There is perceived to be a problem of eutrophication in European marine coastal waters and hence a need to predict the response in terms of enhanced biomass of phytoplankton resulting from the input of anthropogenic nitrogen. This response was investigated indirectly by studying the relationship between nitrate and phytoplankton chlorophyll concentrations in waters of the Scottish west coast. Two-thirds of the 60 data sets analysed gave significant inverse regressions of chlorophyll on nitrate concentration. This result is explained by interpreting synoptic data as representing variation in time, with nitrate decreasing as a result of its assimilation by phytoplankton and conversion into chlorophyll-containing biomass Thus the absolute value of the slope of each significant regression estimates the yield (g) of chlorophyll from nitrate and, indeed, from any form of nitrogen assimilable by microalgae. The median value for q was 1.05 mg chl (mm01 N ) -' ; the range from 0.25 to 4.4 encompassed 95 % of values. Some, but not all, of the variation in q could be explained by error in individual estimates due to chemical-analytical and sampling errors or to inhomogeneities in each sampled phytoplankton population. The remaining variation in q included a seasonal trend, which might have resulted from changes in phytoplankton species composition, nutrient limitation status, or the balance between autotrophic and heterotrophic partitioning of nitrogen. It is suggested that an appropriate value of q can be used to predict the potential maximum increase in phytoplankton which would result from a given anthropogenic nitrogen discharge. The sensitivity of such predictions to error in determination of q is discussed, and the values obtained for q compared with observations in algal culture and mesocosms.

In the western Irish Sea, differences in tidal currents and water depth create seasonal hydrographic regions. The summer stratified region has an associated cyclonic gyre of near-surface water. Each region has a distinct seasonal cycle of phytoplankton production, and the coastal region is one of the most important fish spawning areas in the Irish Sea. The abundances of larval fish and newly metamorphosed pelagic juveniles were negatively and positively correlated with depth, respectively, suggesting that fish that spawn in the coastal region do not spend all of their early life stages inshore. The distribution of larvae and 0-group pelagic fish reflects the close coupling between recurrent hydrographic features and biological production. The coastal region supports early and sustained primary production and generally a higher biomass of zooplankton in early spring than the offshore mixed and stratified regions. Later in the year, pelagic 0-group fish are associated with a greater biomass of zooplankton in the summer stratified region. The movement of pelagic 0-group fish into the summer stratified region may result from the entrainment of a southerly flow of coastal water into the western Irish Sea gyre. The seasonal gyre may physically retain these pelagic fish within the western Irish Sea during the summer.