The aim of the interdisciplinary project KUSTOS (Coastal fluxes of matter and energy: the transition land-ocean in the southeastern North Sea) was to analyze and quantify fluxes of matter and energy from the land to the ocean throughout the coastal region of the German Bight. Objective of my thesis was to investigate the influence of dissolved inorganic N:P ratios above Redfield on the phytoplankton community. In spring 1995 the project focused on the study of turnover processes under high ambient nitrate levels in the Elbe River plume. Biological and chemical changes in a watermass were characterized by tracking the initial water for 10 days with a drifting buoy. Additionally, larger grids of stations were sampled repeatedly on a short time scale. Some grids of stations were sampled before and others during or after the drift investigation. The biological and chemical situation observed in the field was later reconstructed in an enclosure experiment to study the reaction of the spring phytoplankton community to varying salinity and increasing dissolved inorganic N:P ratios in a more controlled environment.
The spring bloom began during the field investigations, transgressing towards stationary state over time. Dithylum brightwellii was the dominant diatom at the drifting buoy station in the plume, while Rhizosolenia species and Cerataulina pelagica were major contributors to the standing stock at less eutrophic stations. Towards the end of the initial bloom DIP and DSi were depleted, while nitrate concentrations remained in a surplus of up to 40 µM, indicating a phosphorus limited system. A hydrodynamic model was applied on the grid data for a broad estimation of the range of biologically induced changes in a large region of the German Bight. These estimates were compared to the consumption of inorganic nutrients and production of POM by the spring bloom during the drift investigation and in the enclosures. The amount of consumed nutrients in the enclosures covered a comparable range as estimated for the drift investigation, while variations were larger for the produced POM, due to e.g. particle sedimentation and grazing in the field. N:P ratios above 20 significantly increased the particulate nitrogen and chlorophyll content as well as the BOD in the enclosures and in some cases resulted in a significant increase in the amount of carbon stored in the plankton. Negative effects of the highest nitrate dosage and an increasing contribution of smaller diatoms and flagellates with rising N:P ratio are speculated. The results of this combined field and enclosure study imply that a rise in nitrate supply above Redfield ratios can have negative effects on a coastal ecosystem. Regions remote from the original source of nutrient input may receive additional loads, as high surplus nitrate amounts in the water and phytoplankton particles with high amounts of nitrogen stored may be transported there by the currents. This needs to be considered in coastal management decisions. Additionally, carbon fluxes in a coastal, non-steady state system, can be strongly underestimated when calculated based on nutrient consumption and the Redfield ratio. Finally, the sensitivity of the Elbe/German Bight system towards eutrophication is discussed in comparison to the Mississippi/Louisiana shelf system.