The impact of thermal stratification on phytoplankton and nutrient dynamics in shelf seas: a model study

Local heating rate within the water column depends on energy input by solar radiation and on heat exchange across the surface as controlled by wind and convection. The heating results in thermal stratification of the water column, which in turn affects the vertical transport of, for example, nutrients. In this paper the implications of the stratification on the biota by focusing on the time of its onset and its variability in time and (vertical) space are evaluated. Especially the consequences of the stratification on the phytoplankton dynamics and the trophic interactions are shown. For this purpose, an integrated ecosystem model is developed which includes a physical submodel and an ecological model. The model is calibrated with data from a mooring project, during which a large number of physical and biological parameters have been measured at a site on the Oyster Grounds in the North Sea over a 15-month period. The physical model is a one-dimensional entrainment/detrainment model. The ecological model consists of submodels which are part of the ERSEM ecological model and which describe the biological and chemical processes in the water column and in the benthos. Results show that stratification has a major impact on the biota. Especially the timing of the onset of the stratification has major consequences for the production and succession of phytoplankton and the structure of the food web during the entire growing season.