Marine Geology of Jobos Bay, Puerto Rico

Jack Morelock, Department of Marine Sciences, University of Puerto Rico, RUM, (retired), Puerto Rico - morelock@tstar.net

Lucy Bunkley Williams, Department of Biology, University of Puerto Rico, RUM PO Box 9012, Mayagüez, Puerto Rico 00861-9012 - lwilliams@stahl.uprm.edu

Abstract

Jobos Bay is a calm 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). 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.

































Figure 1. Jobos Bay. Low angle aerial photograph showing morphology and vegetation.

The Bay can be divided into an east-west trending northern bay with depths to five meters and a north-south 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 less than 0.5 m. These waves impinge on Cayo Puerca and Colchones, producing an area of increased wave energy.












































Figure 2. Jobos Bay bathymetry, CI 2m. (figure)

OCEANOGRAPHY

Tides, Waves and Currents

The south and east coasts of Puerto Rico have a mixed diurnal tide of small amplitude with 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 has an area of 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. The winds are seasonal but blow from northeast through south-southeast 84 percent of the time.

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.

Currents were measured in Jobos Bay with dye injections and by means of surface and two-meter deep drogues (Figure 3). Currents in the nearshore zone in front of the reefs are primarily wind and wave driven. The normal direction is westward flow. 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. Measured velocities were higher in the tidal channels between the Bay and the open ocean.


























Figure 3. Current drogue and dye tracks show west and south transport.

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.

The refraction pattern for Jobos Bay shows only slight changes in wave direction but enough force is exerted on the bottom to affect sediment resuspension. Wind generated waves and sediment resuspension are a common occurrence over the shoal waters of the northern bay.

Water characteristics

Average water temperatures ranged from 25 - 26 o C in the winter to 28 - 29 oC 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 4). These low readings result from a combination of suspended sediment and plankton population.























Figure 4. Average Secchi values over two years.

SEDIMENTOLOGY

Jobos Bay is surrounded by Tertiary and Pleistocene alluvial fan deposits (Berryhill, 1960). The US Department of Agriculture 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 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.

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. The fine-grained sediments are resuspended by wave action. The area in which most of the sediment resuspension occurs is above the three meter contour of the north bay, an area of 1.11 x 106 m2 (Figure 5). 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.

























Figure 5. Areas of sediment resuspension - theoretical and observed.

Resuspension can be retarded by marine grasses that reduce the effectiveness of wave energies in moving sediments. (figure) Thalassia spp cover 30 to 40 percent of the bottom where the depth is less than two meters; where the depth is less than one meter, Thalassia spp cover about 70 percent of the bottom (Kolehmainen, 1972; Vicente, 1975)

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 6). 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).

















Figure 6. Location of sediment traps and sediment grab samples. (figure)

Table 1. Rates of Sediment Influx

Location Sediment Trap Data
Reef channel 23-31 mg/cm2/day
Inner bay 10-19 mg/cm2/day
Cayo Puerca 10-18 mg/cm2/day



The highest rates were recorded in the reef channel, where tidal currents of 20 to 40 cm/sec were recorded. The sediment data 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.


Sediment Texture

Four texture groups were distinguished (Figure 7).































Figure 7. Sediment textures in Jobos Bay.

Sediment Facies

The seaward side of the barrier cays is a reef environment (Figure 8). 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. Three phototransects were made in the survey area and one a mile to the west (Cayo Morillo) to quantify the coral cover (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 8. Sediment facies.























Figure 9. Coral reef survey locations.

Cayos Caribes reef had low coral cover compared to reefs in a clear water area (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. The coral cover by species at five meters on Cayos de Caribes was low compared to the reefs at Cayos de Barca and Cayo Morrillo (Figure 11).




























Figure 10. Total coral cover at La Parguera compared to the cover at Jobos Bay.


























Figure 11. Coral cover by species at 5 m on the reef front at Jobos Bay.

Between 1980 and 1986, the 10 m 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 with rates as high as 2.1 m per year of coastal erosion (Morelock, 1984; Figure 12). 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 Cayo Morillo.




















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 (Ellis & Solander), Porites asteroides (Lamarck), (figure) Agaricia agaricites (Linnaeus), Diploria clivosa (Ellis & Solander), Siderastrea siderea (Ellis & Solander), (figure) and Favia fragum (Esper).





















Figure 13. Puerca reef coral survey.

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 (Pallas), (figure) with some Porites asteroides, Siderastrea siderea, Acropora palmata (Lamarck), Agaricia agaricites, and small uchins. The reef crest - breaker zone was dominated by Millepora spp, Palythoa spp, (figure) 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.





























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. Sufficient wave energy exists so that sediments do not accumulate and the area has not been colonized by coral.

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 in 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.

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 foraminifera 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).

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 is directly controlled by the amount of 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

Suspended 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 turbid areas that have been examined. Bottom sediments are fine grained except for local areas of calcareous shell accumulation.

There is sufficient wind energy to cause frequent resuspension of the bottom sediments, and the turbid water conditions seem to be as much a result of resuspension as 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, the rate of sediment deposition as measured by sediment trap is very high (Morelock, unpubl. Data). 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 Montastrea annularis, Montastraea cavernosa (Linnaeus), Agaricia spp, 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, (figure) Colpophyllia natans (Muller) (figure) and Agaricia agaricites (figure) at La Parguera (Figure 15). Montastraea cavernosa cover is about the same at all sites.


























Figure 15. Coral cover by species at 10 m depth on the reef front at Jobos Bay.

ACKNOWLEDGEMENTS

This work was supported by the NOAA Estuarine Sanctuary Program. Ship time was provided by the Department of Natural and Environmental Resources.

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LITERATURE CITED

  1. 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.
  2. Kolehmainen, S. E. 1972. Ecology of turtle grass (Thalassia testudinum) beds in Jobos Bay. Aguirre Power
  3. Project Environmental Studies Annual Report (PRNC 162). Puerto Rico Nuclear Center
  4. Morelock, Jack. 1984. Coastal Erosion in Puerto Rico. Shore and Beach: 18-27.
  5. 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.
  6. Puerto Rico Nuclear Center, 1972. Aguirre Power Project Environmental Studies, 1972 annual report
  7. Seiglie, George A. 1974. Foraminifers of Mayagüez and Añasco Bays and its surroundings. Part 4. Relationships of foraminifers and pollution in Mayagüez Bay. Caribbean Journal Science 14 (1-2): 1-68.
  8. Vicente, Vance P. 1975. Sea grass communities of Jobos Bay. PRNC Report 196.