CONSULTING. IN~ - Peconic Estuary Program

CONSULTING.
IN~
AND
WOODS HOLE OCEANOGRAPHIC
FINAL
INSTITUTION
REPORT
OXYGEN UPTAKE AND
NUTRIENT REGENERATION
IN THE PECONIC ESTUARY
APRIL 1998
OXYGEN UPTAKE AND NUTRIENT REGENERATION
IN THE PECONIC ESTUARY
Final Report
April 1998
Prepared For:
SUFFOLK COUNTY DEPARTMENT OF HEALTH SERVICES
By:
Brian L. Howes
David R. Schlezinger
Newton P. Millham
George Hampson
Dale D. Goehringer
Center for Marine Science and Technology
University of Massachusetts, Dartmouth
New Bedford, MA02744
and Steve Aubrey
Aubrey Consulting, Inc.
East Falmouth, MA02536
TABLE OF CONTENTS
Pa_a
Preface
io
Executive Summary
ii.
List of Tables
V.
List of Figures
vi.
Introduction
1
Methodology
5
Data Synopsis
10
Conclusions
21
Acknowledgements
22
References
23
Figures
24
Appendix 1. Sediment Oxygenand Nutrient Exchange: SummaryTables
PREFACE
Exchangesof oxygen and nutrients between water columnand sediments were measuredon
six cruises from August1994 -- December1995. Associated measurementsof sediment carbon and
nitrogen content, porosity and grain size and watercolumnrespiration, particulate carbon,particulate
nitrogen and chlorophyll a were conductedto aid in evaluating the role of the sediments in the
oxygendynamicsof the Peconic Estuary. In addition, a mooringwas placed in Great Peconic Bay
to assess the frequencyand duration of potential low oxygenevents in the near bottomwaters. The
goal of the Benthic Flux Studywasto help parameterizethe numericalmodelof the PeconicEstuary
that is being developed by Tetra Tech. Whatfollows is a SummaryReport from the Study. This
report is not meantto present a full biogeochemicalor ecosysteminterpretation of the Peconic
Estuary.
April 1997
Brian L. Howes
Center for MarineScience
and Technology
University of Massachusetts
New Bedford, MA02744
,!:’
EXECUTIVE
SUMMARY
Oxygen Uptake and Nutrient Regeneration
in the Peeonic Estuary
Rates of oxygenconsumptionand nutrient regeneration were measuredannually throughout
the Peconic Estuarine System. Sediment and water columnOxygenuptake were measuredin order
to determine the potential for developmentof bottomwater hypoxiaand the magnitudeof organic
matter cycling throughout the system. Sediment oxygen demandwas found to dominate oxygen
uptake within the upper estuary with water columnprocesses becomingincreasingly important in
the better flushed deeper waters of the outer estuary. Watercolumnrespiration appeared to be
dominatedby phytoplanktonproduction based upon similarity in the observed relationship between
particulate organic matter concentration and oxygenuptake throughout all of the embayments.The
potential for periodic depletions of bottom water dissolved oxygenfocussed on the warmersummer
periods whenoxygen pools are lowest and the rates of consumptionhighest. Within the poorly
flushed upper estuary, observations of periodic water columnstratification
and rates of oxygen
consumptionindi6ated the potential for the depletion of dissolved oxygento ecologically stressful
levels (<4-3 mg/1). Continuous records of bottom water oxygenconcentrations during July and
August1995were consistent with the rate measures,indicating large high-frequencyvariations in
the oxygenfield.
Regeneration of inorganic nitrogen and phosphorus was related to the distribution of
ii
phytoplanktonand organic matter deposition to the sediments. Organicmatter degradation and flux
to overlying waters appears to play an important role in the nutrient economyof Peconic Estuary
waters. Regeneration of nitrogen within the upper estuary significantly magnified inputs from
terrestrial sourcesand helpedto supportgenerallyhigher chlorophylla levels. It appearsthat organic
matter cycling within sediments of the Peconic Estuary is enhancing oxygendepletion of bottom
waters both directly through oxygen uptake and indirectly by the recycling of phytoplankton
nutrients to support further organic matter production and water columnoxygenconsumption.
MANAGEMENT UTILITY
Bottomwater oxygenlevels are a major habitat-structuring feature of shallow eutrophic
systems like the Peconic Estuary. Water column and sediment respiration
and water column
stratification are the proximatecontrols on bottomwater oxygenlevels and relate directly to organic
matter inputs, primarily through phytoplanktonproduction. In addition, sedimentsprovide temporal
storage of nutrients helping to increase the delivery of phytoplanktonnutrients during the higher
regeneration periods in summer/fall.
Quantitative measuresof oxygenuptake and nutrient regeneration can also be used to support
predictions of the potential for bottom water hypoxia. Restoration of benthic communitieswithin
the Peconic Estuary first requires the establishment of adequate oxygenconditions. At present,
within portions of the estuary, oxygendemandperiodically exceeds oxygensupply resulting in
ecologically stressful conditions. The"over production" within inner PeconicBay results directly
IIi
from nutrient stimulation of phytoplanktonproductionwithin bay waters and the degradationof this
organic production within the BaySystem.Oneapproachto reducing eutrophyinglevels of nutrients
within an estuary is to reduce terrestrial inputs. Assessmentof a nutrient reduction approachmust
include the contribution of sedimentrecycling to the nutrient economyof the system. Since organic
matter degradation and nutrient regeneration within water columnand sedimentsappear to play key
roles in both the habitat quality and nutrient economy
of the Pcconicsystem,it is essential that these
parameters be coupled with hydrodynamicsin future water quality modeling.
iv
List of Tables
Table 1. Sedimentflux stations within the Peconic Estuary and dates of sampling.
Table 2. Monitoring parameters.
Table 3. Laboratory analyses.
Table 4. Carbonand nitrogen levels within the sediments at each flux station within the Peconic
Estuary.
Table 5. Grain-size analysis of the surficial sediments (0-2cm)at each of the ten seasonal flux
stations (see also Table1, Figure 1).
V
List of Figures
Figure 1. Location of seasonal and July only sediment flux measurementswithin the Peconic
Estuary. The mooringsite (also a flux station) supported a continuously recording oxygensensor
in July and August1995. See also Table 1.
Figure 2. Seasonal cycle of bottom water temperature within Gardiners Bay (GB), shallow (<5m)
and deep (>5m)flux stations within the Peconic Estuary. Measurements
from the six surveys, 19941995.
Figure 3. Annualoxygenuptake by sediments and water columnwithin the Peconic Estuary. Water
columnrespiration is based upon each basin’s depth. (A) Muddystations in WesternEstuary, (B)
Muddy
stations in Eastern Estuary and (C) Sandystations. I.D.’s refer to stations in Table
Figure 4. Survey of 16 stations throughout the Peconic Estuary, July 5-7, 1995. Rates of water
columnand sedimentrespiration tend to be highest within the muddystations of the western estuary
(inner) and lowest in at the sandy sites (shownby **). Thetrend is mostpronouncedin the rates
water column respiration. ,
Figure 5. Annualnitrogen flux from sediments within the Peconic Estuary. (A) Muddystations
WesternEstuary, (B) Muddystations in Eastern Estuary and (C) Sandystations. I.D.’s refer
stations in Table 1.
Figure 6. Surveyof 16 stations throughout the Peconic Estuary, July 5-7, 1995. Rates of nitrogen
flux tend to be highest within the muddystations of the westernestuary (inner) and lowest in at the
sandy sites (shownby **). The trend is most pronouncedin dissolved inorganic nitrogen fluxes.
Figure 7. Relationship of nitrate and ammonium
fluxes within the Peconic Estuary in (A) winter,
(B) spring and (C) summer.
Figure 8. Relationship of dissolved inorganic nitrogen (DIN)to dissolved organic nitrogen (DON)
within the Peconlc Estuary in during summer.
Figure 9. Relationship of dissolved inorganic nitrogen (DIN) to total dissolved nitrogen
(DON+DIN)
within the Peconic Estuary in during summer.All summerstations measuredin June,
July and August.
Figure 10. Relationship of sediment oxygenuptake to dissolved inorganic nitrogen (DIN) flux
all study sites withing the PeconicEstuary, (A) annual rates and (B) summerrates (June-August).
Figure 11. Relationship of annually integrated sedimentoxygenuptake and July rates at the seven
annual flux stations in the PeconicEstuary.
vi
Figure 12. Relationship of annual sediment oxygenuptake to sediment organic carbon levels. Data
are fromall 16 stations in the PeconicEstuary.
Figure 13. Relationship of water columnrespiration to particulate organic carbonlevels throughout
the Peconic Estuary.
Figure 14. Averagebottom water oxygen levels in Great Peconic Bay from two mooredsensors 25
cmand 45 cmabove the sediment surface.
Figure 15. Tidal variation in bottom water oxygen levels in Great Peconic Bay. Loweroxygen
during ebb tide is consistent with movement
of organic matter rich (possibly low D.O.) waters from
the upper estuary movingseawardin ebb tidal flows.
Figure 16. Bottomwater oxygen, salinity and temperature at sensors 25 cm and 45 cmabove the
sediment surface in Great Peconic Bay during the interval of lowest oxygen.
Figure 17. Decline in water columnand sediment oxygenuptake with increasing distance from the
head of the Peconic Estuary. Therelationship betweendistance and sediment oxygenuptake is most
clear whendepositional sediments are considered separately (sandy sediments denoted by **).
Figure 18. Timeto consumeall of the bottomwater oxygenwithin a 1 m3 parcel of water in contact
with 1 m2 of the sedimentsurface. Data are from the July survey of the 16 flux stations.
Figure 19. Timeto regenerate an amountof dissolved inorganic nitrogen (DIN) equivalent to the
pool of DIN+ PONwithin the water columnoverlying each sediment flux site in July.
Figure 20. Distribution of particulate organic carbonthroughoutthe PeconicEstuary during summer.
Figure 21. Distribution of chlorophyll a fluorescence throughout the Peconic Estuary during
summer.
Figure 22. Distribution of peak water columnrespiration throughout the Peconic Estuary (July).
Note the correlation to POCdistributions (Figure 20).
Figure 23. Distribution of sediment oxygenuptake throughout the Peconic Estuary during summer
(July). Thepatchinessof the rate distribution is related to the distribution of depositionaland nondepositional areas (muddyand sandy sediments).
Figure 24. Distribution of dissolved inorganic nitrogen (DIN)flux from PeconicEstuary sediments.
Thehighly patchy distribution is related to the distribution of depositional and non-depositional
areas (muddyand sandy sediments). The greater variation in the DINversus SODfluxes (Figure
23) relates to secondaryfactors relating to sedimentoxidation whichaffect nitrogen flux dynamics
to a muchgreater extent than oxygenuptake rates.
vii
INTRODUCTION
Sedimentnutrient and oxygenfluxes play major roles in the nutrient and oxygeneconomy
of coastal ecosystems. Marine sediments can represent major sources of nutrients and oxygen
demand.In shallow nearshore systems, sediment-water columnexchange processes can impact the
entire ecosystem.Muchof the reason for the high productivity of phytoplankton,fish and shellfish
in coastal versus offshore environments is due to the smaller volumeof water overlying the
sediments which enhances the cycling between water columnand sediments. However,this lower
dilution of sediment effects can lead to ecosystemdegradation if nutrient loading exceeds the
assimilative capacity of the system.In fact, muchof the assimilative capacity for newnutrient inputs
is ultimately due to sediment recycling processes. Whennutrient loading becomesexcessive, the
anoxic zone of the sediment begins to moveupwardand penetrate the aerated surface layer. The
impactto the water columnis buffered by the aerobic zonewithin the sedimentsand the majorresult
is increased rates of nutrient regeneration and oxygendemand.Underelevated nutrient loading an
accumulation of nutrients and oxygen demanddevelops within the sediments which continues to
impactthe water columnevenafter inputs are reduced. At higher levels of loading, the aerated zone
of the sediments’continuesto diminish until the sediments becomeanoxic at the surface, leading
ultimately to a rapid depletion of oxygenin the overlyingbottomwaters and the decline of in_faunal
communitiesand fish populations whichuse them as a food source. Anoxicor hypoxicevents need
only to occur periodically in order to maintain a depauparatebenthic community.
Sediment oxygen demand(SOD)and nutrient regeneration are, therefore,
comerstone
processes impactingand structuring nearshoresystems. Theyare majorvariables affecting all levels
1
of ecosystemfunctioning, and are part of major feedback processes which can enhanceor destroy
the utility of aquatic coastal ecosystems. The relative importance of these key biogeochemical
processes is generally directly related to the depth of the overlying water and the frequencyand
duration of water columnstratification.
The impactof sedimentson nutrient related water quality
is inversely related to water depth (deeper waters having moreof the nutrient and oxygencycling
occurring in the water column,therefore less in the sediment). Theimpactof stratification is more
complexwith sediment oxygen demandbeing more important in stratified
systems (since oxygen
inputs from the atmosphereare diminished), but sediment nutrient regeneration moreimportant in
vertically mixedsystemswherereleased nutrients are transported to the euphoticzone. In the latter
case, the rapid transfer of regeneratednutrients to the euphotic zone and subsequentstimulation of
primaryproduction maypotentially add to eutrophication. Sedimentnutrient and oxygenfluxes also
vary in response to a variety of environmental factors but are driven primarily by temperature,
organic matter loading and to a lesser extent bioturbation and physical mixing. The complexityof
these interactions, therefore, requires a numberof samplesbe collected throughoutthe year and at
a variety of sites along the nutrient/organic matter loading gradients within a given estuary.
As a result of the growingconcernover the eutrophication of coastal waters and recognition
of the important role of the sediments in nutrient and oxygenbalances, these measureshave been
included in the developing modelfor the Peconic Estuary. Consisting of over 100 embaymentsand
tributaries,
the Peconic Estuary system is nowpotentially threatened by nutrient and pollutant
loading from both point and non-point source discharges. Althoughthe areal extent of the Bays
2
Systemis extensive at ca. 160square miles (about half of whichfall in the GardinersBayprovince),
the threat of nutrient related water quality decline is currently significant in the system’ssmaller
embaymentsand tributaries.
The Peconic Estuary’s corresponding watershed encompassesroughly
110,000 acres and supports both extensive wetlands as well as 15 rare ecosystemsdesignated as
"priority communities"by the NewYorkNatural Heritage Program(Opaluchet al. 1995). Both the
numberof embaymentsand the surface area of the waterbodymakethe Peconic System one of the
major estuaries along the Northeastern coast of the U.S. (eg. BuzzardsBay, MAhas 27 embayments
and is ca. 200 sq. mi.), and therefore of significance in its ecological and economiccontribution to
the coastal Mid-Atlanticregion.
Thehigh aesthetic, recreational and economicvalue of this estuary is primarily the result of
its historically high levels of waterquality. Increasednutrient loadingto the system, however,is of
serious concern as it represents a potential and considerable long-term threat to the estuary’s
ecological health. Thedevelopmentof appropriate management
plans to protect the PeconicEstuary
from environmental decline requires determination of the significance of natural processes in
ultimately determining water quality in the estuary. The importance of natural processes to
evaluating managementobjectives and/or potential remediation measures is reflected in the
significant impact of a browntide bloomon scallop and oyster harvests in the mid-1980’s. The
interaction betweenanthropogenic impacts and natural processes is extremely complexand often
difficult to identify, howeverit is that very aspect whichwill ultimately determinethe environmental
health of our coastal environmentsin the future. Sedimentnutrient and oxygenfluxes can represent
3
a significant componentin the overall ecological health of a coastal water body, influencing both
water column and benthic habitat quality necessary to supporting healthy animal and plant
communities. As well, these processes can reflect/integrate
both natural and humanimpacts such
as alterations to circulation, deposition, nutrient/organic matter loading, etc. Understandingthe
relative contribution of benthic fluxes is importantto long-termprediction of environmentalquality,
especially as increased attention is being given to maintaining the high levels of water quality
generally characteristic of the PeconicEstuary.
To address these issues in the Peconic Estuary, wemademeasurementsof benthic nutrient
and oxygenfluxes across a gradient of nutrient and organic matter loading. Followingan initial
reconnaissance cruise, sampling commenced
with six cruises spanning late August 1994 through
December1995. Ten stations,
ranging from the Peconic River/MeetinghouseCreek to Gardiners
Bay, were measuredon each cruise. To provide better spatial coverage to the measurements,six
additional stations were measuredduring July 1996. Annualflux estimates for these six additional
sites wasestimated from an empirical relationship based uponthe seasonal measurements
throughout
the estuary. Also, to determinethe temporalvariability in bottomwater oxygenconcentrations, a
mooringwas placed in mid-summer
of 1995 to continuously record bottom water dissolved oxygen.
Thesecombinedmeasurements,in concert with other system structuring parameterslike circulation,
provide the base for evaluating the importance of sediment/water columninteractions to the
ecological health of the Peconic Estuary System.
4
METHODOLOGY
SamplingSites: Measurementsof sediment-water columnexchanges of nutrients and oxygen and
associated water columnparameters were conductedthroughout the Peconic Estuary. Ten sites were
selected for seasonal measurementsin order to achieve spatial coverage and measurementsin key
areas (Figure 1). Anadditional, six sites were assayed in July 1995in order to further improve
spatial coverageand to assess the potential importanceof smaller constituent basins (Table 1). Sites
ranged from soft mudto sand and from 1.6-11.5 meters depth. Station location for sampling was
determined using a Northstar 931Xdifferential global positioning system (DGPS)with horizontal
accuraciesof three to five meters.
Sediment Flux Incubations:
Based upon experience in a variety of systems from shallow
embaymentsto deeper harbors (Nantucket to Buzzards Bay) representing a range of nutrient and
organic matter loadings, weused a sedimentcore incubation technique to determinerates of benthic
oxygen and nutrient exchange (Cibik and Howes1995, Howes1997). Large diameter cores (15
i.d.) were collected by SCUBA
diver at each location. Ten sites were measured seasonally (6
samplingsfrom August1994-December
1995) with an additional six sites assayed on July 7, 1995
(Table 1). Thethree to four cores collected per location were held at in situ temperatures during
transport throughthe completionof the incubation period. Coreswere held within their temperature
controlled baths on a gimballedtable to reduce disturbance during transport. Oxygenlevels within
the core headspaces was maintained by continuous aeration. The cores were transferred directly
~omthe ship to the field laboratory at the Suffolk CountyMarineEducationCenter on the shore of
the estuary.
In the field laboratory the cores were maintainedin the dark at in situ temperatures. Water
overlying the sedimentwasreplaced with 0.2 umfiltered water collected at the coring location and
the cores were stoppered to prevent gas exchangewith the atmosphere. At appropriate intervals
(determinedby the rate of oxygenuptake), usually 1-6 hrs, nutrient concentrations were measured.
Oxygentensions were not allowed to drop below 50%of in situ oxygen levels. Nutrient flux
measurementswere madeon six occasions over at least 24 hours. Headspacewaters were filtered
(0.22um Millipore) and held as in Table 2 and analyzed by procedures given in Table 3. When
incubations were completed, samples of the surface (0-2cm) sediments were collected for
sand/silt/clay analysis and for determinationof total organic carbonand nitrogen (Perkin Elmer2400
ElementalAnalyzer). Theselater data were used to determinethe area over whichthe flux data was
to be integrated (based uponsedimentmaps), and the relation of organic matter content to flux.
The rate of sediment-water column exchange for each core was determined by linear
regression of the changein concentration of each analyte through time. Onlythe linear portions of
time-course measurementswere used in the determinations.
Water column Parameters: Water samples were collected by Niskin Sampler at each station.
Triplicate samples of bottom water dissolved oxygen (Winkler titration)
6
and respiration were
collected at each coring site. Water column respiration
samples were incubated at in situ
temperatures and assayed by Winkler titration using a high precision RadiometerTitrator. Water
columnparticulate organic carbon and nitrogen (POC,PON)were assayed by filtering water samples
through pre-combustedglass fiber filters,
with analysis by high temperature combustion(Perkin
Elmer 2400 Elemental Analyzer). In addition, water columnstratification
was checked by CTD
profiling (Seabird) coupledto a fluorometerto determinechlorophyll a levels. Theseprofiles, along
with bottomwater oxygenlevels, wereused to establish the general environmentalconditions at each
station (salinity, temperature,D.O.and stratification).
.,~.
7
TABLE 2
Monitoring Parameters
PARAMETER
VOLUME
CONTAINER
PROCESSING & STORAGE
Nitrate &
TDN
60 ml
polyethylene
(acid washed)
0.22 um membrane
stored on ice (dark)
or frozen to -20 °C
Ammonium
60 ml
polyethylene
(acid washed)
0.22 um membrane
stored on ice (dark);
run within 24 hrs
Ortho-Phosphate
60 ml
polyethylene
(acid washed)
Total Organic
Carbon/Nitrogen
Solids
polyethylene
0.22 um membrane
stored on ice (dark);
run within 24 hrs
combusted GFF, dried
or glass (acid washed)
Flux (oxygen uptake)
15 cm core
Probe
8
TABLE 3
Laboratory Analyses
PARAMETER
MATRIX
UNITS
METHOD
REFERENCE
Nitrate +
Nitrite
water
laM
Autoanalyzer
a
TDN
water
~tM
Digestion/Nitrate
b
Ammonium
water
ttM
Indophenol
e
Orthophosphate
water
ttM
Molybdenumblue
d
Total Organic
Carbon/Nitrogen
sediment
3
ttg/em
Acid treatment, drying
Elemental Analysis
e
Oxygen uptake
sediment
mmol/m2/h
time-course 02 uptake
f
Nutrient flux
sediment
ttmol/m2/d
time-course [cone]
g
a
b
c
d
e
f
g
LachatAutoanalysisproceduresbaseduponthe followingtechniques:
--Wood,E., F. Armstrong
and F. Richards.1967.Determinationof nitrate in sea water by cadmium
copperreduction
to nitrite. J. Mar.Biol. Ass. U.K.47:23-31.
--Bendschneider,K. and R. Robinson.1952.A newspectrophotometric
methodfor the determinationof nitrite in sea
water.J. Mar.Res. 11: 87-96.
D’Elia, C.F., P.A.Stuedler, P.A.and N. Corwin.1977.Determination
of total nitrogenin aqueoussolutions. Limnoi.
Oceanogr.22:760-774.
Scheiner, D. 1976. Determinationof ammonia
and Kjeldahl nitrogen by indophenolmethod.WaterResources10:31-36.
Murphy,
J. and J.P. Reilly. 1962. Amodifiedsingle solution for the determinationof phosphatein natural waters. Anal.
Chim.Acta 27: 31-36.
Perkin-ElmerModel2400CHN
Elemental Analyzer Technical Manual.
Jorgensen, B. 1977. Thesulfur cycle of a coastal marinesediment(LirnfJorden, Denmark).Limnol.Oceanogr.22:814832.; Albro, C., J. Kelley, J. Hermessy,P. Doering,J. Turner. 1993.Combined
Work/Quality
AssuranceProject Plan
(CW/QAPP)
for Baseline Water Quality Monitoring: 1993-1994. MWRA,
Boston MA.
Klump,J. &C. Martens.1983. Benthicnitrogen regenerationIn: Nitrogenin the MarineEnvironment,(Carpenter
Capone,eds.). Academic
Press.
OxygenMooting: The mooring supported an array of two pulsed oxygen sensors (Endeco
T1184)held vertically at 25 cmand 45cmabove the sediment surface. Each sensor also recorded
temperature and salinity necessary for calculation of degree of oxygensaturation. Twomoorings
were deployed July 7, 1995 through August 22, 1995. However,data was only recovered from
the Great Peconic Bay mooringas the Little Peconic Bay mooringwas rendered inoperable by a
state environmentaltrawler soonafter its placement.
9
Conversion Table for Figures and Tables
Reference
DissolvedOxygen(Fig. 14,15,16)
Water ColumnCarbon(Fig. 20)
SedimentOxygenUptake(Fig. 23)
SedimentOxygenUptake (summarytables)
SedimentDINFlux (Fig. 24)
SedimentNH4-NFlux (summarytables)
SedimentNO3-NFlux (summarytables)
SedimentPO4-PFlux (summarytables)
WaterColumnRespiration (summarytable)
Change
uMto mg/1
3 to mg/1
mmol/m
molC/m2/yrto g O2/m2/day
mmol/m2/day
to g/m2/day
mol/m2/yrto mg/m2/day
umol/m2/yrto mg/m2/day
umol/m2/yrto mg/m2/day
umol/m2/yrto mg/m2/day
uM/dayto mg/1/day
Conversion
multiplyvalue by 0.0320
multiplyvalue by 0.0120
multiply value by 0.0877
multiply value by 0.0320
multiplyvalue by 38.4
multiply value by 0.0000384
multiply value by 0.0000384
multiply value by 0.0000877
multiply value by 0.0320
DATA SYNOPSIS
Station Sediments: Sedimentswithin the Peconic Estuary were not of uniform composition.
Sedimentswithin the low velocity regions of the estuary tended to have higher concentrations of
organic carbon and nitrogen and be dominatedby fine sands, silt and clays than the "sandy"
stations which were composedof coarser sands (Tables 4 & 5). All of the sandy stations that
were routinely sampledwere within the upper estuary, Flanders Bay and shallower regions of
Great Peconic Bay. The apparent coarse sand nature of the Peconic River/MeetinghouseCreek
confluence resulted from a prevalence of broken shells (Table 5). The measuredporosities were
typical of sand and mudsediments with values of ca. 40 mL/100cm3 3,
and >70 mL/100cm
respectively.
Thehighest levels of organic matter were associated with the tributaries and smaller
depositional basins. MeetinghouseCreek (7%carbon) was extra-ordinarily organic rich due
nutrient loadings from a variety of sources: 1) historic loadings from agricultural sources such as
the duck farms located in the watershed; 2) developmentactivities within the watershed; and 3)
10
direct input of organic material from the bordering marshhabitats. The high organic carbon
levels within the freshwater/saltwater zone within the UpperPeconicRiver are likely due to
deposition ofriverine transported material. West Neck Bay and Reeves Bay sediments show
high organic matter levels most likely as a result of their restricted basins, lowwater exchanges
and high productivities (i.e. high organic deposition). The moreopen regions of the Estuary
showedsedimentcharacteristics typical of coastal sediments throughoutthe region.
AnnualCycle: Samplingappears to have captured the full seasonal cycle of water temperatures
(Figure 2). Whileall sites showedsimilar seasonal temperature excursions, there were consistent
differences due to location and water depth. Thedeeper waters of Gardiners Bay (11.5m) were
consistently cooler during summerand cooled moreslowly in the fall than the shallower waters
of the Inner PeconicEstuary (Figure 2). Similarly stations within the Estuary with depths greater
than five meters were cooler in summerthan shallower stations (<5m). Althoughthe differences
are relatively small (<2°C), the pronouncedeffect of temperatureon respiration rates (Q~0=2-3)
suggests a mechanismfor enhancedrespiration in the shallower water of the Estuary (other
factors being equal).
Sedimentoxygenuptake and nutrient regeneration rates are primarily determinedby
temperature and the amountof deposited labile organic matter and secondarily mediated by the
composition of in.faunal communities.Water columnrespiration is also dependant upon
temperature and labile organic matter. However,in a system like the Peconic Estuary where
muchof the particulate organic matter within the water columnis producedin situ by
photosynthesis, rates of respiration are likely morevariable than the sedimentwhererates are
"smoothed"due to their storage of deposited organic matter. Combiningwater columnand
sediment respiration rates can be used as a measureof total system organic matter
11
remineralization and to estimate the rate of re-aeration required at night to maintainbottomwater
oxygenlevels.
Wecan construct a composite annual cycle of sediment oxygen uptake and water column
respiration from the four seasonal measurementsand the summervalues from July 1995 and
August/September1994 (Figure 3). At all ten monitoring sites the maximum
rates of oxygen
uptake were found at the highest temperatures in either July or August. However,at somesites
the pattern of oxygenuptake departed significantly from temperature variations. Maximum
rates
were observed in the July 1995 sampling for the shallow, relatively warmerUpperor Western
portion of the estuary, the muddyand sandy sites from the mouthof the Peconic
River/Meetinghouse Creek, through Reeves Bay, Flanders Bay and Great Peconic Bay (Figure
3a,b,c). In contrast, the deeper, cooler and better flushed Eastern Estuary (NoyackBay, West
Neck Bay and Gardiners Bay) all showedcycles of oxygen uptake which moreclosely followed
the temperature cycle (Figure 2).
The difference in total system metabolismbetweenthe Upper(western) and Lower(eastern)
estuary maybe associated with the BrownTide bloomwhich occurred in 1995 but not 1994
(SCDHS
Cell Counts, 1995). The effect of the dense bloomwould be to increase oxygen uptake
within the Upper.shallowerregions of the estuary and haveless of an effect in the deeper more
well flushed portions of the estuary wherecell counts were generally lower. Giventhe similarity
in temperatures betweenJuly and August, differences in oxygenuptake can be viewedas
differences in organic matter mass. Within the regions of muddysediments of Peconic
River/MeetinghouseCreek, Reeves Bay, Great Peconic Bay and the sandy sediments of Flanders
and Great Peconic Bays (Figure 3), both water columnand sediment respiration rates appeared
be enhancedin the bloomyear. The higher total respiration within Gardiners Bay, WestNeck
12
Bay and NoyackBay in August1994 in the non-browntide year was primarily the result of
higher rates of water columnrespiration, possibly due to a bloomof a different phytoplankton.
Overall, the only stations to showdramatically lower Augustthan July rates of sediment
respiration were the very innermost stations of Peconic/MeetinghouseCreek, ReevesBay and
Great Peconic/Flanders Bay. The composite annual yearly rates of sediment oxygen uptake
represents an averaging of the observed interannual differences. However,even for the
innermostthree stations the difference in integrated annual rates between1994and 1995should
be relatively small, since the major portion of the bloomwasfor only about one month.
Sedimentrespiration wasrelatively similar throughout the estuary. However,sandy stations
showedconsistently lower (ca. 50%)rates than nearby muddystations and there wasa trend for
the deeper portions of the estuary (eastern) to have lower rates than the shallowerinner regions
(western). Thesetrends were especially clear in the July survey of 16 stations (Figure
Sedimentnutrient regeneration rates paralleled rates of sediment oxygenuptake (Figure
5a, b,c). Rates of dissolved inorganic nitrogen (DIN) release were dominatedby ammonium
all muddysites. In contrast, at sandy sites (Great Peconic/Flanders Bay &Flanders Bay) both
ammonium
and nitrate fluxes were near the detection limit for most of the year and showeda
slight net uptake.. DINuptakeat these stations is likely the result of a combinationof algal
uptake and denitrification, as algae were observedat these sites and their sandynature may
facilitate nitrification/denitrification reactions. However,it shouldbe cautionedthat the uptake
rates are near the detection limit. As with oxygenuptake, DINfluxes tended to follow the trend
of WesternEstuary>EasternEstuary>>Sandy
Sites, and is best seen in the July survey of 16 sites
(Figure 6). It appears that factors other than remineralization rates maybe mediatingnitrogen
fluxes (see below).
13
Total dissolved nitrogen fluxes were measuredas the combinationof dissolved inorganic and
organic nitrogen. The methodsfor assaying of dissolved organic nitrogen (DON)are muchless
sensitive than for DINand the exchangesmust be interpreted accordingly. There was no
consistent pattern of DONflux betweenthe sedimentstations with location or sediment type.
Overall, the rates tended to be smaller than DINfluxes with almost all sites showingperiodic
uptake and release (Figure 5). Annuallyintegrated rates of oxygenuptake, DINand DONflux
are presented in AppendixI.
July Survey and AnnualOxygenUptake: Seven of the ten annual flux stations had fine or
muddysediments. At these seven stations there wasan excellent correlation betweenthe July
oxygenuptake rate and the annually integrated rate (RZ=0.85;Figure 11). Fromthis empircal
relationship, it is possible to estimate annualoxygenuptake rates for the six additional sites
measuredonly in the July survey. Thesesites can then be used to increase the spatial coverage
for a system-wideorganic matter balance (AppendixI).
Sediment oxygen and nitrogen flux relationships:
At all stations,
ammonium
dominated DIN
flux in spring and summer,while during winter ammonium
and nitrate fluxes were about equal
in the stations showingpositive nitrate fluxes (a positive flux indicates release from the
sediments). In winter and spring about half of the measurementsshowedsediment nitrate uptake
and the innermost stations occasionally showedsediment ammonium
uptake and nitrate release
(Figure 7). This seasonality in the partitioning of DINflux follows the cycle of moreoxidizing
sediments in winter and morereducing sediments in summer.The result of this oxidationreduction cycle is that moreof the DINis oxidized to nitrate before release in spring/winter,
while ammonium
release greatly exceeds nitrate flux in summer.However,there was no
unifying pattem of ammonium
to nitrate fluxes at any period within the study. This was
14
expected due to the numberof mediating factors which determine the partitioning of the DIN
flux (e.g. bioturbation, oxidation-reductionpotential, rate of remineralizationetc.).
Similarly there wasno clear pattern betweenthe rate of DINand DONfluxes (Figure 8).
However,it appears that DINgenerally represents a relatively constant fraction of the total
dissolved nitrogen flux during the summerseason (Figure 9). WhenDINand TDNare compared
throughout the estuary, the TDNis only 1.1 times the DINflux alone (R2=0.78). The lack
relation in direct comparisonsbetween DINand DONfluxes most likely stems from a
...,?
combinationof greater variability in the DON
assay coupledto the "real" natural variability in
DON
flux (bioturbation, infaunal direct release, phytoplanktonleaching, heterotrophic release
etc.). In addition, DONand DINfluxes are only weaklyassociated, since for DINflux to occur
organic matter must be remineralized, but DONflux only needs cells to lose soluble organic
matter. Similarly, DINfluxes can be reduced by uptake by benthic algae, whosepresence which
maylead to increased DONfluxes through leaching. In summary,the dynamicscontrolling DIN
and DON
fluxes are not tightly coupledand therefore the lack of similar patterns in rates are not
unexpected.
In contrast, there was a muchclearer relationship betweenoxygenconsumptionto
remineralize organic matter and the flux of remineralized nitrogen (DIN). Assumingthat one
carbon atom is remineralized for each two atoms of oxygenconsumedin the process (RQ=I), the
measuredDINand SODfluxes can be used to suggest the C/Nratio of the organic matter being
remineralized. Rates of carbon to nitrogen remineralization for July and Augustwhen
decompositionand remineralization are most active yield an average flux ratio of ca. 17. This
flux ratio is roughly twice the C/Nratio (ca. 8) of water columnparticulate matter within the
estuary’s waters (Valiela 1994) and indicates that during decomposition,relatively morecarbon
15
is remineralizedthan DINis released. This C/Npatter likely results fromcouplednitrificationdenitrification of nitrogen regenerated within the sediments(with a small additional amountof
buried and possibly somesediment DINuptake). Rates of denitrification accounting for about
50%of the nitrogen remineralized within coastal sediments is common
(Seitzinger 1988,
Nowickiet al. in press, Howes1997). Whilethese data are not sufficient to quantify
denitrification for an estuarine massbalance, they providea clear indication of the potential
importanceof sedimentdenitrification to the nitrogen balance of the PeconicEstuary.
Organic Matter and Respiration: Sedimentoxygenuptake was directly related to the amountof
organic carbon within the upper twocmof the sediments at each of the 16 sites within the
Peconic Estuary. The relationship wasfairly robust except for the very highly organic sediment
within the Upper Peconic River (PRU)and MeetinghouseCreek (MC).All of the stations
exposedto a similar temperatureregime. Therefore, the similarity in respiration per unit organic
matter suggests a relatively constant fraction of the carbonpool is labile (Figure 12).
Similarly, water columnrespiration wasalso directly related to the particulate organic carbon
pool within the water column(R2=0.93, Figure 13). As in the sediment comparison,the
similarity in temperaturebetweenthe sites west of Shelter Island indicates a relatively constant
fraction of the POCpool is labile. Note that water columnrespiration includes both the
respiration of heterotrophs and phytoplanktondark respiration. Relationships of both sediment
and water columnrespiration to organic carbon levels allow modelingof oxygenuptake from
moreeasily assayed parameters(e.g. POC).However,it is likely that the goodnessof fit of the
sedimentrelationship results from the relatively depauperateinfaunal populations. The lack of
healthy infaunal populations removesa major variable in sediment carbon pool-oxygenuptake
dynamics.
16
BottomWater OxygenFluctuations: The mooringplaced within the basin of Great Peconic Bay
(Figure 1), yielded data on oxygen, salinity and temperature of bottomwaters from July
through August 22, 1995. Duplicate meters were deployed at 25 cmand 45 cmabove the
sediment surface. Bottomwater oxygenlevels were relatively low throughout the deployment.
However,the major depletion occurred shortly after the major bloomof BrownTide. Based
upon the SCDHS
cell counts, there is an agreementbetweenthe timing of the bloomand the
timing and duration of the larger hypoxicperiod in Great Peconic Bay(Figure 14).
There was a clear tidal cycle super-imposedupon the longer diurnal and bloomcycles
(Figure 15). Loweroxygenwas typically found during ebb versus flood tides. This is consistent
with moreorganic rich waters from the UpperEstuary being transported seawardin the ebbing
tidal waters. In addition, the transported UpperEstuary waters maybe lower in D.O. as a result
of the higher respiration rates due to the import of organic matter into those sub-systems.
Thesediments were clearly involved in the low oxygenlevels at the Great Peconic Bay site.
Oxygenlevels recorded by the bottom sensor were typically lower than those recorded by the
sensor placed only 20 cmhigher in the water columnsupporting the concept of an important
oxygensink within the sediments (Figure 16). Whenlower oxygen was found at the deeper
sensor it wasgenerally matchedby slightly higher salinities and lower temperatures. Thesalinity
and temperature data suggests that the bottomwaters mayhave been stratified,
s
thus reducing re-
aeration and increasing the potential for oxygendecline. Duringthe interval (July 20-25) when
salinity and temperaturewere similar at the two sensors, oxygenlevels were also similar (Figure
16).
17
Distribution of Organic Matter Remineralization: The respiration results suggest a consistent
gradient in labile organic matter running from the Peconic River and MeetinghouseCreek to
Gardiners Bay. The gradient is most clearly seen in water columnrespiration (Figure 17) which
decreases linearly with the log of the distance from the head of the estuary (Peconic River).
While sediment respiration follows a similar relationship with distance from the headwaters,
there appears to be morevariability in the data. Thevariability is due to the presence of sandy
(non-depositional) sediments within portions of the estuary (shownby "**" in Figure 17).
However,whenonly the depositional sediments are considered the relationship of decreasing
respiration within increasing distance along the estuary is very clear.
Based uponthe sediment oxygenand nutrient flux and water columnrespiration data it is
possible to determine the potential of various regions of the Peconic Estuary to develop low
oxygenconditions. Fromthe July rates of oxygenuptake at all 16 stations, we can calculate the
time required to consumeall of the oxygenwithin a 1 m3 parcel of water in contact with 1 m-" of
the sediment surface (Figure 18). Wecalculated the total amountof oxygen in one cubic meter
of bottomwater and divided the result by the rate of uptake to determine the time required to
removeall of the oxygenfrom the cubic meter of water. Fromthis simple analysis it is clear that
within the upper regions (headwaters to ReevesBay) even brief periods of stratification
(<1 day)
can result in hypoxic conditions. Althoughrates of oxygenuptake continue to decline
throughout the remainder of the estuary from Flanders Bay to Gardiners Bay, hypoxic conditions
are generally possible with periodic water columnstratification
events of <1 day.
As expected from the relationships between DINflux, distance from the estuary headwaters
and sediment oxygen uptake (Figure 10 & 18), the DINflux also decreases with increasing
distance from the headwaters (as do nutrient concentrations). Based upon the sediment DIN
18
fluxes and water columnpools of DINand particulate organic nitrogen (PON),it is possible
gauge the role of the sediments in the nitrogen economyof the water column.Wecan calculate
howlong it takes for sediment DINregeneration to replace the water columnDIN+PON
pool
(--turnover). Althoughthe N concentrations decline movingseaward, the water depth, hence
dilution, also increases. The result is that sediment nitrogen regeneration from muddysediment
areas takes weeksto monthsto replace the entire water columnpool (Figure 19). However,
within the muddystations of the upper estuary (PRU,PMS,MC,RB)the slow rates of water
exchangeand the high rates of regeneration suggest that these sediments mayplay an important
role in magnifyingthe impacts of nitrogen loading within this region.
Spatial distribution: Baseduponthe sedimentsurveys and the bathymetryit is possible to gain
an understandingof the likely areal coverageof benthic metabolic activity and controlling
substrate pools. There appear to be four provinces of water columnorganic matter levels
following the gradient from the head of the Estuary to Gardiners Bay. Particulate organic carbon
levels varied morethan an order of magnitudefrom the very high levels in ReevesBay and
Flanders Bay to the low levels in the near-oceanic waters of Gardiners Bay (Figure 20). West
NeckBay appears to be a net phytoplanktonproducer as it had three times the POClevels as the
incoming waters. However,the POCdistribution was only roughly correlated with water column
chlorophyll a fluorescence (Figure 21). Chlorophyll a showeda morepatchy distribution than
POC,but with clear bloomconditions within the inner regions of the estuary and WestNeck
Bay.
Sedimentoxygenuptake is determined both by the production (or import) of organic matter
within the water columnand its deposition to the sediments. Thelower rates of oxygenuptake
(and associated DINfluxes) within Flanders Bay and Reeves Bay comparedto Little Peconic
19
Bayparallels the pattern of depositional and non-depositionalsediments. It appears that the
higher organic matter loadings to the UpperEstuary, which result in the highest water column
respiration rates in the inner bays and tributaries, are not depositedwhollywithin the local
sediments(Figure 22). Instead, the particulate organic matter producedwithin the inner estuary
is partially transported seawardto enhanceloadings to the depositional basins (Figure 23).
addition, the sedimentsof Flanders Bay and at the Flanders/Great Peconicjunction are generally
sandy and exhibit very low or negative DINrelease (uptake, see Figure 24). Theselow rates are
likely due both to uptake by benthic photoautotrophswithin the shallower waters wherealgae
ii:ii
were observed and to microbial denitrification.
Thevery high rates of organic matter
mineralization and DINflux within MeetinghouseCreek mayresult from a continuing
depuration of the sediments, but on-going nitrogen and organic matter inputs maybe playing an
even moreimportant role. Baseduponthe available information, it is not possible to determine
the relative importanceof historic versus on-goinginputs to current fluxes. In contrast, it is most
likely that mineralization rates within the Baysare supportedby current inputs and production as
influenced by hydrodynamics.Spatial patterns of water columnrespiration appear primarily
influenced by the Estuary’s gradient in production and water exchange, while sediment
mineralization, although similarly influenced, is affected by additional structuring factors. The
additional complexityin the spatial pattern of sedimentfluxes stems from the need for organic
matter to be deposited and the mediating influence of sediment oxidation status on coupled
nitriflcation-denitrification
?i¯ ~¯
and therefore DINfluxes. In addition, infaunal communitiescan alter
the patterns of DINfluxes. As water quality improveswithin the PeconicEstuary, it is likely that
significant changesmayoccur in the rates and distribution of oxygenuptake and DINrelease.
Therecolonization of the estuary’s benthoswill likely result in a short-term increase in nutrient
loading due to "mining"of the sediments through infaunal bioirrigation. Themagnitudeof this
infaunal effect cannot be gaugedat the present time.
20
CONCLUSIONS
1) Sedimentand water columnrespiration represent significant sinks for oxygenwithin the
Peconic Estuary. Oxygenuptake and water columnoxygen depletion is most pronouncedin the
summerand generally decreases from the Peconic River to Gardiner’s Bay. However,the
gradient in respiration is most pronouncedin the water columnas there were pronounced"hot
spots" in measuredsediment oxygen uptake.
2) Benthic nitrogen fluxes also generally declined from the Peconic River to Gardiner’s Bay.
During summerwhenphytoplanktonnutrient levels are reduced, benthic nitrogen fluxes
throughout the estuary are sufficient to replace the most active nitrogen pools (PON+DIN)
about
once. However,within the UpperEstuary benthic regeneration mayreplace this nitrogen pool
every 2-3 weeks.
3) Sedimentfluxes were correlated with organic matter deposition, while water column
respiration wascoupledto particulate concentrations.
4) Sedimentswithin the upper estuary (MHC,PRUand RB)and its tributaries exhibit highly
eutrophic nitrogen/oxygenfluxes. This effect is morenoticeable in the muddysediments.
5) Water columnrespiration accounts for two-thirds of the carbon turnover within the Peconic
Estuary.
6) Sediment OxygenDemandand bottom water respiration are high throughout the Upper
Estuary, with rates sufficient to producebottomwater hypoxiaif vertical mixingis restricted for
periods of 0.5-1.5 days.
7) Bottomwaters of the UpperEstuary (Great Peconic Bay) are currently exhibiting a high
degree of temporal variablility with periodic short-term hypoxicevents in the summermonths.
8) Sedimentdenitrification is a major gap in our understandingof the nitrogen cycle of the
Peconic Estuary. Thesedimentflux data suggest that denitrification rates will have a very patchy
distribution and therefore require a survey approach.
9) There appears to be a relationship of BrownTide to inter-annual variations in sediment
oxygenand nutrient fluxes in the western portion of the estuary. However,the mechanism
cannot be discerned from the existing data.
21
ACKNOWLEDGEMENTS
Wethank Mr. Vito Minei, Director Peconic Estuary Program, for his guidance in the
planning and evolution of this research. This project could not have been conductedbut for the
logistical and field support of Suffolk CountyDepartmentof Health Services, particularly, Bob
Nuzzi, MacWaters, Ed Nicholson, Mark Reushel and Gary Chmurzynski. Wethank Susan
Brown-leger (WHOI),David White (WHOI)and Brendan Zinn (WHOI)for assistance
laboratory analyses. Weare indebted to GreggRivara of the Suffolk CountyMarineEducation
Center for laboratory space for the flux incubations. Supportfor this study wasthrough the
Peconic Estuary Program, Administrated by Suffolk CountyDepartmentof Health Services.
22
REFERENCES
BrownTide Cell Counts. December1995. Suffolk County Department of Health Services.
Cibik, S.J., B.L. Howes,C.D. Taylor, D.M. Anderson, C.S. Davis and T.C. Loder. 1996. Annual
water columnmonitoring report. Massachusetts Water Resources Authority. 200 p.
Howes, B.L. 1997. Sediment metabolism within Massachusetts Bay and Boston Harbor relating
to sediment - water column exchanges of nutrients and oxygen in 1995. Massachusetts Water
Resources Authority. 70 p.
Nowicki, B.L., E.R.D. Van Keurin and J.R. Kelly. 1997. Nitrogen losses through sediment
denitrification in Boston Harbor and MassachusettsBay. Estuaries, 20:626-639.
Opaluch, J.J., T.A. Grigalunas, J. Diamantidesand M. Mazzotta. 1995. Environmentaleconomics
in estuary management.Maritimes, Winter 1995.
Seitzinger, S.P. 1988. Denitrification in freshwater and coastal marine ecosystems:Ecological and
geochemical significance.
Limnologyand Oceanography,33: 702-724.
23
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Figure 16. Bottomwater oxygen, salinity and temperature at sensors 25 cm and 45 cm above the
sedimentsurface in Great Peconic Bayduring the interval of lowest oxygen.
Distribution
of Watercolumnand Sediment Oxygen Uptake
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Figure 17. Decline in watercolumnand sediment oxygen uptake with increasing distance from the head
of the Peconic Estuary. The relationship between distance and sediment oxygen uptake is most clear
whendepositional sediments are considered separately (sandy sediments denoted by **).
Peconic Estuary: 1994-1995
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Oxygen Depletion by Sediment and Watercolumn Uptake.
Figure 18. Time to consumeall of the bottom water oxygen within a 1 m3 parcel of water in contact
with 1 m2 of the sediment surface. Data are from the July survey of the 16 flux stations.
Peconic Estuary: 1994-1995
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Figure 19. Timeto regenerate an amountof dissolved inorganic nitrogen (DIN)equivalent to the pool
of DIN+ PONwithin the watercolumnoverlying each sedimentflux site in July.
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