IMPORTANCE OF TERRESTRIALLY-DERIVED, PARTICULATE PHOSPHORUS TO PHOSPHORUS DYNAMICS IN A WEST-COAST ESTUARY

Allochthonous inputs of suspended particulate matter from freshwater e nvironments to estuaries influence nutrient cycling and ecosystem meta bolism. Contributions of different biogeochemical reactions to phospho rus dynamics in Tomales Bay, California, were determined by measuring dissolved inorganic phosphorus exchange between water and suspended pa rticulate matter in response to changes in salinity, pH, and sediment redox. In serum bottle incubations of suspended particulate matter col lected from the major tributary to the bay, dissolved inorganic phosph orus release increased with salinity during the initial 8 h; between 1 -3 d, however, rates of release were similar among treatments of 0 psu , 16 psu, 24 psu, and 32 psu. Release was variable over the pH range 4 -8.5, but dissolved inorganic phosphorus releases from sediments incub ated for 24 h at the pH of fresh water (7.3) and seawater (8.1) were s imilarly small. Under oxidizing conditions, dissolved inorganic phosph orus release tvas small or dissolved inorganic phosphorus was taken up by particulate matter with total P content <50 mu moles P g(-1); rele ase was greater from suspended particulate matter with total phosphoru s content >50 mu moles P g(-1). In contrast, under reducing conditions maintained by addition of free sulfide (HS-), dissolved inorganic pho sphorus was released from particles at all concentrations of total pho sphorus in suspended particulate matter, presumably from the reduction of iron oxides. Since extrapolated dissolved inorganic phosphorus rel ease from this abiotic source can account for only 12.5% of the total dissolved inorganic phosphorus flux from Tomales Bay sediments, we con clude most release from particles is due to organic matter oxidation t hat occurs after estuarine deposition. The abiotic, sedimentary flux o f dissolved inorganic phosphorus, however, could contribute up to 30% of the observed net export of dissolved inorganic phosphorus from the entire estuary.