Phosphorus budget of Apalachicola Bay:a river-dominated estuary in the northeastern Gulf of Mexico

Abstract

Estuarine nutnent budgets are used to understand the transformation of nver-born nutrients delivered to the sea, and to gain insights into the impact of terrestnally-denved nutnents on coastai ecosystems. To construct a phosphorus budget for Apalachicola Bay, we measured soluble reactive, dissolved organic, and particulate phosphorus in the Apaiachicola River and estuary on a monthly basis over a 2 yr penod. The results of the chernical time-senes measurements were coupled with nver flow data and results of a 3-dimensional hydrodynamic model of the estuary to estimate phosphorus input to the estuary and phosphorus exchange with the Gulf of Mexico (GOM). On average, the Apalachicola River accounted for 70 and 78 % of the water and tot& phosphorus (TP) input to the estuary, respectively, while exchange with the GOM accounted for the remainder. TP input to the estuary was 4.57 r 0.21 (r 1SD) g P m-2 yr-', the majonty of which was in the particulate form (2.69 r 0.19 g P m-2 F-'). Dissolved organic phosphorus (DOP) represented 24 % of the total input, while soluble reactive phosphorus (SRP) represented the smallest fraction (17 %) of the total input. Particulate phosphorus export to the GOM (2.75 r 0.19 g P m-2 yr-') was similar to particulate phosphorus input to the estuary. However, only 64 and 58% of the DOP and SRP input to the estuary, respectively, were exported to the GOM. Phosphorus accumulation in the sediments accounted for 25 % (1.10 r 0.37 g P m-2 yr-') of the TP input to the estuary. Phytoplankton phosphorus demand for the study period was 6.75 g P m-2 yr-I, of which 11 % could be supplied by net SRP and DOP input to the estuary. The phosphorus budget for Apalachicola Bay was balanced to within 9 % of TP input.