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

Estuarine nutrient budgets are used to understand the transformation of riv er-born nutrients delivered to the sea, and to gain insights into the impac t of terrestrially-derived nutrients on coastal ecosystems. To construct a phosphorus budget for Apalachicola Bay, we measured soluble reactive, disso lved organic, and particulate phosphorus in the Apalachicola River and estu ary on a monthly basis over a 2 yr period. The results of the chemical time -series measurements were coupled with river flow data and results of a 3-d imensional hydrodynamic model of the estuary to estimate phosphorus input t o the estuary and phosphorus exchange with the Gulf of Mexico (GOM). On ave rage, the Apalachicola River accounted for 70 and 78% of the water and tota l phosphorus (TP) input to the estuary, respectively, while exchange with t he GOM accounted for the remainder. TP input to the estuary was 4.57 +/- 0. 21 (+/- 1 SD) g P m(-2) yr(-1), the majority of which was in the particulat e form (2.69 +/- 0.19 g P m(-2) yr(-1)). Dissolved organic phosphorus (DOP) represented 24 % of the total input, while soluble reactive phosphorus (SR P) represented the smallest fraction (17%) of the total input. Particulate phosphorus export to the GOM (2.75 +/- 0.19 g P m(-2) yr(-1)) was similar t o 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 GO M. Phosphorus accumulation in the sediments accounted for 25 % (1.10 +/- 0. 37 g P m(-2) yr(-1)) of the TP input to the estuary. Phytoplankton phosphor us demand for the study period was 6.75 g P m(-2) yr(-1), of which 11% coul d be supplied by net SRP and DOP input to the estuary. The phosphorus budge t for Apalachicola Bay was balanced to within 9 % of TP input.