep- Sep- Sep- Sep- Sep- Sep- Sep- Sep- Sep- Sep- Sep- Sep74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92
Year
Figure 17. Total Suspended Sediment (TSS) concentrations (mg/L) reported by USGS at Atkinson.
al. (1974) report inorganic elemental data from throughout the estuarine gradient. Low pH
values typify the freshwater end of the estuary because of high concentrations of organic matter.
Values exhibit a gradient from 5.71 in fresh water to 7.64 at the ocean. Average values for
dissolved elements in the estuary, which are suggested to reflect the composition of the drainage
basin, are Zn (2.1 ppb), Cu (5.7 ppb), Cd (0.8 ppb), Hg (0.07 ppb) and Ni (1.5 ppb). Dissolved
iron (370 ppb) and Mn (42 ppb) both show a seaward depletion, decreasing in concentration by a
factor of 2-3 during the transition from fresh to seawater zones. This depletion suggests
flocculation and precipitation of both these species at higher salinities, pH and Eh (Windom et
al., 1971; Gao and Zepp, 1998). Windom (1975) suggests that effectively all Fe transported
TSS
TSS (mtons x 103/y)
140
120
100
80
60
40
20
0
1974
1979
1984
1989
Year
Figure 18. Total Suspended Sediment (TSS) loads calculated for the USGS station at Atkinson
(error bars represent standard deviations).
27
down the Satilla and most of the Mn is sequestered in the estuary in salt marsh sediments.
However, all dissolved Hg and most of the dissolved Cd and Cu is transported through the
estuary to the sea. In contrast, the concentrations of particle-bound metals associated with
suspended matter (average concentration ~40 mg/l) are orders of magnitudes higher: Zn (561
ppm), Cu (42 ppm), Ni (169 ppm), Cr (470 ppm), Co (21 ppm), Cd (10 ppm), Hg (0.4 ppm) and
Fe (6.2 %). In the particulate phase, only Mn shows an obvious seaward trend in concentration,
exhibiting a concomitant increase in concentration in the solid phase as the dissolved phase
decreases (average 812 ppm, increases from 350-1300 ppm from fresh to salt water).
Dissolved Inorganic Nutrients
There are very few observations of dissolved inorganic nutrients within the Satilla prior
to the LMER program. From 1973-1992, the Georgia EPD sponsored a monitoring program in
which surface salinities were sampled regularly at fixed stations in many of Georgia’s estuaries.
In the Satilla, the station was located at 30° 57' 52" N latitude and 81° 30' 29" W longitude. The
EPD data set is the only one that measured nutrients, and this was done at one station only. The
Georgia Rivers Land Margin Ecological Research Project, funded by the National Science
Foundation, was a multi-investigator six-year project designed to measure the differences and
similarities in material flow and processing in the estuarine areas of the five major rivers which
enter the Atlantic Ocean along the Georgia Coast (Appendix 1). The Satilla River was one of the
rivers studied. A summary of the observations of nutrient concentrations made during the
LMER program is listed in Table 2.
Over the course of these observations, the concentrations of ammonia, nitrate, and nitrite
in the freshwater end of the estuary averaged 0.017, 0.060, and 0.001 mg N/l, respectively.
Concentrations in the estuary were higher, and averaged 0.022, 0.076, and 0.006 mg N/l,
respectively. Concentrations of ammonium and nitrate were at their minimum at the seawater
end, averaging 0.010 and 0.015 mg N/l, respectively, whereas nitrite concentrations were at their
maximum, averaging 0.010 mg N/l. The NO3- distribution along the salinity gradient tended to
have a maximum in the mid-salinity zone (5 to 15 psu), after which concentrations dropped
quickly to near zero at the sea end-member. This pattern did not vary seasonally, although the
surveys conducted from July to October generally showed higher average concentrations of NO3than in February and April. During July to October nitrite concentrations were also much higher
than those in February and April and a NO2- peak was evident, occurring mostly downstream of
the NO3- maximum (around 20 – 30 psu). In contrast, NH4+ exhibited near-zero concentrations
(close to or below detection limit) along the salinity gradient from July to October and high
concentrations in February and April, when a bell shape (a concentration peak at the midsalinity) was clear. These data are currently being written up for publication (Wang et al. in
prep.) and are also available via the Georgia Rivers LMER web page:
(http://lmer.marsci.uga.edu/datapage/riverdata.html).
28
Table 2. Nutrient samples taken in the Satilla River Estuary and White Oak Creek by the Georgia Rivers
Land-Margin Ecosystems Research (LMER) Project (W. J. Wiebe and J. E. Sheldon, unpublished data).
All samples were taken during low tide.
Date
(mm/dd/yy)
11/06/94
04/08/95
04/14/95
04/15/95
04/16/95
10/17/95
10/25/95
10/26/95
06/08/96
06/09/96
06/10/96
06/11/96
06/11/96
07/17/96
07/18/96
07/20/96
07/21/96
08/20/98
08/23/98
08/28/98
02/05/99
Channel
Satilla
Satilla
Satilla
Satilla
Satilla
Satilla
Satilla
Satilla
White Oak
White Oak
White Oak
White Oak
Satilla
Satilla
Satilla
Satilla
White Oak
Satilla
White Oak
Satilla
Satilla
+
-
-
3-
NH4 NO2 NO3 TDN PO4
Station
Si
(km from mouth) µM N µM N µM N µM N µM P µM Si
-14 – 33
*
*
*
*
*
*
2 – 34
*
*
*
*
*
*
16 anchor
*
*
*
*
*
*
1 – 34
*
*
*
*
*
*
16 anchor
*
*
*
*
*
*
0 – 48
*
*
*
*
*
*
0 – 32
*
*
*
*
*
*
25 anchor
*
*
*
*
*
*
1 – 16
*
*
*
*
*
1 – 16
*
*
*
*
*
1 – 16
*
*
*
*
*
1 – 15
*
*
*
*
*
26 – 41
*
*
*
*
*
0 – 47
*
*
*
*
*
4 – 32
*
*
*
*
*
0 – 49
*
*
*
*
*
0 – 16
*
*
*
*
*
0 – 39
*
*
*
*
*
0 – 16
*
*
*
*
*
-4 – 34
*
*
*
*
*
0 – 47
*
*
*
*
*
The University of Georgia Marine Extension Service monitored nutrients at 7 sites in the
lower Satilla River over a one-year period beginning in February 2000. They found an average
ammonium concentration of 0.034 mg N/L at low tide and 0.045 mg N/L at high tide within the
Satilla River estuary. High tide ammonium averages often resembled the low tide average for
the same month, except in July when the high tide average jumped to almost 0.140 mg N/L from
near 0 mg N/L at low tide a week earlier. Ammonium concentrations varied from station to
station on the Satilla River with no apparent trend.
Samples collected at low tide from the Satilla River and analyzed for nitrate showed an
increasing trend moving from the mouth, up the river for both surface and bottom water.
Additionally, samples collected from several stations at high tide show lower average nitrate
concentrations than samples collected from these stations at low tide. Together, these
observations suggest that nitrate is coming from upstream. The average nitrate concentration in
the Satilla River from February 2000 to January 2001 was 0.09 mg N/L at low tide and 0.06 mg
N/L at high tide. The average nitrite concentration was 0.01 mg N/L at both low tide and high
tide. The minimum and maximum nitrate values observed during the sampling year were 0.0
and 0.34 mg N/L, respectively, with a median value of 0.05 mg N/L.
29
Average orthophosphate concentrations from the Satilla River showed a slight increase
with distance from the mouth in the lower estuary. The average orthophosphate concentration
observed was 0.101 mg/L at low tide and 0.107 mg/L at high tide. The peak orthophosphate
concentration was observed at high tide in July with an average of 0.185 mg/L. Orthophosphate
concentrations were highest during July, August and September.
In summary, Satilla River data from February 2000 through January 2001 reveal complex
relationships between these various parameters. Nitrate and orthophosphate concentrations
increase up river, but specific sources have not been identified. Ammonium and orthophosphate
concentrations both peaked during high tide in July 2000. Nitrite, which in excess can be toxic
to fish and other species, remained relatively low.
Dissolved Inorganic Carbon
The low-salinity regions of the Satilla River are characterized by unusually high pCO2
values that are frequently coupled to undersaturation of oxygen (Cai et al. 1999). Cai and Wang
(1998) and Cai et al. (1998) show that alkalinity and pH rises rapidly in the brackish water of the
Satilla (salinity <3-5 PSU) from 100 µM to 2 mM and 5.5 to 8.0, respectively. High pCO2 levels
(~5000 µatm; 2-20 times atmospheric) are observed in the river as well, where CO2 fluxes are
calculated to be 10-100 times those in the open ocean. The observed gas concentrations cannot
be explained by within-estuary processes (i.e., respiration in the estuarine water column and
sediments). Instead, mass balance calculations in Cai et al. (1999) show that these high levels
are driven by organic matter respiration in the aerobic and anaerobic subsystems of the intertidal
marshes which are flooded at high tide and supply a signal of inorganic carbon that is taken back
to estuarine waters as the tide falls. The high productivity of the intertidal marshes thus affect
the estuaries by exporting endproducts from within-marsh decomposition (CO2, HCO3), with
outwelling of organic matter to the estuaries a minor process by comparison (Cai et al. 2000).
Cai et al. (2000) also demonstrates that the Satilla marsh-estuarine complex is a quantitative sink
for nitrate.
Dissolved and Particulate Organic Material
The behavior of dissolved and particulate organic material (DOM/DOC/POC) has been
another focus of research in the Satilla, with major advances in the past decade (Beck et al.,
1974; Gao and Zepp, 1998; Hopkinson et al., 1998; Moran et al., 1999; Moran et al., 2000; Otero
et al., 2000; Otero et al., 2003). Beck et al. (1974) describe characteristics of the Satilla River as
an example of a black water river, where DOC levels range from 25-50 ppm. They point out that
it has an unusual ratio of DOC:DIM (1:1), whereas this ratio is typically 1:10 in other rivers of
the world. Far up in the freshwater portions of the river, DOM/POM reflects its soil origin and is
chemically similar to fulvic acids, whereas in the lower reaches of the river, the contribution of
swamp-derived humic acids become more important. Otero et al. (2003) further examined these
substances to show that the DOC, which reaches maximum values of 29 mg C/l at the head of
the estuary, is dominated by humic substances in salinities below 15 PSU. Seaward along the
estuary, the DOC decreases to ~5 mg C/l at the ocean and humic substances become less
important, making up 10-20 % of the DOC at the ocean. Otero et al. (2003) further examined
30
the δ13C of the DOC and humic substances to suggest that both riverine and in-situ sources of
carbon (decomposed marsh detritus and microalgal production) are important to the Satilla River
Estuary.
Otero et al. (2000) describe the character of the POC, which is between 2-50% of the
total organic carbon. The POC is dominated by particles derived from C4 plants (40%) and C3
plants (40%), with minor contributions from microalgal production (20%). The δ13C of particles
becomes more depleted toward the ocean, representing the dilution of riverine material with
estuarine and oceanic particles.
Burkholder and Burkholder (1956) found appreciable amounts of B12 (0.8-1.2 ug/g
solids) in Satilla River suspended sediments and water. In settling experiments, the bulk of the
B12 is associated with the organic fraction of solids in sea water. Enrichment experiments show
bacteria to be the dominant producer of vitamin B12 in marine muds and waters.
Photodegradation is an important factor in the processing of organic matter. Moran et al.
(1999, 2000) show that this process allows rapid bacterial degradation of dissolved organic
matter, leading to losses of >30% of the DOC in fresh water and biodegradation of a maximum
of 11% of DOC in marine waters. These low values of DOC degradation in the Satilla system
reflect the lack of labile organic matter in blackwater rivers (2-3%) as compared to values of
19% in typical world rivers. Gao and Zepp (1998) demonstrate that the high levels of Fe in the
Satilla catalyzes the photodegradation of DOM. This degradation leads to decreases in pH and
dissolved Fe concentrations and increased DIC (380 µM ), CO and ammonia, which has
implications for carbon biogeochemistry.
Many terrestrially-derived compounds present in DOM fluoresce strongly when exposed
to UV light, and this fluorescent material (FDOM) provides a sensitive measure of terrestrial
DOM concentration and source. This property has been exploited in the Satilla River by Moran
and Sheldon to study the dynamics and mixing patterns of the estuary. In surveys conducted in
the upper estuary (10-30 km from the ocean), FDOM was conservatively mixed during 1995 and
1996 cruises. In 1997, however, relative FDOM concentration gradually decreased in this region
by up to 15%, suggesting removal (e.g. flocculation, degradation) or fluctuations in FDOM input
to the estuary. In contrast, highly non-conservative mixing was observed in the lower reach of
the estuary (0-10 km) during all three cruises. Major changes in relative FDOM concentration (40% in 1995, +25% in 1996, +40% in 1997) occurred in proximity to the Intracoastal Waterway
(ICW). Additionally, data from 1997 indicate that a mixture of Satilla River and ICW water,
with intermediate FDOM content, is transported to the shelf. These results suggest that
significant lateral transfers of FDOM occur between the Satilla River and ICW that vary both in
magnitude and direction. Additional data on DOC and DOM in the Satilla River estuary is
available on the Georgia Rivers LMER web page
(http://lmer.marsci.uga.edu/datapage/riverdata.html).
31
Suspended Sediment Characterization
Distribution and Concentration
Suspended sediment concentration (SSC), which spans a range from ~10-5000 mg/L in
the Satilla (Kennedy, 1964; Windom et al., 1971; Blanton et al., 1999; Alexander, unpublished
data; Alber, unpublished data), vary greatly in space and over time at any given location.
Typical surface water values range from 10-50 mg/l in slack water surveys of the river (Figure
19A). On these same slack water surveys, SSC in the lower meter of the water column is
typically quite high, ranging from 25-800 mg/L, with maximum concentrations as high as 5,000
mg/L being observed (Figure 19B). However, anchor stations at several sites, which document
the range of concentrations present at any one location over a tidal cycle, exhibit much higher
concentrations with values ranging up to 200 mg/L in surface waters. Thus, the slack water
surveys give an indication of the concentration of materials that are always present in the water
column of the Satilla, whereas the anchor stations show us the dynamic nature of sediment
resuspension, transport and deposition over each tidal cycle (Blanton et al., 1999; Alber 2000).
The anchor station data highlights the behavior of sediments within the river, where tidal
forcing plays a very important role is the resuspension of material twice daily from the bottom.
There are two pulses of resuspension that are observed related to tidal forcing, one at early-mid
ebb and one at early-mid flood tide. At both of these times, water column sediment
concentrations rise and rapidly peak. Over the course of a tidal cycle, concentrations will rise
from 10’s of mg/L to 1-2 orders of magnitude higher at mid tide, only to decrease once again in
the last half of the tide to the previous, lower values. Fall velocities of particles, a measure of
particle size and density, also change over a tidal cycle. At slack water, fall velocities are
typically less than 0.2 mm/s, indicating that very slowly settling, low density particles dominate
the water column at this stage of the tide. At early to mid tide, fall velocities increase to 1-2
mm/s, indicating the presence of large, high density flocs, both formed in the water column by
particle interactions, or aggregates derived from semiconsolidated material resuspended from the
bottom. These more rapidly settling particles are able to settle out of the water column as current
velocities decrease. The turbid bottom layers observed during the anchor stations all contain fast
settling particles while those particles higher in the water column settle much more slowly
(Blanton et al., 1999). Above a concentration threshold of about 200 mg l-1, the particles all
settle fast enough to reach bottom within an hour or so in undisturbed water. At lower
concentrations, the particles settle so slowly that they cannot reach bottom within a slack-water
cycle. Thus the fast settling particles spend most of their time at or near the bottom, while the
slowly settling particles remain in the water column as they are constantly resuspended by
energetic conditions and unable to settle to the bottom.
32
Figure 19. Suspended sediment distribution in the Satilla River as a function of salinity. Open
symbols represent data from high and low water slack surveys of the river, whereas closed
symbols represent data collected at anchor stations.
A. Distribution in surface waters (upper 1 meter). Note that slack water surveys demonstrate an
elevated concentration at salinities below about 8 PSU, possibly representing a diffuse,
energetically mixed ETM. Anchor station data (filled points) demonstrate the wide range of
concentrations that are observed at any one site over a tidal cycle. Note that values range up to
200 mg/l.
B. Distribution in bottom waters (lowermost 1 meter). Slack water surveys document
concentrations that are at least 3 x higher than in surface waters. An increase in concentration
similar to that observed in the surface waters may exist in the 0-8 PSU range. Anchor station
data document very high concentrations (up to 5,000 mg/l) of particles present due to active tidal
mixing.
33
There is a neap/spring signature to the concentration of suspended sediments as well.
Data from 1997 spring and neap tide, slack water transects of the river illustrate this behavior
(Figure 20). At neap tides, suspended sediment concentrations are 10-25 mg/L and 50-150 mg/L
within the upper and lower meter of the water column, respectively. During spring tides, the
extra tidal energy in the system is more effective at eroding and resuspending sediment and
concentrations are 25-50 mg/L and 150-600 mg/L within the upper and lower meter of the water
column, respectively. Thus, about 4 times more sediment is maintained in the water column at
spring tide as compared to neap tide.
Figure 20. Spring-neap variability in suspended sediment concentration. Note that surface SSC
values at spring tide are similar to bottom SSC values at neap tide.
The Estuarine Turbidity Maximum
Mixing in the Satilla is extremely effective and thus the Satilla does not exhibit a tightly
constrained boundary between fresh and salt water. Hence, a classical turbidity maximum may
form, but will have wide-ranging influence. We have yet to find clear evidence for a consistent
geographic location in the Satilla for an estuarine turbidity maximum (ETM). Based on the
results of more than 30 longitudinal surveys of the Satilla, there seem to be two zones of
consistently higher concentration. Sixteen OBS maxima were found between a salinity of 0 to 8
PSU. We also found 14 other maxima between 12 and 32 PSU. The heterogeneous distribution
of bottom sediment types in the Satilla is likely to reflect multiple ETMs tied more to estuarine
circulation, driven by a combination of gravitational and tidal forces. Muddy deposits found in
Crow Harbor Reach (km 24-28) and between kilometers 4-6 in St. Andrews Sound may
represent the upriver and seaward end results of this tidal excursion, or a zone of convergence for
estuarine water masses.
34
There is some evidence for shifts in highly turbid zones related to discharge. The
response of turbid zones to river discharge has been reported in other estuarine systems (Allen et
al., 1980; Gelfenbaum, 1983, Fettweis, Sas, and Monbaliu, 1998; Uncles et al., 1998). During
April 1995, discharge was about 120 m3 s-1 and maximum turbidity was found approximately 13
km from the ocean (Blanton et al., 1999). By contrast, in a similar survey in July 1996, when
discharge was below 20 m3 s-1, maximum turbidity was located 10 km farther upstream. The
ETM location in both of these surveys were located within the 0 - 15 PSU regime which would
be consistent with the position of the “null” zone for the gravitational circulation mode.
However, there are too few studies at present to establish a clear relationship of an ETM in the
Satilla to river discharge.
Suspended Sediment Quality
To determine whether there were differences in sediment quality between fast- and
slowly-settling particles, Alber (unpublished data) examined some biological properties of the
different fractions during a spring-tide anchor station when property variations were maximized
(Table 3). Chl-a and phaeopigment concentrations were higher in resuspended samples. When
normalized to suspended sediment concentrations, however, surface samples had 0.023%
chlorophyll and 0.012% phaeopigment, while bottom samples representing resuspension
averaged only 0.008% chlorophyll and 0.007% phaeopigment. These differences in turn yielded
carbon:chlorophyll ratios that were several times higher in the re-suspended samples.
Thus, there appear to be qualitative differences between surface samples representing the
unsettled component of suspended sediments and bottom samples representing resuspended
sediments. Re-suspended (fast-settling) fractions tend to be of poorer quality, with consistently
lower proportions of chlorophyll and phaeopigment but with higher carbon:chlorophyll ratios.
Table 3. Biological properties of surface “suspended” and bottom “resuspended” sediments measured
during a spring tide anchor station in 1997. SSC = suspended sediment concentration, POC = particulate
organic carbon.
Parameter
Surface
Bottom
SSC (mg l-1)
POC (mg l-1)
% carbon
Chlorophyll-a (mg l-1)
Phaeopigment (mg l-1)
Carbon:Chl-a
29-70
1.8+/-1.1
4.8
8.1+/-2.2
4.6+/-1.2
103
100-348
9.9+/-2.1
4.9
11.7+/-1.7
13.4+/-7.5
710
POC concentrations were also routinely measured as part of the LMER project. These generally
ranged between 1 and 9 mg C/l, with variable peaks along the length of the estuary (e.g. highest
concentrations were sometimes observed at the head of tide, sometimes mid-estuary, and
sometimes towards the mouth).
35
BIOLOGICAL COMPONENTS
Primary Producers
Marsh Vegetation
Aerial photographs and GIS analysis were used by Higinbotham et al. (submitted) to map
the distribution of tidal marsh vegetation along the salinity gradient of the Satilla River Estuary
and to determine whether there had been vegetation changes related to salinity intrusion over
time. Vegetation maps were constructed from 1993 USGS DOQQs, 1:77000-scale color infrared
photographs taken in 1974 and 1:24000-scale black and white photographs taken in 1953, and
changes between years were identified with a GIS overlay analysis. Four vegetation
classifications were identified and groundtruthed with field surveys: salt marsh (areas containing
primarily Spartina alterniflora); brackish marsh (S. cynosuroides and S. alterniflora), Juncus
(Juncus roemerianus), and fresh marsh (Zizania aquatica, Zizaniopsis miliacea, and others)
(Table 4).
Table 4. Species of vegetation found within various classification categories.
# Obs
Salt Marsh
Spartina alterniflora
Distichlis spicata
18
Juncus
Juncus roemerianus
101
Brackish Marsh
Spartina alterniflora
Spartina cynosuroides
42
Fresh Marsh
Zizania aquatica
Zizaniopsis milicae
Spartina cynosuroides
Solidago sempervirens
Lilliopsis sp.
Juncus effusus
Cladium jamaicense
2
% Cover
(Range)
% Cover
(Avg.)
Heights (m)
(Range)
Heights (m)
(Avg.)
80% - 100%
0% - 20%
99%
1%
0.3 - 0.6
0.4 - 0.6
0.5
0.5
80% - 100%
0% - 20%
93%
7%
1.1 - 1.8
1.5 - 1.6
1.5
1.6
0% - 100%
50% - 100%
39%
71%
1.3 - 1.4
1.5 - 1.8
1.4
1.6
20% - 50%
0%- 20%
30% - 80%
35%
10%
55%
1.4 - 1.7
1.7 - 1.8
1.4 - 1.8
1.6
1.7
1.7
The vegetation in the Satilla exhibits characteristic zonation patterns, with salt marsh
areas closest to the mouth, brackish marsh areas in the middle section of each estuary, and fresh
marsh areas farthest upstream (Figure 21). Juncus is found in areas of higher elevation near the
36
upland vegetation as well as in areas that border salt, brackish, and fresh marsh. When this
distribution was compared with observations of salinity in the main channel of the estuary, the
division between salt and brackish marsh occurred where high tide salinities averaged 15 psu;
Juncus began at 21 psu; and the division between fresh marsh and both Juncus and brackish
marsh occurred at 1 psu. These observations fit in well with observations by Odum (1988) that
salt marshes are generally found in polyhaline areas, where salinities are greater than 18 PSU,
and fresh marshes are confined to areas where salinities are less than 0.5 PSU.
Most of the marsh vegetation in the estuary was classified in the same category in all
three years considered (1953, 1974, 1993). Many of the changes that did occur were located
near or at the edges of large areas that remained stable over the entire 40 year study period. The
vast majority of the observed changes involved transitions back and forth between Juncus and
either brackish or salt marsh. However, there were no obvious patterns that suggest that
particular areas are expanding or decreasing, nor was there evidence for a directional shift over
time such as might be expected if there had been a long-term increase in salinity. These results
suggest that the broad distribution of tidal marsh vegetation in the Satilla is driven by salinity,
but that at the local scale marsh plants communities are dynamic, with frequently changing
borders.
Figure 21. Vegetation zonation in the Satilla River estuary. From Higinbotham et al., submitted.
37
Phytoplankton
Chlorophyll a and phaeopigment were measured as part of the LMER program.
Chlorophyll concentrations generally ranged from 1-20 mg m-3. Concentrations are generally
higher at the mouth than further upstream, although there is often evidence for a mid-estuarine
peak. Phaeopigment concentrations were generally less than 10 mg m-3 and varied inversely
with chlorophyll. An example transect plotted against both salinity and distance from the mouth
is shown in Figure 22. Additional information can be found on the Georgia Rivers LMER web
page (http://lmer.marsci.uga.edu/datapage/riverdata.html).
LMER Chlorophyll
8/20/98
chl a
phaeo
25
mg m-3
20
15
10
5
0
0
10
20
Salinity
30
40
25
mg m-3
20
15
10
5
0
0
8
16
24
32
Distance from mouth (km)
40
Figure 22. Example Chloropyll transect from LMER program. Concentrations of chlorophyll (open
circles) and phaeopigment (filled circles), in mg m-3, are plotted against salinity (top) and distance along
the estuary (bottom). Data of M. Alber
More recently (February 2000-January 2001), the Marine Extension Service measured in
vivo [chlorophyll a] using a Turner Designs 10-AU fluorometer as part of their monitoring
program. In general, data from the Satilla River show a slight increase upriver from the mouth.
Station averages ranged from 13.74 to 20.34 ug/L. Monthly averages ranged from 8.81 ug/L in
38
September to 58.33 ug/L in May. The average in vivo [chlorophyll a] concentration measured
during the year was 17.84 ug/L.
Animals
Appendices A2–A3 contain species lists for the Satilla River estuarine area and other
resources for information on animals. Specific genetic work has been done on the Atlantic eel,
Anguilla rostrata, which is found in the Satilla River. Atlantic eel mtDNA is distinctly different
from that of the European eel, indicating that they do not come from the same panmictic
population. MtDNA of Atlantic eels from along the 4000-km long North America coastline
shows no differentiation.
SUMMARY AND RECOMMENDATIONS FOR FUTURE STUDIES
The Satilla River estuary has received greatly increased study since the advent of the
Georgia Rivers LMER program in 1994. The LMER project provided synergism for additional
support from the Georgia Department of Natural Resource's Coastal Incentive grant program and
the Georgia Sea Grant College Program. Although the more recent studies are beginning to offer
important insight into the dominant processes within the Satilla River estuarine system, they lack
sufficient detail to answer questions about how the components of this system interact (i.e., salt
and brackish marshes, tidal creeks, tidal flats, major tributaries, the ocean and the riverine
channel). These types of questions are inherently biogeochemical and physical in nature,
crossing disciplinary boundaries.
The next generation of efforts in the Satilla should, when practical, integrate modeling
efforts with field measurements. This integrated effort should include field measurements in
tidal creeks and salt marshes. In addition to the Satilla River proper, White Oak Creek should
receive particular attention in future research efforts as this tributary is a poorly known, but
major, source of material flux to the Satilla system. Material fluxes (e.g., salt, sediment,
submarine groundwater discharge, freshwater runoff) from the White Oak Creek system should
have a signal large enough to be distinguished from signals in the main trunk of the Satilla River
given the creek’s size.
Recommendations for Future Study
Throughout this document, gaps in our knowledge of the Satilla River estuarine system have
been highlighted in appropriate sections. To summarize these data gaps, we recommend that
priority be given to research efforts that seek to:
•
•
Measure the hypsometric curve of the entire tidal watershed of the Satilla River estuary.
Provide detailed side-scan sonar imagery of the Satilla River including White Oak Creek.
Conduct ground-truth surveys for existing data of the Satilla River to accurately interpret
bottom habitats. Conduct sidescan surveys (with ground-truth) of White Oak Creek to
identify areas of distinctive bottom character and benthic habitats.
39
•
•
•
•
•
Measure the salt inventory and area of intertidal flooding over a range of river discharges
along a substantial length of the estuary that includes major intertidal areas such as White
Oak Creek and the marshes surrounding Bailey Cut, and determine the relationship
between flow and salinity distribution in the estuary.
Determine the distribution of biological resources (i.e., fish, shrimp) along the length of
the estuary, and link this information to discharge-induced changes in the salt field.
Determine the magnitude, extent and impact of submarine groundwater discharge in the
Satilla River estuary and White Oak Creek. Determine how groundwater inputs affect
the salt and nutrient balance. Include evapotranspiration studies of adjacent salt marshes
and tidal creeks.
Determine the relative importance of channel-associated mudflats and marsh-draining
tidal creeks on the carbon and sediment balance. Include studies to determine the sizes
and constituents of suspended sediments, how these correlate with the circulation field,
and how biogeochemical processes affect their sizes and composition. Determine how
retention capacity of the marshes affects the overall salt and sediment balance.
Integrate modeling and field projects at the outset so that all models can be validated for
prediction purposes. These efforts should extend into intertidal areas. Include models
that can handle the transport of cohesive sediments. Include studies that determine the
influence of lateral circulation processes on the salt and sediment balance.
40
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46
Appendix 1
Listed below are additional data sources.
Georgia Rivers LMER - Much of the recent information about the Satilla River comes from the
Georgia Rivers Land-Margin Ecosystem Research Project, which was funded by the National
Science Foundation from 1994 through 2000. This project involved routine cruises in the Satilla
estuary during which physical, geological, chemical and biological parameters were measured.
In addition, there were several intensive experiments undertaken as part of the project to
characterize the tidal currents, bottom stress, and residual currents in the estuary. Much of the
data from this project can be accessed via the Georgia Rivers LMER website:
(http://lmer.marsci.uga.edu/datapage/riverdata.html)
Human impacts on Georgia estuaries - The Georgia Coastal Management Program funded two
studies between 1998 and 2002 that examined the effects of humans and land use change on the
Georgia estuaries. These projects included intensive field studies of modern salinity patterns in
the Satilla river estuary as well as an analysis of precipitation, runoff and discharge. Summaries
of those projects and a list of publications can be accessed at:
http://www.marsci.uga.edu/coastalcouncil/wiegert_humanimpacts2.htm and
http://www.marsci.uga.edu/coastalcouncil/wiegert_landuse2.htm.
Water quality - The UGA Marine Extension Service has been funded by the Georgia Coastal
Management Program to monitor water quality in Georgia riverine estuaries on a rotating basis
(one estuary per year). The Satilla River Estuary was monitored at 7 stations from 2/00 - 1/01.
Hydrodynamic Modeling - The Georgia Sea Grant Program and the Georgia Coastal
Management Program are supporting the development of a 3-D finite volume model of the
Satilla River estuary. A full description of the model and selected results can be accessed at
http://codfish.smast.umassd.edu/research_projects/SATILLA/home.html
47
Appendix 2
The following is a list of species, which were collected from station 821 (Satilla River,
Todd Creek, Lat: 31°58.06N Long: 81°30.66W) while conducting a fishery-independent
assessment of Georgia’s commercial shrimp and crab resources from 1995 – 1998. A total of 43
finfish species were collected from station 821 (Figure 2, Pg. 7, Interstate Fisheries Management
Planning and Implementation). Roughly 30 additional finfish species were collected in the St
Andrews sound system (Table 9, Interstate Fisheries Management Planning and
Implementation). These additional finfish species (as well as others) do exist in the Satilla
proper.
Interstate Fisheries Management Planning and Implementation
Award No. NA57FG0170
Final Report
Project Period; April 1, 1995 - March 31, 1998
By Alex Ottley, Carolyn N. Belcher, Brooks Good, James L. Music, Jr., Clark Evans,
Commercial Fisheries Program
48
Finfish
Anchoa hepsetus
Anchoa mitchilli
Ancylopsetta quadrocellata
Arius felis
Bagre marinus
Bairdiella chrsoura
Brevoortia tyrannus
Carcharhinus limbatus
Chaetodipterus faber
Chloroscombus chrysurus
Citharichthys spilopterus
Cynoscion nothus
Cynoscion regalis
Dasyatis americana
Etropus crossotus
Ameiurus catus
Larimus fasciatus
Leiostomus xanthurus
Lepisosteus osseus
Lepophidium brevibarbe
Menticirrhus americanus
Micropogonias undulatus
Monacanthus setifer
Opisthonema oglinum
Paralichthys dentatus
Paralichthys lethostigma
Peprilus alepidotus
Peprilus triacanthus
Pogonias cromis
Prionotus carolinus
Prionotus evolans
Prionotus tribulus
Scorpaena brasiliensis
Selene vomer
Sphoeroides maculatus
Sphyrna tiburo
Stellifer lanceolatus
Symphurus plagiusa
Syngnathus louisianae
Trichiurus lepturus
Trinectes maculatus
Urophycis floridana
Urophycis regia
Striped Anchovy
Bay Anchovy
Ocellated Flounder
Hardhead Catfish
Gafftopsail Catfish
Silver Perch
Atlantic Menhaden
Blacktip Shark
Spadefish
Atlantic Bumper
Bay Whiff
Silver Seatrout
Weakfish
Southern Stingray
Fringed Flounder
White Catfish
Banded Drum
Spot
Longnose Gar
Blackedge Cusk-Eel
Southern Kingfish
Atlantic Croaker
Pygmy Filefish
Atlantic Thread Herring
Summer Flounder
Southern Flounder
Harvestfish
Butterfish
Black Drum
Northern Searobin
Striped Searobin
Bighead Searobin
Barbfish
Lookdown
Northern Puffer
Bonnethead Shark
Star Drum
Blackcheek Tonguefish
Chain Pipefish
Atlantic Cutlassfish
Hogchoker
Southern Hake
Spotted Hake
49
Crustaceans
Callinectes sapidus
Callinectes similis
Farfantepenaeus aztecus
Farfantepenaeus duorarum
Litopenaeus setiferus
Blue Crab
Lesser Blue Crab
Brown Shrimp
Pink Shrimp
White Shrimp
Cheliceratas
Limulus polyphemus
Horseshoe Crab
Cephalopodas
Lolliguncula brevis
Brief Squid
Gastropoda
Busycon carica
Knobbed Whelk
50
Appendix 3
Listed below are the species that have been commercially harvested from the
Satilla River since 1972.
catfish
black drum
red drum
flounders
whiting
mullet
spotted sea trout
sheepshead
sturgeon
crab
whelk
oysters
american eel
american shad
hickory shad
white and brown shrimp
51