A MODELING INVESTIGATION OF THE DISTRIBUTION OF STRATIFICATION AND PHYTOPLANKTON ABUNDANCE IN THE IRISH SEA

Thermal stratification and phytoplankton abundance are modelled on a 5 km grid covering the Irish Sea. The water column is approximated by three layers. The top layer is uniformly mixed by wind stirring and the bottom by tidal energy, while linear gradients can occur in the middle layer. The model is forced with hourly meteorological data and mean tidal energies. Primary production is represented by a model with a single nutrient and a single phytoplankton population. The results from the model show good agreement with data collected on a Ministry of Agriculture, Fisheries and Food (MAFF) cruise in May 1992 and with historical data. When advection is included, driven by depth-averaged currents, the surface temperature patterns are improved but bottom temperatures in deep water are raised and high concentrations of chlorophyll are carried offshore from coastal regions. This indicates a limitation of using depth-averaged currents and a need to account for differences in phytoplankton species composition in coastal and offshore waters. Calculations demonstrate the importance of salinity variations to stratification and phytoplankton growth. Smoothing the wind mixing energy has the effect of delaying the onset of the spring bloom in areas where wind mixing is significant. Removing the diurnal cycle of solar heating also delays the spring bloom. The chlorophyll gradient in the middle layer has a large impact on the response of the model to short-term variability in the meteorological forcings.