Nitrogen budget of Apalachicola Bay, a bar-built estuary in the northeastern Gulf of Mexico

Apalachicola Bay, a bar-built, sub-tropical estuary in the northeastern Gul f of Mexico (GOM), USA. receives freshwater from the Apalachicola River and exchanges water with the GOM at 4 sites. The output from a 3-dimensional c irculation model and nitrogen measurements in the river and the estuary ove r a 2 yr period were used to determine nitrogen input to the estuary and ex change with the GOM. The Apalachicola River was the major nitrogen source t o the estuary and accounted for 92 and 73% of the total dissolved inorganic and organic nitrogen (DIN and DON) input, respectively. Nitrogen input fro m the GOM provided the remainder. DIN comprised 61% of total dissolved nitr ogen (TDN) input to the estuary from the river and 26% of TDN input from th e GOM. Maxima in TDN input to the estuary occurred during the high river-fl ow period (October to February). In contrast, TDN input minima occurred dur ing the summer (May to September), when river flow was low. Benthic ammoniu m flux to the water column was maximum during the summer. However, it was n ot large enough to satisfy phytoplankton nitrogen demand during that period . The fraction of DIN input that was exported to the GOM increased linearly as estuarine water residence time decreased from 2 wk during summer to les s than 3 d during winter. Seasonal nitrogen budgets indicated that DIN expo rt to the GOM was maximum during winter (87 +/- 5.4 mg N m(-2) d(-1)) and m inimum during summer (9 +/- 1.4 mg N m(-2) d(-1)). In contrast to the large decrease in DIN export to the GOM between these 2 periods, DON export to t he GOM decreased from 81 +/- 5.8 mg N m(-2) d(-1) during winter to only 45 +/- 3.2 mg N m(-2) d(-1) during summer. Estimated denitrification rates ind icated that 9% of the annual total nitrogen input to the estuary was remove d by that mechanism. The f ratio in Apalachicola Bay varied from 0.11 durin g the summer to 0.74 during the winter and averaged 0.19 for a 2 yr period. On an annual basis, DON input to the estuary and export to the GOM were in balance, while 66% of DIN input to the estuary was exported to the GOM. Th e nitrogen budget for Apalachicola Bay was balanced to within 6% of total n itrogen input.