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

Abstract

Despite the quantitative importance of dissolved organic nitrogen (DON) in aquatic sys- tems, information on the spatial and temporal distribution of DON in estuaries is lacking. We employed a combination of property-surface plots, hydrodynamic model results, and property-salin- ity plots to examine nitrate and DON distributions in Apalachicola Bay, Florida, USA, during Apalachicola 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 concentrations. Therefore, DON production by phytoplankton exceeds the combined effects of dilution plus DON processing by the food web at these stations. Depending 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 declined within the estuary as salinity increased. Budget calculations indicate that DON retention within the estuary was insignificant during all months sampled. Therefore, DON that is exported from Apalachicola Bay to the Gulf of Mexico (GOM) consists of DON that enters the estu- ary and is not utilized in this short residence-time estuary plus a fraction produced within the estuary by the food web. Nitrate and DON exported from the estuary comprise sources of new nitrogen for the northeastern GOM food web.