Dissolved organic nitrogen and nitrate in Apalachicola Bay, Florida: spatial distributions and monthly budgets

Despite the quantitative importance of dissolved organic nitrogen (DON) in aquatic systems, information on the spatial and temporal distribution of DO N in estuaries is lacking. We employed a combination of property-surface pl ots, hydrodynamic model results, and property-salinity plots to examine nit rate and DON distributions in Apalachicola Bay, Florida. USA, during Apalac hicola River high-flow and low-flow conditions. DON enters Apalachicola Bay in river water and in seawater through several passes. DON concentrations in water at some stations within the estuary exceed end-member concentratio ns. Therefore, DON production by phytoplankton exceeds the combined effects of dilution plus DON processing by the food web at these stations. Dependi ng on estuarine hydrodynamics, nitrate concentrations were influenced both by dilution with low-nitrate water entering the estuary from the passes and by biological processes. In contrast to DON, nitrate concentrations declin ed within the estuary as salinity increased. Budget calculations indicate t hat DON retention within the estuary was insignificant during all months sa mpled; Therefore, DON that is exported from Apalachicola Bay to the Gulf of Mexico (GOM) consists of DON that enters the estuary and is not utilized i n this short residence-time estuary plus a fraction produced within the est uary by the food web. Nitrate:and DON exported from the estuary comprise so urces of new nitrogen for the northeastern GOM food web.