Seasonal dynamics of the North Sea sediments using a three-dimensional coupled sediment–water model system

A high-resolution early diagenetic model for the North Sea sediments has been coupled with a pelagic ecosystem model to quantify the three-dimensional processes in the coupled sediment–water system from the sea surface to a sediment depth of 11 cm focussing on the processes in the sediments of the North Sea. The pelagic ecosystem model ECOHAM1 simulates the 1986 phytoplankton dynamics considering circulation, temperature, nutrient availability in the water column and the resulting flux of particulate organic carbon (POC) onto the sediment. These seasonal and regional variable water column processes are considered as forcing for the early diagenetic model C. CANDI, which calculates the processes in the upper sediment column, represented by 14 dissolved and 6 solid species, resolved with 84 vertical levels. With the coupled model the daily benthic fluxes of POC, oxygen, nitrate, phosphate, and sulphate at the sediment–water interface for each of the 1158 horizontal cells covering the whole North Sea area has been determined. The simulations show the seasonal and regional variations in the pelagic and the sediment system. The coupled model reproduces very well measured oxygen and nitrate penetration depths at selected validation stations. A vertical section from Fair Isle into the German Bight in summer demonstrates high spatial phosphate variability in the water column and in the sediment. Observations on the sediment–water interface fluxes and of concentration distributions in the sediment are very sparse. The results of this high-resolution model allows the calculation of budgets at the sediment–water interface for the whole simulation area. The annual cumulated phosphate flux across the sediment–water interface exhibits strong fluxes concentrated in a narrow band off the continental coast in shallow waters over 40 mmol m −2 yr −1, whereas in the central North Sea fluxes are lower than 30 mmol m −2 yr −1. Simulated annual cycle of fluxes at the sediment–water interface at a position located in the central North Sea showed a phosphate flux shifted by 1 month compared to the organic matter flux, whereas the sulphate flux into the sediment showed an overall time lag of about 5 months.