THE EFFECT OF DEPOSITION OF ORGANIC-MATTER ON PHOSPHORUS DYNAMICS IN EXPERIMENTAL MARINE SEDIMENT SYSTEMS

The effect of deposition of organic matter on phosphorus dynamics -in sandy marine sediments was evaluated using an experimental system (box cosms) and three different strategies: (1) no supply (2) one single ad dition (3) weekly additions of a suspension of algal cells (Phaeocysti s spec.). Macrofauna (3 species, 6 individuals of each) were added to half of the boxes. Both in the case of the single and weekly additions a clear effect of increased organic matter loading on phosphorus dyna mics was found. Following the organic matter addition, porewater phosp hate concentrations in the upper sediment layer increased, phosphate r elease rates from the sediment increased by a factor 3-5 and in the bo xes to which a single addition was applied NaOH-extractable phosphorus increased substantially. The increase in phosphate release rates from the sediment was attributed to mineralization of the added material a nd to direct release from the algal cells. No clear effect of the pres ence of macrofauna on sediment-water exchange of phosphate could be di scovered. The macrofauna were very effective at reworking the sediment , however, as illustrated by the organic carbon profiles. It is hypoth esized that the sediment-water exchange rates of phosphate were regula ted by the layer of algal material which was present on the sediment s urface in the fed boxes. In the boxes to which the single addition was applied porewater phosphate concentrations were lower and NaOH-extrac table phosphorus was higher in the presence of macrofauna, suggesting that macrofauna can stimulate phosphate binding in the sediment.