The effects of rainfall on the distribution of inorganic nitrogen and phosphorus in Discovery Bay, Jamaica

Abstract

We sought to define the ambient concentrations of inorganic nitrogen and phosphorus in the coastal waters of Discovery Bay, Jamaica, and to investigate the effects of groundwater and rainfall on those concentrations. Ambient nutrient concentrations, in areas away from groundwater sources, were generally low during the 4.5-yr study period, with nitrate, soluble reactive phosphorus (SRP), and ammonium varying between 0.2 and 1 µmol L-1, 0.02 and 0.04 µmol L-1, and 0.05 and 0.2 µmol L-1, respectively. Groundwater nitrate diluted conservatively within the bay, with the average concentration being predicted, from dilution curves, to be 90 µmol L-1. Groundwater SRP also diluted conservatively from a predicted average concentration of 0.6 µmol L-1, although there was suggestion of a source of SRP within the bay, presumably recycled phosphorus. Episodic, heavy rainfall events (100-200 mm over a few days) in the vicinity of the bay were generally linked to immediate decreases in bay salinities but only marginal changes in nutrient concentrations. Two consecutive and very heavy basin-wide rainfall events (>200 mm over several days) that occurred within 4 months of each other resulted in salinities that gradually decreased, whereas nitrate concentrations increased by factors of 2-5; these elevated concentrations were sustained for 3-4 months. Increases in ammonium and SRP also occurred but were not sustained. Dissolved inorganic nitrogen : SRP ratios suggest that the system can become phosphorus-limited following very heavy rainfall.