Geochemistry of Florida Bay sediments: Nutrient history at five sites in eastern and central Florida Bay

Recent seagrass dieoff and massive microalgal blooms have focused attention on the health of the Florida Bay ecosystem. Changes in nutrient input and the nutrient dynamics of Florida Bay are hypothesized to be linked to these problems, but crucial baseline information is still lacking. Efforts to re store Florida Bay to its natural condition will require information on the nutrient history of the bay. The purpose of this study was to examine distr ibutions of organic C, total N, and total P in carbonate sediments from sit es of continuous and known sedimentation rate (Pb-210 and Cs-137 dated), in eastern and central Florida Bay. These sediments provide a record of histo rical changes in the C, N, and P load to the eastern and central bay. Analy ses were conducted on sediments from cores collected at five sites, and on buried seagrass fragments at two sites. At three of the sites, sediments fr om seagrass-covered and adjacent barren areas were examined to determine di fferences in sedimentary geochemistry. Stable isotope analyses (delta(13)C and delta(15)N) of sedimentary organic C and total N and of buried seagrass fragments were also carried out at two sites to examine possible changes i n nutrient sources to the estuary. Results were consistent with recent increases in N and P in eastern Florida Bay, beginning in the early to mid 1980's. The timing of the increase in n utrient load observed in the sediment data directly preceded the first obse rvations of massive microalgal blooms and seagrass dieoff in Florida Bay in 1987. The observed nutrification was greater for P than N, and was most pr onounced at the most northeasterly site sampled (Pass Key). Isotope data (d elta(15)N) suggested that an increase in algal production accompanied the i ncrease in N load at the Pass Key site. A long record of organic C, total N , and total P distributions fi om Whipray Basin in central Florida Bay show ed historical peaks (mid 1700's and late 1800's) in organic C and total N, but not total P; these enrichments were nearly equivalent to recent inputs to the estuary. Barren areas were observed to have generally lower concentr ations of organic C, total N, and total P in near surface sediments compare d to seagrass-covered areas, but had generally similar concentrations in de eper sediments. This suggested that barren areas adjacent to seagrass-cover ed sites were places where relict sediment was physically transported and c overed seagrass beds. This dataset provides an historical view of changes i n nutrient inputs to Florida Bay, and baseline information needed for nutri ent modeling of the bay.