Modelling research on the production cycle of phytoplankton in the Southern Bight of the North Sea in relation to riverborne nutrient loads

Abstract

The physico-chemical and biological composition of North Sea water off the Dutch coast is influenced by various factors including meteorological conditions, wind and tide induced flows and mixing, river pollution loads, waste loads and various physical, chemical and biological processes. The long term variations (> ∼1 month) of the biological composition are mainly caused by fluctuations in meteorological conditions and concentrations of nutrients for algae. The loads of phosphorus and nitrogen compounds on the North Sea have increased significantly since the early 20th century. Especially the discharge of phosphorus (P) and nitrogen (N) transported by the river Rhine has increased about sixfold since 1930. The nutrient rich river water originating from the rivers Scheldt, Meuse and Rhine, mixes with sea water in such a way that 50% of the river discharge is transported in a 15 km wide strip along the Dutch and Belgian coast. Especially in this coastal area, the increase in nutrient loads has had a marked influence on the nutrient concentrations. In fact, the increase in N and P and the essentially constant discharge of reactive silicate (Si) caused a change in nutrient, first depleted by phytoplankton, from N or P some 50 years ago to Si nowadays. The question, to what extent the increase in nutrient enrichment has given rise to enhanced phytoplankton growth, or to changes in species composition, is difficult to answer because of a lack of reliable data from the past. Nevertheless it is conceivable that at present primary productivity may occasionally be higher than in the past. The upper limit to the current diatom blooms in Dutch coastal waters is set by the amount of available silicate. An increase in the upper limit is not to be expected due to the constancy of the Si-load during the last 50 years. Some flagellate species, however, appear to be phosphorus limited in spring and locally dominate diatoms. Nitrogen compounds maintain low concentrations throughout the summer. Therefore, it is conceivable that the abundance of the specific flagellate species is closely related to the riverborne P- and N-loads. The research as described in this paper intends to simulate the phytoplankton biomass and composition in relation to meteorological and hydrophysical conditions, nutrient loads and zooplankton biomass. It can be considered as a theoretical contribution towards a better understanding of the production and eutrophication processes in this polluted and exploited part of the North Sea.