Modeling the fate and transport of nitrogen and sediment within coastal embayments on St. John, U.S. Virgin Islands

Abstract

The recent rises in resident and tourist populations on St. John in the U.S. Virgin Islands have spurred the construction of new roads and housing developments throughout much of the island. As a result, a number of pollutants, including sediment and nitrogen, are being delivered to the coastal waters around the island at increasing rates and are believed to be having adverse impacts on the health of coral reef ecosystems. Understanding the impacts of increased sediment and nitrogen loads on coral reefs is critical for the protection of these valuable natural resources. In the present study, two mass balance models for sediment and nitrogen were developed to evaluate the impact of watershed development on the water quality of coastal embayments. The sediment model considered inputs by watershed loading and losses by gravitational settling and tidal flushing. The degree of sediment stress inflicted upon each bay was evaluated using three stress metrics. The results of the sediment model agreed with historical turbidity measurements collected by the National Park Service in terms of the relative degree of sediment stress observed in each bay.

(cont.) A mass balance model for nitrogen was also developed accounting for input fluxes by groundwater discharge, atmospheric deposition, and nitrogen fixation and output fluxes by tidal flushing and denitrification. The biological fluxes of nitrogen were orders of magnitude greater than the physical fluxes suggesting that nitrogen availability in these ecosystems may be controlled mainly by microbial processes and not by external inputs. Accounting only for the physical fluxes, steady-state inorganic nitrogen concentrations agreed with historical measurements of inorganic nitrogen collected by the National Park Service. The model results indicated elevated levels of sediment and nitrogen in bays with watersheds that are more developed and have larger areas. Overall, this study provides strong evidence that both watershed development and watershed size are closely related to deteriorating water quality in coastal bays around St. John, and further studies of nitrogen and sediment pollution are warranted.