HR: 0800h
AN: OS51C-1323    [Abstracts]
TI: Interannual Variability in Carbon and Nitrogen Stable Isotopic Signatures of Size-Fractionated POM from the South Florida Coastal Zone
AU: * Evans, S L
EM: samantha.evans@fiu.edu
AF: Department of Earth Sciences, Florida International University 11200 SW 8th ST, Miami, FL 33199 United States
AU: Anderson, W T
EM: william.anderson@fiu.edu
AF: Department of Earth Sciences, Florida International University 11200 SW 8th ST, Miami, FL 33199 United States
AU: Anderson, W T
EM: william.anderson@fiu.edu
AF: Southeast Environmental Research Center, Florida International University 11200 SW 8th ST, Miami, FL 33199
AU: Jochem, F J
EM: frank@jochemnet.de
AF: Marine Biology Program, Florida International University 3000 NE 131 ST, North Miami, FL 33181 United States
AU: Fourqurean, J W
EM: fourqure@fiu.edu
AF: Department of Biology, Florida International University 11200 SW 8th ST, Miami, FL 33199 United States
AB: Environmental conditions in South Florida coastal waters have been of local and national concern over the past 15 years. Attention has focused on the ecosystem impacts of salinity increases, seagrass die-off, increased algal bloom frequency, waste water influence, groundwater discharge, and exchange between Florida Bay, the Gulf of Mexico, and the Atlantic Ocean. Changes in water quality and productivity levels may be reflected in the isotopic signatures of coastal zone primary producers. Recent work with seagrasses in South Florida has demonstrated high seasonal and spatial variability in C and N isotopic signatures and decoupling between the two isotopic systems as they vary. To better understand the sources of seasonal and spatial fluctuation, size fractionated POM (particulate organic matter) samples have been collected on a quarterly basis since Sept. 2002. Fractions collected include $>$150$\mu$m, 50-150$\mu$m, and 0.1-50$\mu$m using Nitex mesh sieves and a portable pump system deployed from a small boat at 10 sites around the Florida Keys and Florida Bay. It was hypothesized that planktonic groups respond more quickly to changes in water quality then seagrasses, and thus variations may be more clearly attributed to environmental parameters. Significant spatial and temporal variability is evident both within site between size fractions and between sites. Seasonal oscillations of up to 4$\permil$ were observed in N isotopic values and 6$\permil$ in C isotopic values of the 50-150$\mu$m size fraction, which is dominated by diatoms and dinoflagellates. $\delta$$^{13}$C values are depleted in the late winter/early spring sampling period possibly reflecting decreased productivity stress on available C pools. $^{13}$C depletion is generally coincident with $\delta$$^{15}$N enrichment in the late winter/early spring, possibly demonstrating changes in DIN pools (NO$_{3}$$^{-}$ and NH$_{4}$$^{+}$ concentrations) or changes in decomposition or denitrification rates. Broad groupings appear to separate Atlantic coral reef sites, interior Florida Bay sites, and western boundary Florida Bay/Florida Keys channel sites in isotopic signature and the magnitude of seasonal variation. Additionally, results utilizing flow cytometry in conjunction with isotopic analyses of the $<$50$\mu$m POM fraction from Florida Bay show a correlation between low DIN availability, cyanobacterial population abundance, and more depleted $\delta$$^{15}$N signatures, possibly indicating N-fixation as a supplemental nutrient source, favoring N-fixing cyanobacterial blooms.
DE: 9800 GENERAL OR MISCELLANEOUS
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting