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Coastal Eutrophication in the Southeastern United States: Nitrogen versus Phosphorus Limitation and the Contribution of Organic Nitrogen

PIs: Deborah A. Bronk and Marta P. Sanderson, Dept of Marine Sciences, Univ of Georgia, Athens, GA, USA  (currently at the Virginia Institute of Marine Science, College of William and Mary)

Support: Georgia Sea Grant College Program (Project Number R/WQ-10)

Timeframe: 3/1/98 - 2/28-02 (complete)

Project Overview:
This was a study of two Georgia rivers, the Altamaha and the Satilla, with the following objectives: 1) to investigate nitrogen (N) versus phosphorus (P) limitation using stoichiometric ratios and bioassays; 2) to measure uptake rates of organic N (urea, dissolved primary amines, and humic-N) and inorganic N (ammonium, nitrate, nitrite); 3) to quantify the potential role of groundwater as a source of eutrophication; 4) to compare the findings with studies on the utilization of dissolved organic N (DON) in New Jersey and Maryland; to continue and expand the dialogue with local and regional shareholders and the general public on coastal eutrophication issues.

  • Thrice yearly sampling cruises at four stations along the two rivers and piezometric sampling of surficial groundwater were followed by standard wet chemical techniques to quantify inorganic and organic N and P species present.

  • Bioassays (determination of chlorophyll a) were performed after ammonium, nitrate and phosphate additions.

  • N flux rates were monitored using tracer 15N.


  • In the Altamaha River, DON could represent up to 90% of the ambient dissolved N in the water column; and that 80% of this DON was humic-N.

  • Organic N accounted for upwards of 47% of the N taken up by the plankton community; 31% of total N uptake from humic-N and 21% from urea.

  • The research results were compared with three other Sea Grant funded projects (Chesapeake Bay [Pat Glibert, University of Maryland], New York, [Julie LaRoche, Brookhaven National Laboratory], and New Jersey [Sybil Seitzinger, Rutgers University]). Consistent with the belief that the Georgia Rivers represented more pristine natural state dominated by marshes, preliminary findings from the comparison indicate that the Altamaha and upper Savannah had the lowest C:N ratio indicative of refractory organic matter from marshes.


Bronk, D.A., Glibert, P.M., Malone, T.C., Banahan, S., and Sahlsten, E. 1998. Inorganic and organic nitrogen cycling in Chesapeake Bay: autotrophic versus heterotrophic processes and relationships to carbon flux. Aquatic Microbial Ecology.

Bronk, D. A. 1999. Rates of NH4+ uptake, intracellular transformation, and dissolved organic nitrogen release in two clones of marine Synechococcus spp. J. Plankton Res. 21: 1337-1353.

Bronk, D.A. 2002. Dynamics of organic nitrogen. In Biogeochemistry of Marine Dissolved Organic Matter. Hansell, D.A. and Carlson, C.A., Eds. Academic Press, San Diego. pp. 153-247.

Koopmans, D. J. and Bronk, D. A. 2002. Photochemical production of inorganic nitrogen from dissolved organic nitrogen in waters of two estuaries and adjacent surficial groundwaters. Aquatic Microbial Ecology. 26: 295-304.

Wiegner, T. N., Seitzinger, S. P., Glibert, P., Bronk, D. A. and LaRoche, J. (Submitted). Influence of watershed land use on riverine dissolved organic matter composition and bioavailablility.

Bronk, D. A., Lomas, M. W., Glibert, P. M., and Sanderson, M. P. (In revision). Total dissolved nitrogen analysis: Comparison between the persulfate, UV, and high temperature oxidation methods. Mar. Chem.

Wingate, L. A. and Bronk, D. A. (submitted). Effect of vacuum pressure on post-filtration dissolved nitrogen concentrations: the importance of cell morphology in field and culture settings. J. Exp. Mar. Biol. Ecol.

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This page was updated October 13, 2006