Transformation of particle-bound phosphorus at the land-sea interface

The distribution of phosphorus was examined along the salinity gradient of the Chesapeake Bay estuary during spring and summer in 1989 and 1990. Particulate phosphorus (PP) was the dominant form of phosphorus in the estuary accounting for 23–90% of total phosphorus (TP). Particulate phosphorus was highest in the upper estuary (0.42–1.84 μM) and rapidly decreased in concentration in the upper bay with only slight decreases observed down-estuary. Dissolved organic phosphorus (DOP) often comprised a somewhat larger fraction of the TP (8–43%) than did dissolved inorganic phosphorus (DIP; 5–40%) with DOP and DIP concentrations highest in bottom waters during summer anoxia. Both DIP and DOP concentrations were maintained at relatively low levels in surface waters by plankton, especially during the spring phytoplankton bloom. The distribution of particle-bound phosphorus in the suspended matter of the Chesapeake Bay was partitioned by a sequential chemical leaching technique into three major fractions: (1) an organic and loosely-bound fraction (Org-P), (2) an iron-associated fraction extracted with citrate-dithionate-bicarbonate (CDB-P), and (3) a detrital (apatite rich) fraction extracted with hydrochloric acid (HCl-P). The particle-bound phosphorus was associated mainly with Org-P (43–61%), followed by CDB-P (32–46%), with HCl-P generally comprising only a small fraction of the total particle-bound phosphorus (5–13%). The majority of changes in the composition of the particle-bound phosphorus occurred early in the transition from fresh- to saltwater with declines in both iron-associated phosphorus (CDB-P) and apatite-associated phosphorus (HCl-P). The ratios of CDB-Fe to CDB-P in suspended matter were relatively low suggesting that the particulate matter of Chesapeake Bay has a low capacity to absorb additional phosphorus through interactions with iron. Biological processes in the bay were much more important to the biogeochemical cycle of phosphorus than in many other estuaries previously examined for phosphorus biogeochemistry. Although inorganic exchange reactions may occur, they do not have the capacity to ‘buffer’ DIP concentrations throughout the Chesapeake Bay estuary.