Phosphorous Inputs and Eutrophication on the Florida Reef Tract

Lapointe, Brian; Matzie, William; Clark, Mark

Report comes from “Proceedings of the Colloquium on Global Aspects of Coral Reefs: Health, Hazards and History.” The Florida Reef Tract (FRT) has experienced dramatic change and loss of coral cover during the past decade following many decades of intense human activities on the south Florida watershed. Comparative studies during 1990 showed that chlorophyll a, turbidity, and total dissolved phosphorus were 2-3 fold higher on bank reefs of the FRT compared to similar reefs in the less developed western Caribbean region. Sampling during summer 1992 showed high phosphorus concentrations in reduced salinity coastal waters along the eastern Gulf of Mexico, indicating land-based sources of phosphorus input. Existing data and observations also show that prevailing currents of the inner shelf waters of the eastern Gulf of Mexico move southerly, into Florida Bay, and then seaward, providing a mechanism for phosphorus transport to the FRT. Because the carbonate-rich waters of the FRT are phosphorus-limited, cumulative phosphorus inputs from far-field sources (as well as near-field sources) mix with N-rich waters in the Florida Keys and provide a balanced nutrient supply needed to sustain nuisance algal blooms. Enrichment above a threshold of 0.10 uM total dissolved phosphorus appears adequate to support nuisance algal blooms. A Landsat satellite image taken in late May 1992 showed a dense phytoplankton bloom in the same area where we observed phosphorus-rich (N-limited) waters of the eastern Gulf of Mexico flowing into N-rich (P-limited) waters of western Florida Bay. Increased phytoplankton and turbidity resulting from mixing of these water types result in widespread algal blooms, which stress reef systems by reducing submarine light while simultaneously increasing sedimentation that smothers corals, increases coral disease, and enhances coral overgrowth by benthic macroalgae. Temperature extremes have long been assumed to exert the primary stress on coral reef development along the FRT. However, our data and observations suggest that phosphorus inputs from human activities in south Florida represent an upstream source contributing to eutrophication on tile FRT.

Phosphorous Inputs and Eutrophication on the Florida Reef Tract

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