Marine Geology of Jobos Bay, Puerto Rico

Marine Geology of Jobos Bay, Puerto Rico
Jack Morelock, 1 Lucy Bunkley Williams, 2 Roberto Acevedo 1
1
Department of Marine Sciences, University of Puerto Rico, Rum
PO Box 908, Lajas, Puerto Rico – [email protected]
2
Department of Biology, University of Puerto Rico, Rum
PO Box 9012, Mayagüez, Puerto Rico 00861-9012 – [email protected]
Abstract
Jobos Bay is a quiet water area closed off from the ocean by barrier reefs and mangrove growth. Both
wave energy and current flow are low within the Bay. The temperatures, salinity and oxygen content are
normal marine. Sediments are mainly fine grained and subject to resuspension in the shallow northern
part of the Bay. Living coral is restricted to the ocean side of the barriers except for limited cover at Cayo
Puerca.
Introduction
Jobos Bay estuary is a
semi-enclosed body of
water separated from the
open ocean by barrier
reefs (Figure 1).
Figure 1. Jobos Bay. Low
angle aerial photograph,
1985.
Lack of land management in the surrounding low lying alluvial plain has resulted in increased sediment
influx into the Bay, and an increased turbidity of the water column. This influx has been the major factor
in changing the physical environment and in causing changes in the benthic community.
The Bay can be divided into an east-west trending northern bay with depths to five meters and a northsouth southern bay with depths to 10 m (Figure 2). Connection to the open ocean is via a few channels
cutting across the barrier reef and mangrove complex of Cayos Caribes, Cayos de Barca, Cayo Morrillo
and Cayos de Pájaros. The connections are less than one meter deep except at Boca del Infierno (four meters) between Cayos de Barca and Cayo Morrillo (eight meters) and between Cayo Morrillo and Cayos de
Pájaros (more than eight meters). The channels allow exchange of water with the open ocean for this protected environment. The sill depth at Boca del Infierno restricts incoming wave height to 0.5 m. These
waves impinge on Cayo Puerca and Colchones, producing an area of increased wave energy.
Figure 2. Jobos Bay bathymetry.
Oceanography
Tides, waves and currents
The south and east coasts of Puerto Rico have a mixed diurnal tide of small amplitude. Tide measurements at Jobos Bay have a pattern similar to the tide table data. PRNC, 1972 A diurnal tidal cycle was measured with a maximum ebb flow of 88 cm/sec and a flood tide maximum of 56 cm/sec.
The northern bay is 7 km2 with an average depth of 2.5 m, and the southern bay with 4.4. km2 has an average depth of 4.9 m, for a total tidal prism of about 38 million m3. An exchange and renewal of water is
accomplished by wind, tidal and geostrophically controlled flow, with a mean residence time for the water mass of approximately 5.5 days
A wave refraction program was run for the north bay using a Snell's Law model described in the USACE
Shore Protection Manual, to evaluate the effect of waves on resuspension of sediments as shoaling occurs.
The refraction pattern for Jobos Bay shows only slight changes in wave direction but enough force is exerted on the bottom to also affect sediment resuspension (Figure 3). Wind generated waves and sediment
resuspension are a common occurrence over the shoal waters of the northern bay.
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Figure 3. Wave orthogonal
pattern, northern Jobos Bay
Currents were measured in Jobos Bay with dye injections and by means of surface and two-meter deep
drogues (Figure 4). Currents in the nearshore zone in front of the reefs are primarily wind and wave
driven. The normal direction is westward flow. The winds are seasonal but blow from northeast through
south-southeast 84 percent of the time.
Figure 4. Current drogue and dye
tracks show west and south transport.
There is a general westerly flow
of surface water that is replaced
by upwelling of bottom water
entering through the deep channel
at the west end. The deeper water
normally flows eastward during
flood tide, the direction of flow is
reversed during strong ebb tides.
The surface water of the same
area flows to the west during both
flood and ebb tide with normal to strong winds from the east or southeast. The velocities of the surface
and deep currents in Jobos Bay vary with wind speed and direction and with the tides. Surface currents in
the Bay normally average 10 cm/sec with a range from zero to 26 cm/sec with west-southwest flow.
Measured velocities were higher in the tidal channels between the Bay and the open ocean.
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The west end of the Bay terminates against a shallow mangrove area which creates gyres and variable
local currents as the wind generated surface current flows from the east end of the Bay toward the west
shore. The shoreline deflects surface currents to the south.
Water characteristics
Average water temperatures ranged from 25 to 26 o C in the winter to 28 to 29 o C in summer. Water column stratification was not observed during most of the measurements, because of the inner bay shallowness and the wind-induced wave action – the wave action can mix the water column within one hour.
Heated water enters the Bay from the power plant on the north shore. Because discharge is intermittent,
no record was obtained of temperature changes associated with the discharge.
Salinity variations were slight, and no significant differences were noted between the north and south
bays. The monthly variations in salinity were not significant. Oxygen profiles in the Jobos Bay water column showed five to seven ppm oxygen with moderate variations between field measurements and between depths.
Secchi values were three to four
meters (Figure 5). These low
readings result from a combination
of suspended sediment and
plankton population.
Figure 5. Average Secchi values
Sedimentology
Jobos Bay is surrounded by Tertiary and Pleistocene alluvial fan deposits. Berryhill, 1960 A complex of different soils has been formed in the area. USDA Soil Serv., 1977 The USDA calculations for sediment loading from
soil erosion were used to calculate the increase in sediment input into the environment from the stripping
of adjacent land areas. Piest, et al, 1975 For Jobos Bay, the result is an increase of 15 kg/m2 additional sediment.
The area in which most of the resuspension occurs is above the three meter contour of the north bay, an
area of 1.11 x 106 m2 (Figure 6). In evaluating resuspension of bottom sediments, it is interesting to note
that the passage of a power boat exceeding 8 km/hr speed, will generate a wave similar to the wind generated wave field.
Resuspension can be retarded by marine grasses that reduce the effectiveness of wave energies in moving
sediments. Thalassia spp cover 30 to 40 percent of the bottom where the depth is less than two meters;
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where the depth is less than one meter, Thalassia spp cover about 70 percent of the bottom. Kolehmainen, 1972;
Vicente, 1975
Figure 6. Areas of sediment resuspension
The limited circulation and closed nature of the Bay has resulted in it being a sink for much of the influx
of fine grained terrigenous sediment. This has increased the amount of fine material in the Bay.
Deposition in Jobos Bay is of material from both terrigenous and marine sources. The carbonate fraction
is shell material and fine carbonates from algal precipitation and from bioerosion of carbonate shells and
reef debris
Sediment traps were placed in three locations: a tidal channel in the back reef of Cayos Caribes, in front
of Cayo Puerca reef, and in the north center of the Bay (Figure 7). The sediment trap was 1.5 m above the
bottom, so that it is receiving suspended sediment load, which includes resuspension of bottom sediments.
These sediment traps results are representative of the annual suspended sediment budget (Table 1).
The highest rates were recorded in the reef channel, where tidal currents of 20 to 40 cm/sec were recorded. These values can be compared to La Parguera (relatively clear water reef environment) values of
2 mg/cm2/day and Mayagüez (river sediment plumes) 11-100 mg/cm2/day.
Although the sediment facies are mixed terrigenous and carbonate particles, the trapped suspended sediments were much higher in terrigenous material.
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Figure 7. Location of sediment traps and
sediment grab samples
Table 1.
Reef channel
Inner bay
Cayo Puerca
23-31 mg/cm2/day
10-19 mg/cm2/day
10-18 mg/cm2/day
Sediment texture
Four texture groups were distinguished (Figure 8).
•
Sands and gravelly sands were found on the reef flat and behind the reef flat. These are carbonate biogenic shell sands with minor amounts of carbonate mud. Sufficient wave energy and currents cross the reef flat to prevent deposition of fine-grained sediments. The sands associated with
mud reefs have a higher concentration of fines. Some of the sand and gravelly sand facies occur
in local sites where the amount of broken shell debris is high in relation to fine-grained material.
•
A silty sand was associated with mud reef mounds, and was also found in shoal areas where local
concentrations of shell debris occur. Although most of the occurrences are in shallow water or
over mud mound shoals, some random occurrences in deep water were found, related to local
shell concentrations.
•
The most common texture found in the Bay was a sandy mud that covers most of the northern
and southern bays and is found in the mangrove environments. The sand fraction is carbonate
shell debris. The fine fraction contains both terrigenous and carbonate mud. The fine fraction of
this facies in the shallow waters of the northern bay is the most commonly resuspended material.
•
A silty clay was found in the deeper and central part of the northern and southern bay, and at the
east end of the northern bay. The occurrence of this texture is rare, and seems to reflect an absence of sand sized shell debris rather than bathymetric or physical controls on the distribution.
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Figure 8. Sediment textures in Jobos Bay
Sediment facies
The seaward side of the barrier cays is a reef environment. The barrier reefs have a well-developed reef
crest and backreef flat lying behind a forereef that extends from the surface to 15 m depth (Figure 9). The
only reef inside the Bay is at the seaward front of Cayo Puerca. A Porites mud mound is located in the
inner bay.
Figure 9. Coral reef survey
locations
Cayos Caribes reef had low coral cover compared to the reefs at La Parguera (Figure 10). The reef at the
Cayos Caribes ocean front is reduced to a hard ground, with the coral colonies having a very flat surface.
At five meters, the coral cover on Cayos Caribes was 3.0 percent living coral cover with only six species
present.
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Figure 10. Total coral cover at la Parguera
compared to the cover at Jobos Bay
The coral cover by species at five meters on
Cayos de Caribe was low compared to the
reefs at Cayos de Barca and Cayo Morrillo
(Figure 11) and between 1980 and 1986 the
10 meter depth coral cover was buried at
Cayos Caribes by sand deposited on the reef
front as shallow as eight meters. It appears
that sand and finer sediments eroded from the
coastline east of this reef are being transported to this site by longshore currents with disastrous results to
the living reef. This
shoreline was eroding
from 1951 to 1971, with
rates as high as 2.1 m per
year of coastal erosion
(Figure 12). Morelock, 1984
A change in the alignment
of the reef front between
Cayos Caribes and Cayos
de Barca gives some
protection to Cayos de
Barca, and the greater
distance from the source
of sediments reduces the
effect on Cayos de Barca.
Figure 11. Coral cover by
species at 5 m on the reef
front at Jobos Bay.
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Figure 12. Eroding
coastline as source of
sediments covering Cayos
Caribes reef
The dye study showed
an exchange of water
across the reef between
the open ocean and
Jobos Bay. The
sediments in the tidal channel immediately behind the reef flat are terrigenous, with a heavy organic content. The transport of silt-laden bay waters over the reef has been a long-term process that has also contributed to coral loss on the reef front.
At Cayo Puerca, the "reef front" is only a couple of meters high (Figure 13). The base of the reef and the
bay floor are covered with heavy growth of Thalassia spp and macro algae. The coral cover that existed
here was related to the entrance of clear
water through Boca del Infierno. Photo
transect measurements on Cayo Puerca
gave a total cover of 0.6 percent living
coral. Only three meters of the survey
(14 m total length) had more than one
percent cover. The species present
were: Montastraea annularis, Porites
asteroides, Agaricia agaricites,
Diploria clivosa, Siderastrea siderea,
and Favia fragum,.
Figure 13. Puerca reef
The backreef area of Cayos de Caribes (Figure 14) was dominated by Thalassia spp at a depth of
30 to 70 cm, with abundant algae living between blades. The reef flat is a fairly wide algal pavement
developed on top of the actively growing reef dominated by Porites porites, with some Porites asteroides,
Siderastrea siderea, Acropora palmata, Agaricia agaricites, and small urchins. The reef crest - breaker
zone was dominated by Millepora complanata, Palythoa spp, and Zoanthus spp. The coral and associated
cover on the reef flat is comparable to other Caribbean reefs, in contrast to the loss of cover that occurs on
the reef front.
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Figure 14. Cayos Caribes
backreef
Mud reefs are found in many protected environments in the Caribbean. These reefs do not have a structural framework; they are secondary framework reefs that have developed because of the binding and
trapping action of Thalassia spp and the ability of Porites porites to grow in sediment influenced environments. One Porites mud reef was mapped in the inner bay at Jobos.
The shoal pass of Boca del Infierno is a hard ground facies. It has not been colonized by coral, and sufficient wave energy exists so that sediments do not accumulate.
The carbonate sand and gravel facies that is found behind the reef flat and in areas of mollusk shell accumulation is primarily restricted to areas where there is sufficient wave energy for removal of the fine fraction. This would be wave or current velocities in excess of 0.9 cm/sec for suspended material and 15
cm/sec for fine material on the bottom. We measured higher velocities during our study. The reef skeletal
sands are high energy environments relative to the rest of the Bay. The composition of the sand and
gravel component in all of the facies is calcium carbonate shell debris (Figure 15).
Muddy sands and gravels are found in quieter water areas, where there is not enough energy to remove
the fine fraction. The sand-gravel component is contributed by carbonate shell debris. The finer fraction is
dominantly terrigenous; but some carbonate muds are present. This facies does not have an orderly distribution related to water depth or wave energy; but is instead a facet of the sandy mud facies, where local
concentrations of sand and gravel sized carbonate debris occurs.
Sandy muds dominate most of the area of surface sediments in the Jobos Bay Estuarine Sanctuary. These
are fine-grained sediments (silts and clays) that are in equilibrium with the energies of the Jobos Bay environment. The fine fraction is carbonate as well as terrigenous. The sand fraction is carbonate shell material that ranges from five to 30 percent of the sample. Mollusk shell fragments, Halimeda spp fragments,
foraminifera, and spicules are common constituents of the carbonate fraction.
Diversity of the plankton community was consistent throughout the Bay except for the waters in the immediate vicinity of Central Aguirre that had high sediment and nutrient influx. The diversity of the fo-
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raminifera population decreases at stations in front of the Aguirre sugar mill, indicating pollution of the
bottom environment. These locations show an increase in Quinqueloculina rhodfensis, a species favored
in a polluted site. Sieglie, 1974
Figure 15. Sediment facies
The supply of nutrients is high in the north bay, and moderate in the south bay. Sieglie, 1974 Water circulation
is adequate to prevent eutrophic conditions, and the limiting factor for biogenic production is the low solar penetration into the highly turbid bay and shelf waters. In general, the benthic environment in Jobos
Bay, and on the shelf barrier reef system has a reduced population relative to many other areas of the
south coast, but comparable to disturbed environments at Ponce, Guayanilla, and Guanica. Morelock, unpubl. data
The inner bay is covered with fine organic rich terrigenous muds with a fairly high carbonate fraction.
Some areas have local concentrations of coral or mollusc debris. Even the fine-grained areas of silty clay
have as much as 30 percent carbonate. The texture of the sediments discussed above is directly controlled
by the carbonate shell debris. The outer bay and the cays have more carbonate debris, but the tidal channels cutting across the Cayos Caribes inside the reef flat are dominated by terrigenous muds. A very great
local variation in carbonate content was observed. Sediments on the reef front are carbonate sands with a
high percentage of terrigenous mud and sand.
Local areas free of shell debris and dominantly clay sized material are mapped as muds. These are mixed
terrigenous and carbonate muds. These are generally deeper parts of the Bay, but are probably due to local deficiency in shell debris rather than physical energy processes.
Conclusions
Sediment levels are very high in the waters of Jobos Bay and light transmission is very low, even compared to Mayagüez, Guayanilla, and Ponce, three other areas that have been examined. Bottom sediments
are very fine grained except for local areas of calcareous shell accumulation.
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There is sufficient wind energy to cause frequent resuspension of the bottom sediments, and the turbid
water conditions seem to be more a result of resuspension than an influx from outside sources. The urban,
commercial, and agricultural development surrounding the Bay has stripped enough soil cover that there
is an influx of fine terrigenous sediment during heavy rainfall conditions.
The water column is fairly stable and the current regime does not lead to a rapid exchange of the water
mass. Transport of turbid water from the Bay has probably had less effect on the outside reefs than turbidity arising from coastal erosion and longshore transport. Morelock, 1977
The general physical and geological environment of Jobos Bay is severe as a biological habitat. Any increase in the present stresses will probably further decrease its viability as a marine habitat for sediment
sensitive organisms.
Compared to other locations, Morelock, unpubl. data the rate of sediment deposition as measured by sediment trap
is very high. This, together with the stress of sediment on the outside reef introduced by coastal erosion
east of Cayos Caribes, and the construction of an effluent discharge plant east of the Bay adds up to a
high level of adverse conditions.
The reef front has very low coral cover compared to reefs in clear water areas of Puerto Rico and the Caribbean. The most common coral species are Montastraea cavernosa, Agaricities sps, and Siderastraea
siderea. The coral cover by species at five meters depth for La Parguera versus Jobos Bay reef front is
low at most five meter sites, but there are marked differences at 10 m – mainly due to higher levels of
cover by Montastraea annularis, Colpophyllia natans and Agaricia agaricites at La Parguera (Figure 16).
click here to
return to the
coral reef papers
Figure 16. Coral cover by species at 10 m depth
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References
Berryhill, H. L. 1960. "Geology of the Central Aguirre quadrangle, Puerto Rico." U.S. Geol. Surv. Misc.
Inv. Map I-318, U.S. Geological Survey.
Kolehmainen, S. E. 1972. Ecology of turtle grass (Thalassia testudinum) beds in Jobos Bay. Aguirre
Power
Project Environmental Studies Annual Report (PRNC 162). Puerto Rico Nuclear Center
Morelock, Jack. 1984. Coastal Erosion in Puerto Rico. Shore and Beach: 18-27.
Piest, R. F., C. F. Miller, and V. A. Vanoni. 1975. Sediment sources and sediment yields, Sediment Engineering, Manuals and Reports on Engineering Practice. Am. Soc. Civil Engr. 54: 383-93.
Puerto Rico Nuclear Center, 1972. Aguirre Power Project Environmental Studies, 1972 annual report
Seiglie, George A. 1974. Foraminifers of Mayaguez and Anasco Bays and its surroundings. Part 4. Relationships of foraminifers and pollution in Mayaguez Bay. Caribbean Journal Science 14, no. 1-2: 1-68.
U.S. Department of Agriculture Soil Conservation Service. 1977. Soil Survey of Humacao Area of Eastern Puerto Rico
Vicente, Vance P. 1975. "Sea grass communities of Jobos Bay." PRNC Report 196.
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