Limiting Factors for Phytoplankton in the North Sea

Abstract

Underwater light intensity and nutrients are important for the abundance and species composition of phytoplankton. In winter the development of phytoplankton in the North Sea is limited by light. In the central parts of the North Sea the spring bloom starts earlier than in coastal areas. This difference is caused by the higher light attenuation coefficients in the coastal zone. In summer the surface mixed layer (SML) of the stratified areas shows light saturation of phytoplankton photosynthesis. During the growing season the continental coastal waters of the North Sea are potentially phosphorus or silicon limited when tested with a bioassay of the diatom Skeletonemacostatum. The offshore waters are potentially nitrogen limited. There are indications that the spring diatom bloom in the coastal waters ends because of silicon depletion and that the nitrogen and phosphorus concentrations are still large enough to allow the development of a large bloom of Phaeocvstiscf. pouchetii. In summer diatoms disappear from the SML of the stratified areas, but they survive under the thermocline. Possible effects of a reduction of the nutrient loads to the North Sea are discussed in the context of experience in the Delta region (SW-Netherlands).