Influence of variability in water transport on phytoplankton biomass and composition in the southern North Sea: a modelling approach (FYFY)

Abstract

A model for phytoplankton composition and succession coupled to a transport model for the southern North Sea is presented. This model is used to examine the time and spatial variability in phytoplankton biomass and succession. Long term time variability due to the variability in horizontal water transport is studied by using daily varying transport fields for the period 1970–1981. These transport fields result from simulations with a circulation model driven by realistic wind fields for this period. Selective factors for phytoplankton are resource competition and zooplankton grazing. This leads to a general abundance of edible phytoplankton groups in the whole southern North Sea, while poorly edible groups mainly occur in the eutrophicated coastal areas. Apart from this, phytoplankton groups which are specialized in growth under nitrogen-limited conditions are selected in open sea while, near the Dutch coast and the German Bight, phosphate-specialized groups are selected. From a comparison of simulations with yearly averaged and daily varying transport fields, it is concluded that differences with respect to the annual mean phytoplankton biomass are negligible. However, large differences are found for the distribution and abundance of specific phytoplankton groups. A simulation for the period 1970–1981 shows that part of the observed variability in spring biomass as well as the variability in the duration of dominance and abundance of species near the Dutch coast can be attributed to the variability in the horizontal water transport.