Distribution of phosphorus in sediments from Tomales Bay, California

Abstract

The distribution of phosphorus among six sequentially extracted fractions was determined in suspended load and bottom sediments from Lagunitas Creek and Tomales Bay, California. The sequential extraction scheme was a modification of the sedex scheme of Ruttenberg (1992), using the surfactant sodium dodecyl sulfate (SDS) to extract organic phosphorus. SDS effectively removed organic phosphorus from sediments with little interference from inorganic phosphorus containing phases. The distribution of phosphorus in the sequentially extracted fractions was similar in suspended load and bottom sediments collected from Lagunitas Creek and the bay. The concentration of phosphorus in CDB-extractable iron oxide phases was below detection in all samples. Most of the phosphorus (70–80%) contained in these sediments was found in the organic and residual phases. Porewaters from sediment cores were analyzed for dissolved inorganic carbon, ammonium, phosphate, sulfate, iron, calcium and fluoride. Within bay sediments, porewaters were depleted in phosphate relative to dissolved inorganic carbon and ammonium, suggesting that phosphate released from organic matter decomposition is being removed from the porewaters. Sequential extraction results suggest that approximately 15% of the phosphate released from organic matter decomposition could be removed from the porewaters by adsorption into the exchangeable phosphorus fraction and an additional 3% may be precipitated as authigenic phosphate minerals. The remaining phosphate (82%) released from organic matter decomposition must be either taken up within the sediment mixed layer (0–40 cm) or escapes to the overlying water column.