WATER COLUMN AND SEDIMENT NITROGEN AND PHOSPHORUS DISTRIBUTION PATTERNS IN THE FLORIDA-KEYS, USA

Measurements of the distribution patterns of nutrients (ammonium, nitr ate, orthophosphate, total N and total P) and chlorophyll concentratio ns were conducted under an interdisciplinary program known as SEAKEYS, initiated because of concern that anthropogenic nutrients may be impa cting Florida coral reefs. Samples were collected along transects that extended from passes or canals to 0.5 km offshore of the outermost re efs. Seven of the transects were either in the Biscayne National Park (BNP) and Key Large (upper keys) or Seven Mile Bridge/Looe Key (upper part of lower keys) areas, which have the best present-day reef develo pment; the two in the middle keys off Long Key were in an area of mini mal reef development where passes allow estuarine Florida Bay water to flow onto the Florida reef platform. Off the upper keys, water column concentrations of N and chl a were elevated near marinas and canals ( 1 mu M NO3, 1 mu g/1 chl a), but returned to oligotrophic levels (e.g. , chl a less than or equal to 0.25 mu g/l; NO3 less than or equal to 0 .25 mu M; NH4 less than or equal to 0.10 mu M) within 0.5 km of shore. Phosphorus concentrations, however, were often higher offshore (great er than or equal to 0.2 mu M PO4). Sediment interstitial nutrient conc entrations decreased from inshore to the offshore reef areas (e.g., gr eater than or equal to 100 mu M NH4 inshore to less than or equal to 5 0 mu M NH4 offshore) and were comparable to those of some presumably p ristine coastal and reef carbonate sediments. Sediment bulk N was high er nearshore and decreased steeply offshore (greater than or equal to 60 mu g-at N/gm sediment to less than or equal to 20 mu g-at N/gm sedi ment, respectively); bulk P concentrations (less than or equal to 6 mu g-at P/gm sediment) varied little or exhibited the reverse pattern. S ediment N:P ratios were consistently lower offshore (1-10 vs. 20-40 ne arshore). Higher offshore P concentrations are attributed to periodic upwelling along the shelf edge. In the middle keys water column nutrie nts and chl a concentrations were both higher than those in the upper keys, and there was less of an inshore-offshore decrease than that not ed in the upper keys. Sediment nutrients were higher also, and nearsho re and offshore areas did not differ. Water column and sediment nutrie nt concentrations and distribution patterns in the upper part of the l ower keys were most similar to those measured in the upper keys. Overa ll, the present data do not support the contention that reef areas in the upper keys are accumulating elevated loads of land-derived nutrien ts via surface water flow, but does document moderately elevated nutri ent and chi a levels in many developed nearshore areas. Most of the an thropogenic and natural nutrients entering the coastal waters from sho re appear to be taken up by near shore algal and seagrass communities before they reach patch reef areas. Further work is needed to determin e whether nutrient-enriched ground waters reach the reefs, however the se would be expected to cause an enrichment of reef sediments, which w as not observed.