HR: 15:45h
AN: NB24F-02 INVITED     [Abstracts]
TI: Hydrologic and Biological Controls on the Fate of Watershed-derived Nitrogen in Estuaries: Insights from "Whole-estuary" Stable Isotope Enrichments
AU: * Tobias, C
EM: tobiasc@uncw.edu
AF: University of North Carolina at Wilmington, Earth Sciences Department 601 S. College Rd, Wilmington, NC 28403 United States
AU: Peterson, B
EM: peterson@mbl.edu
AF: Marine Biological Laboratory, 7 MBL St., Woods Hole, MA 02543 United States
AU: Deegan, L
EM: ldeegan@mbl.edu
AF: Marine Biological Laboratory, 7 MBL St., Woods Hole, MA 02543 United States
AU: Vallino, J
EM: jvallino@mbl.edu
AF: Marine Biological Laboratory, 7 MBL St., Woods Hole, MA 02543 United States
AU: Holmes, R
EM: rholmes@mbl.edu
AF: Marine Biological Laboratory, 7 MBL St., Woods Hole, MA 02543 United States
AB: In situ nitrogen isotope tracer additions are becoming an increasingly widespread tool for understanding nitrogen fate and turnover in stream ecosystems. The approach however has only been attempted twice in estuaries. Isotope enrichment of these systems represents a several-fold increase in scale, and added challenges of bidirectional flow. Despite these potential limitations, the approach has yielded insights into nitrogen uptake, trophic transfer, and recycling on the scale of tens to hundreds of thousands of cubic meters of water and up to 4 kilometers of benthic area. The large-scale use of ¹⁵N tracer additions to estuaries has provided the unique ability to quantify multiple nitrogen (N) flow pathways under natural hydrologic and geochemical conditions. Estuarine ¹⁵N enrichment studies have been performed in two estuaries in the Plum Island LTER (Parker and Rowley Rivers) that differed in geomorphology, hydrology and biological structure. In each experiment ¹⁵N-nitrate tracer was added at the head of the estuaries in order to label watershed N entering the estuary and provided a "tag" for that N as it was processed in the estuary. ¹⁵N tracer released into the long water residence time Parker River was processed primarily (approx. 100 percent) through phytoplankton which subsequently supported high rates of secondary production in both the water column and benthos. In contrast, tracer released into the short water residence time Rowley River was primarily (75-80 percent of added ¹⁵N) advected, unaltered out of the estuary as nitrate. The ¹⁵N sinks that did exist within the Rowley were divided evenly between assimilation by benthic autotrophs (including uptake by marsh macrophytes) and denitrification. Recycling of the assimilated ¹⁵N back into the water column was a more important mechanism of N transfer than support of secondary production in the Rowley. Collectively, the use of large-scale stable ¹⁵N isotope additions in these contrasting estuaries has allowed us to examine how geomorphology, hydrology, and biotic structure interact to regulate estuarine N processing.
DE: 0400 Biogeosciences
SC: North American Benthological Society [NB]
MN: 2005 Joint Assembly