Spatiotemporal Variability in Seawater Chemistry of a shallow lagoon in Discovery Bay, Jamaica, driven by Submarine Groundwater Discharge
Description
This study was initiated to investigate the spatial and temporal variabilities in water quality in the shallow western quadrant of Discovery Bay, Jamaica. Previous studies had indicated significant submarine groundwater discharge (SGD) into the bay, possibly transporting elevated levels of nutrients; however, many of these studies were conducted as one-off exercises or over very short time periods. Salinity-depth profiles were obtained at 11 sites along four north-south transects, between June 1999 and March 2000. Water samples were analysed for ammonium, total oxidised nitrogen (nitrate+nitrite, TOxN), total dissolved nitrogen (TDN), soluble reactive phosphorus (SRP), total dissolved phosphorus (TDP) and chlorophyll a concentrations using the manual methods outlined in Parsons et al., 1984. The sampling period encompassed both wet (October – November 1999) and dry seasons. Large differences were observed between surface and depth salinities, particularly at the sites close to the Discovery Bay Marine Laboratory, where at least 5 submarine springs were identified, with greater salinity stratification observed following heavy rainfall. Surface salinities ranged from about 25 to 34 over the period. A strong negative correlation was observed between TOxN and TDN and salinity. Ammonium concentrations were very low with no significant correlation with salinity, suggesting that the groundwater was not a significant source of ammonium to the bay. In fact, slightly higher concentrations of ammonium were often detected in higher salinity samples, pointing to an alternative source within the bay, possibly from regeneration by benthic organisms or anaerobic diagenesis within the sediment. SRP and TDP concentrations, although typically very low, exhibited a significant negative correlation with salinity on most occasions. The correlation was not as significant as those obtained for TOxN and TDN possibly due to more variable fluxes from the sediments. Bay waters were strongly phosphorus-limiting with high N:P ratios. Chlorophyll a concentrations showed no correlation with salinity and were generally low. The results were indicative of oligotrophic to mesotrophic conditions in this region of the bay over the period of the study. The study confirmed that the groundwater entering Discovery Bay is enriched in nitrogen relative to phosphorus, but the observed concentrations were typical of rural, unpolluted, tropical groundwaters. The groundwaters did not supply significant amounts of ammonium or organic nutrients. Compared to historical data, it did not appear that Discovery Bay was experiencing “chronically” high nutrient concentrations; however episodic increases in nutrient concentrations were possible after rainfall events which may enhance the growth of macroalgae and affect coral reef health. This effect may be exacerbated by the low densities of herbivores in this bay due to disease and over-fishing.
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Institutions
- University of the West IndiesKingston, Kingston