This report is a contribution to the Strategic Environmental Assessment (SEA6) conducted by the Department of Trade and Industry (now Department of Energy and Climate Change). This report summarises information on the ecology of planktonic species found in the SEA6 area. The Irish Sea is very diverse not only in the physical-chemical regimes operating upon it, but in the ecology of planktonic organisms found there. Data on the nutrient chemistry of the Irish Sea shows that the eastern Irish Sea is more heavily impacted by nutrients owing to freshwater run-off from land, which is far greater than in the western Irish Sea. Nutrients increased from the 1950's to the 1980's after which time the concentrations have levelled off and in some case declined. The phytoplankton biomass appears to have mirrored the influence of the nutrients both in time and space. Highest biomass (inferred from chlorophyll analysis) is generally found in regions of low salinity and tends to be greatest in the eastern Irish Sea. The phytoplankton community has also been shown to vary throughout the seasons and also within different regions or 'ecohydrodynamic' domains of the Irish Sea.

The zooplankton community of the Irish Sea has also undergone significant change over the last thirty or so years. The most noticeable of these changes being a significant decrease in abundance of most of the species recorded. Some species distributions and abundances have been shown to be influenced by climate and it is highly likely that other species of plankton in the Irish Sea are also affected in this way. Climate, or more specifically the North Atlantic Oscillation (NAO), certainly has a major impact upon the physical-chemical environment of the region and this has a direct influence upon the ecology of planktonic organisms found in the Irish Sea.