William H. Orem¹, Georgiana L. Wingard¹, Charles W. Holmes², Harry E. Lerch¹, Anne L. Bates¹, Margo D. Corum¹, Marisa C. Beck¹ and Marci Marot^3
1U.S. Geological Survey, National Center, Reston, VA, USA
²U.S. Geological Survey, Center for Coastal Research, St. Petersburg, FL, USA

Excess nutrients are causing eutrophication and microalgal blooms in many of the estuaries in south Florida, including Biscayne and Florida Bays. The effects of excess nutrient input on biotic assemblages within the estuaries are not fully understood, but eutrophication and microalgal blooms may be responsible for seagrass dieoff in Florida Bay, and coral mortality in the Florida Keys. In Biscayne Bay, seagrass dieoff, changes in microalgal population structures, and changes in other benthic species may be occurring. The sources of excess nutrients to these estuaries are not well established, and may include: canal discharge, urban runoff, submarine groundwater discharge, internal recycling of nutrients, and offshore sources. Historical studies can sometimes provide information on sources of nutrients by establishing links between events (e.g. canal construction or urbanization), and the onset of eutrophication. Historical studies can also constrain the degree of anthropogenic effects on estuaries by providing information on the range of past nutrient levels in estuaries.

The major objective of this study was to determine the historical record of nutrient elements (C,N, and P) in Biscayne and Florida Bays through analysis of dated sediment cores. Results were also compared with faunal and floral data to link the timing of changes in nutrient input to that of changes in the biological community. In Biscayne Bay, significant trends in C, N, and P concentrations were observed in all cores, representing diagenetic recycling of nutrient elements, and historical changes in the flux of nutrient elements to the sediments. The most interesting historical trend is the large increase in P concentrations in surface sediments at many sites, beginning in the 1970's. The increase in sedimentary P is largest in the south and lowest in the north, suggesting that the apparent increased P flux to the sediments resulted from inputs from canal structures in the southern part of Biscayne Bay, especially the C111 canal.

In Florida Bay, results suggest recent increases in nutrient concentrations at all sites beginning in the early to mid 1980's. Sudden and sometimes dramatic shifts in the concentrations of C, N, and P were observed. The timing of the increased nutrient flux to the sediments directly precedes the first observation of massive microalgal blooms and seagrass dieoff in Florida Bay in 1987. Increased concentrations were greater for P than for N, and increased to the northeast. Accumulation rates for nutrients were much higher at all sites in Florida Bay compared to the coastal zone in the vicinity of Taylor Slough. This indicates that Taylor Slough is unlikely to be a source of nutrients to Florida Bay. The longer sediment record in Florida Bay revealed earlier events of higher nutrient loading or enhanced productivity in eastern and central Florida Bay, including pronounced maxima of decadal length from 1730 to 1800. The decadal scale of these events suggest control by long scale climatic factors.

Contact Information: William H. Orem, U.S. Geological Survey, 956 National Center, Reston, VA 20192 USA, Phone: 703-648-6273, Fax: 703-648-6419, Email: borem@usgs.gov

(This abstract is from the 2006 Greater Everglades Ecosystem Restoration Conference.)

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