Copepod abundance in the western Irish Sea: relationship to physical regime, phytoplankton production and standing stock

Abstract

The distinct patterns of stratification in the North Channel and stratified region of the western Irish Sea influence the seasonal abundance of phytoplankton. The 3-4 month production season in the stratified region was characterized by production and biomass peaks in the spring (up to 2378 mg C nr2 day-' and 178.4 mg chlorophyll nr2) and autumn (up to 1280 mg C nr2 day-' and 101.9 mg chlorophyll nr2). Phytoplankton in the North Channel exhibited a short, late production season with a single summer (June/July) peak in production (4483 mg C nr2 day1) and biomass (-160.6 mg chlorophyll m~2). These differences have little influence on copepod dynamics. Both regions supported recurrent annual cycles of copepod abundance with similar seasonal maxima (182.8-241.8 x 10 3 ind. m~2) and dominant species (Pseudocalanus elongatus and Acartia clausi). Specific rates of population increase in the spring were 0.071 and 0.048 day1 for the North Channel and stratified region, respectively. Increased copepod abundance in the stratified region coincided with the spring bloom, and was significantly correlated with chlorophyll standing stock. Increased copepod abundance preceded the summer production peak in the North Channel. This increase was not correlated with chlorophyll standing crop, suggesting that a food resource other than phyto- plankton may be responsible for the onset of copepod production prior to the spring bloom. Hetero- trophic microplankton as an alternative food source, and advection of copepods from the stratified region, are proposed as possible explanations for copepod abundance increasing in advance of the summer peak in primary production.