Do mussel beds intensify the phosphorus exchange between sediment and tidal waters?

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Abstract
Fluxes of total phosphorus (TP) and dissolved inorganic phosphorus (DIP) between an intertidal mussel bed and the overlying tidal water were measured with an in situ-flume during 5 tidal cycles in summer 1989. Simultaneous control measurements of phosphorus fluxes were made in a parallel flume lane without cover of mussels. Fluxes of total phosphorus showed an irregular way of release and uptake by the mussel bed, whereas in the control lane release during flood and uptake during ebb tide were observed. DIP was mainly released by the mussel bed. This release of DIP was significantly higher than that of the control lane. Additional flume measurements of DIP-fluxes over a mussel bed - deeper inundated during high tide - were made during 6 tidal cycles in 1986. These measurements showed release processes of DIP during the beginning and the end of the tidal cycles, whereas around high tide, uptake processes were observed, which reflected the interaction of DIP with particles and phytoplankton in the water column. The difference between the concentration of TP and DIP showed the amount of organic phosphorus (OP) bound in particles as well as in dissolved organic matter. On average OP was taken up by the mussel bed, whereas over the control sediment a similar pattern as that for TP was found. During 2 tidal cycles the mussel bed showed a net uptake of TP, during 3 tidal cycles a net release. The different directions of TP-flux were explained by different phytoplankton contents in the water column. Although phytoplankton is only a minor part in total particulate matter, a high amount of phytoplankton increased filtration activity by mussels and led to a distinct input of particulate P from the water column to the mussel bed. During low phytoplankton content the resuspension and diffusion of phosphorus components are not balanced by uptake processes.