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.