Chlorophyll standing crop and phytoplankton production in the western Irish Sea during 1992 and 1993

Abstract

Chlorophyll standing crop and phytoplankton production were studied in the western Irish Sea over a 21 month period during 1992 and 1993. For both years, the start of the production season was fint observed in Dundalk Bay and occurred progressively later in more northerly coastal and off- shore waters. Standing crop and production exhibited marked spatial heterogeneity with 12-5- to 19- fold differences in crop observed over distances of 20-30 km. Distinct regional differences in the length of the production season were apparent.The longest season, 6-7 months with a production of 194 g C nr2, occurred in Dundalk Bay. The season lasted 3-4 months in the summer stratified region with a production of 140 g C nr1. Northerly, offshore mixed waters and coastal waters of Northern Ireland supported a short (2-3 months) season and production of 194 and 140 g C nr2, respectively. The simi- larity in seasonal production between Dundalk Bay and coastal waters of Northern Ireland, and between the summer stratified and northern mixed regions, is attributed to the intensity of production during the summer. Between 59 and 79% of seasonal production in the northern mixed region and coastal waters of Northern Ireland took place during June and July, compared to 29-40% in Dundalk Bay and the summer stratified region. Lower summer production in the latter two may be due to nutri- ent limitation and this has implications for the sensitivity of these two regions to anthropogenic nutri- ent enrichment.