Phytoplankton community structure at contrasting sites in the Irish Sea: a modelling investigation

Factors controlling phytoplankton community structure were investigated at two sites in the western Irish Sea using an ecosystem model coupled to a one-dimensional physics model. The ecosystem model includes two compartments for phytoplankton (diatoms and non-diatoms) and zooplankton (meso- and microzooplankton), two nutrients (nitrogen and silicate) and a parameterisation of sediment dynamics. Diatoms were assigned higher maximum growth rates than non-diatoms in the model such that they are superior competitors, providing silicate is non-limiting. Resource limitation was formulated using a multiplicative model and temperature effects on biological processes were not included on account of a lack of a clear approach to do so. The model successfully reproduced seasonal cycles of primary production, chlorophyll and nutrients at both sites. The predicted dominance of diatoms during the summer at the coastal site was due to the supply of silicate from the sediment and top-down control of non-diatoms by microzooplankton grazing. Seasonal stratification at the offshore site prevented silicate of sedimentary origin from reaching surface waters, causing the mixed spring diatom-flagellate bloom to be replaced by non-diatoms with occasional diatom resurgence. Higher diatom abundance resulted in reduced grazing on microzooplankton by mesozooplankton and explains the higher biomass of microzooplankton at the coastal station. It is concluded that under conditions of nutrient re-supply, top-down grazing helps to control the balance between diatoms and non-diatoms. In deeper, stratified waters, there is only bottom-up control (nutrient limitation) of phytoplankton community structure.