The importance of sediments in the transformation and turnover of nutrients and organic matter in the Wadden Sea and German Bight

Abstract

From 1994 through 1996 transformation processes in the water column of the German Bight and the adjacent Wadden Sea were investigated in the projects TRANSWATT and KUSTOS. On the basis of a review of carbon and nutrient budgets we examine the role of processes in the sediment for overall carbon and nutrient cycling in the Wadden Sea and adjacent German Bight. We distinguish two aspects: the sediment as the site where organic matter is rapidly turned over and the sediment as the site where organic matter and nutrients are immobilized. The relative importance of the sediment for the remineralisation of organic matter depends on the water depth: The review of carbon budgets suggests that in the Wadden Sea (2 - 3 m) about 50% of the remineralisation occurs in the sediment. In the German Bight (20 m), 10 - 20% of the primary production is remineralised in the sediment. The budgets further show that the Wadden Sea is heterotrophic. About 100 gC m-2 y1 is imported from the coastal zone. This implies a net autotrophy of the coastal zone, which is in line with the results from the projects TRANSWATT and KUSTOS. Within the Wadden Sea, organic matter has to be turned over two to three times and in the German Bight three to four times to explain the annual primary production. This is lower than in the offshore North Sea where annual turnover rates up to five have been found. Several processes remove nutrients on longer time-scales from the biogeochemical cycle. The importance of the local formation of phosphorus containing minerals like apatite as a phosphorus sink is shown. A discussion of several denitrification estimates concludes that in the German Bight and adjacent Wadden Sea on average about 8 -16% of the total nitrogen influx (from the coastal zone, from rivers and via the atmosphere) is lost to the atmosphere.