REGIONAL DIFFERENCES IN STRATIFICATION AND ITS EFFECT ON PHYTOPLANKTON PRODUCTION AND BIOMASS IN THE NORTHWESTERN IRISH SEA

Differences in tidal mixing result in the formation of offshore mixed and stratified regions in the NW Irish Sea during spring and summer. Stratification resulted from vertical gradients in temperature, although vertical gradients in salinity were important during the early stages of stratification. The northern coastal and offshore mixed regions were characterized and distinguished from the southern coastal and summer stratified regions by the presence of more saline, cool near-surface water and incomplete depletion of dissolved inorganic nutrients. Distinct regional differences in the production season of phytoplankton were observed. This lasted 6 months with a seasonal production of 155 g C m(-2) in the southern coastal region. A shorter season, 4 and 2 months, and lower production of 101 and 96 g C m(-2) occurred in the summer stratified and northern mixed regions, respectively. The southern mixed region supported the shortest season (<2 months) and lowest production (66 g C m(-2)). It is estimated that a daily light exposure of similar to 200 Wh m(-2) is required for the onset of the production season and it is concluded that the subsurface light climate as a function of solar radiation and surface mixed layer depth, rather than nutrient availability, controls its duration. The existence of a short)ate production season in the offshore regions is confirmed and this contrast with the season in the North Sea is attributed to differences in the subsurface light climate. Sustained production in the southern coastal region may play an important role in fish recruitment and offset any effect of the short late offshore production season.