HR: 08:45h
AN: H51E-03    [Abstracts]
TI: Stable Isotopic Detection of Ammonium and Nitrate Assimilation by Estuarine Phytoplankton
AU: * York, J K
EM: jyork@bu.edu
AF: Boston University Marine Program, Marine Biological Laboratory, Woods Hole, MA 02543 United States
AU: Valiela, I
EM: valiela@bu.edu
AF: Boston University Marine Program, Marine Biological Laboratory, Woods Hole, MA 02543 United States
AU: Repeta, D J
EM: drepeta@whoi.edu
AF: Woods Hole Oceanographic Institution, Marine Chemistry & Geochemistry Dept., Woods Hole, MA 02543 United States
AB: We used a stable isotopic approach to determine the relative contribution of nitrate and ammonium to nitrogen requirements of an estuarine phytoplankton community, in Childs River, MA. To obtain a phytoplankton, rather than a mixed seston isotopic signature, we purified chlorophyll a from seston samples and measured its nitrogen isotopic composition. The isotopic signature of phytoplankton increased from winter to summer, but did not vary with salinity. In contrast, the isotopic signatures of nitrate and ammonium decreased linearly with increasing salinity, and only the isotopic signature of ammonium increased in warmer months. We compared the nitrogen isotopic composition of nitrate, ammonium, and phytoplankton, and found that although nitrate dominated the DIN pool, ammonium constituted the majority (60 to 97%) of the DIN taken up by phytoplankton. The fractionation factor for phytoplankton ammonium uptake was 4 \permil. Surprisingly, it appears that phytoplankton took up N mainly as ammonium, and thus acquired their isotopic signature, in the upper reaches of the estuary, and divided and increased abundance during down- estuarine transport based on that initial uptake of nitrogen.
DE: 4805 Biogeochemical cycles (1615)
DE: 4845 Nutrients and nutrient cycling
DE: 4235 Estuarine processes
DE: 1040 Isotopic composition/chemistry
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting