Benthic nutrient regeneration in the ERSEM ecosystem model of the North Sea

Abstract

In any ecosystem modelling approach to shallow seas an adequate description of the sediment-water interactions to all the essential nutrients is necessary. With this aim a fairly simple concept has been developed and applied for the modelling of the nutrient cycles of N, P and Si which includes the essential diagenetic processes such as vertical transport, oxic and anoxic mineralization, silicate dissolution, adsorption, nitrification and denitrification. All these processes are explicitly formulated for one or more distinct layers in the sediment. The model contains a low number of variables and is efficient with respect to computer time. The model is part of the European Regional Seas Ecosystem Model (ERSEM) of the North Sea, a joint effort of several institutes around the North Sea. The benthic-pelagic coupling of the ERSEM model is discussed with emphasis on the consequences for the benthic nutrient cycling and the interrelations between the different benthic nutrient submodels. The results show that the model is able to describe the seasonal variation of nutrient fluxes including the sediment-water exchanges and the vertical profiles in the pore water. From the model results is concluded that changes in organic matter deposition directly influence the benthic nitrification and denitrification through changes in the oxygen availability to the nitrifiers and, due to this, the supply of nitrate to the denitrifiers. Short fluctuations in organic deposition (e.g. after the spring bloom) are not reflected in the fluxes due to sorption buffering of phosphate in the oxidized surface layer of the sediment.