Investigations on the nitrogen cycle in the coastal North Sea

This thesis focuses on the nitrogen cycle in Dutch coastal waters and sediments. The main hypothesis of this study was that the different steps of the nitrogen cycle occur spatially and temporally separated from each other rather than that the cycle is closed in the same place and time. To verify this hypothesis, multiple experimental approaches were applied to investigate N2 fixation, denitrification, anammox and nitrification in the some selected ecosystems (coastal microbial mat, water column and the bottom sediments of the Southern North Sea) of the Dutch coast. We concluded that N2 fixation, denitrification anammox and nitrification and the responsible microorganisms occur on different spatial and temporal scales in ecosystems of the Dutch coast. A number of factors may drive the observed changes in the main processes in the nitrogen cycle. These may include composition and nature of the diazotrophic community and physicochemical factors including temperature, salinity, oxygen, and availability of substrates. Future studies are recommended in order to provide knowledge on the effects of environmental conditions on the nitrogen cycle in coastal environments, using quantitative experiments in mesocosms and laboratory experiments with the key microorganisms.