TRANSFORMATION OF PARTICLE-BOUND PHOSPHORUS AT THE LAND SEA INTERFACE

The distribution of phosphorus was examined along the salinity gradien t of the Chesapeake Bay estuary during spring and summer in 1989 and 1 990. Particulate phosphorus (PP) was the dominant form of phosphorus i n the estuary accounting for 23-90% of total phosphorus (TP). Particul ate phosphorus was highest in the upper estuary (0.42-1.84 mu M) and r apidly decreased in concentration in the upper bay with only slight de creases observed down-estuary. Dissolved organic phosphorus (DOP) ofte n comprised a somewhat larger fraction of the TP (8-43%) than did diss olved inorganic phosphorus (DIP; 5-40%) with DOP and DIP concentration s highest in bottom waters during summer anoxia. Both DIP and DOP conc entrations were maintained at relatively low levels in surface waters by plankton, especially during the spring phytoplankton bloom. The dis tribution of particle-bound phosphorus in the suspended matter of the Chesapeake Bay was partitioned by a sequential chemical leaching techn ique into three major fractions: (1) an organic and loosely-bound frac tion (Org-P), (2) an iron-associated fraction extracted with citrate-d ithionate-bicarbonate (CDB-P), and (3) a detrital (apatite rich) fract ion extracted with hydrochloric acid (HCl-P). The particle-bound phosp horus 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 to tal particle-bound phosphorus (5-13%). The majority of changes in the composition of the particle-bound phosphorus occurred early in the tra nsition from fresh- to saltwater with declines in both iron-associated phosphorus (CDB-P) and apatite-associated phosphorus (HCl-P). The rat ios of CDB-Fe to CDB-P in suspended matter were relatively low suggest ing that the particulate matter of Chesapeake Bay has a low capacity t o absorb additional phosphorus through interactions with iron. Biologi cal processes in the bay were much more important to the biogeochemica l 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 concentratio ns throughout the Chesapeake Bay estuary.