COPEPOD ABUNDANCE IN THE WESTERN IRISH SEA - RELATIONSHIP TO PHYSICALREGIME, PHYTOPLANKTON PRODUCTION AND STANDING STOCK

The distinct patterns of stratification in the North Channel and strat ified region of the western Irish Sea influence the seasonal abundance of phytoplankton. The 3-4 month production season in the stratified r egion was characterized by production and biomass peaks in the spring (up to 2378 mg C m(-2) day(-1) and 178.4 mg chlorophyll m(-2)) and aut umn (up to 1280 mg C m(-2) day(-1) and 101.9 mg chlorophyll m(-2)). Ph ytoplankton in the North Channel exhibited a short, late production se ason with a single summer (June/July) peak in production (4483 mg C m( -2) day(-1)) and biomass (similar to 160.6 mg chlorophyll m(-2)). Thes e differences have little influence on copepod dynamics. Both regions supported recurrent annual cycles of copepod abundance with similar se asonal maxima (182.8-241.8 x 10(3) ind. m(-2)) and dominant species (P seudocalanus elongatus and Acartia clausi). Specific rates of populati on increase in the spring were 0.071 and 0.048 day(-1) for the North C hannel and stratified region, respectively. Increased copepod abundanc e in the stratified region coincided with the spring bloom, and was si gnificantly correlated with chlorophyll standing stock. Increased cope pod abundance preceded the summer production peak in the North Channel . This increase was not correlated with chlorophyll standing crop, sug gesting that a food resource other than phytoplankton may be responsib le for the onset of copepod production prior to the spring bloom. Hete rotrophic microplankton as an alternative food source, and advection o f copepods from the stratified region, are proposed as possible explan ations for copepod abundance increasing in advance of the summer peak in primary production.