Transformation of phosphorus in the wadden sea: Apatite formation

Abstract

Between February 1992 and June 1993 the distribution of particulate and dissolved phosphorus in the Ems estuary was investigated in approximately monthly intervals. Furthermore, the phosphorus content of sediments was assessed. These data as well as literature data were used to construct a phosphorus budget of the Ems estuary. Fluxes were dominated by the particulate fraction representing 77% of the total phosphorus input. 40% of the phosphorus input is lost to the sediment. In suspended matter from the coastal zone and from rivers, organic and iron-bound phosphorus dominate (> 75%). Within the sediments phosphorus is mainly associated with calcium minerals like apatites (65%). The presence of authigenic apatites was confirmed with a sequential extraction method. Combination of the phosphorus budget with the data on the composition of particulate phosphorus shows that the organic and iron bound phosphorus is not stable on longer time scales: about 80% is either transformed into dissolved phosphorus and exported from the estuary or transformed to apatites and buried. It is suggested that formation and retention of authigenic apatites is the major process withdrawing phosphorus from the coastal biogeochemical cycle.