A two-year survey of phosphorus speciation in the sediments of the Bay of Seine (France)

The distribution pattern and temporal variations of concentrations of phosphorus forms were studied in the sediments of the Bay of Seine in seven cruises over the period of 1992-1994. Calcium-bound phosphate (Ca-P) contributed over 50% of the sedimentary phosphorus pool, while exchangeable phosphate (exch-P) and Fe/Al-bound phosphate (Fe/Al-P) respectively made up less than 16% and 38% of total phosphorus. Concentrations of Ca-P were quite constant all over the bay, a consequence of the marine origin of the sediments' calcareous fraction. However, exchangeable phosphate and Fe/Al-bound phosphate (and thus total phosphorus) concentrations showed a mean east/west gradient highlighting the influence of particulate inputs from the Seine plume. Concentrations of inorganic phosphorus forms showed few temporal variations and are an almost permanent feature of the sediments. Finally, organic phosphorus (orga-P) was he only form to exhibit significant spatio-temporal variations. Its strong increase from winter to spring and summer can be related to phytoplankton growth, but remineralization prevents orga-P from accumulating. Potentially bioavailable phosphorus, constituted by orga-P and exch-P, represented mean proportions of no more than 20-40% of the total sedimentary phosphorus pool. This means that enrichment assessments based on total phosphorus alone are questionable. (C) 1997 Elsevier Science Ltd.