Sediment–water exchange of nutrients in the Marsdiep basin, western Wadden Sea: Phosphorus limitation induced by a controlled release?

To quantify the release of inorganic phosphorus from the sediments and assess its contribution to present primary production, a basin-wide study of the Marsdiep (western Wadden Sea, The Netherlands) was performed. Two distinct sedimentary zones were identified: a depositional area characterized by a high content of silt and organic carbon and a small grain size and the majority of the area, composed of fine/medium sand and a low organic carbon content. The sediment–water exchange was higher in the fine grained depositional area and based on a relationship found between the release of inorganic phosphorus and the silt content, a total annual release of 1.0×107 mol P was estimated for the whole Marsdiep basin. A spatial variability in the processes controlling the nutrient release was found. The exchange in the depositional area resulted mainly from molecular diffusive transport, with mineralization and sorption determining the concentration of inorganic phosphorus in the porewater. For the coarser sediment stations the activity of macrofauna clearly enhanced the fluxes. Given the relative demand of nutrients (N:P:Si) for phytoplankton growth, the release was phosphorus deficient during most of the year. Nevertheless, it increased from February until September, in parallel with the increase in temperature and light, thus having the potential to fuel primary production during their seasonal growth period. In terms of absolute values, our results show that the present exchange, enhanced by the activity of macrofauna has the potential to fuel a significant fraction of the recent levels of primary productivity