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. S tratification resulted from vertical gradients in temperature, althoug h vertical gradients in salinity were important during the early stage s of stratification. The northern coastal and offshore mixed regions w ere characterized and distinguished from the southern coastal and summ er stratified regions by the presence of more saline, cool near-surfac e water and incomplete depletion of dissolved inorganic nutrients. Dis tinct regional differences in the production season of phytoplankton w ere 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 mon ths, and lower production of 101 and 96 g C m(-2) occurred in the summ er stratified and northern mixed regions, respectively. The southern m ixed region supported the shortest season (<2 months) and lowest produ ction (66 g C m(-2)). It is estimated that a daily light exposure of s imilar to 200 Wh m(-2) is required for the onset of the production sea son and it is concluded that the subsurface light climate as a functio n of solar radiation and surface mixed layer depth, rather than nutrie nt availability, controls its duration. The existence of a short)ate p roduction season in the offshore regions is confirmed and this contras t with the season in the North Sea is attributed to differences in the subsurface light climate. Sustained production in the southern coasta l region may play an important role in fish recruitment and offset any effect of the short late offshore production season.