Biological studies in the vicinity of a shallow-sea tidal mixing front V. Composition, abundance and distribution of zooplankton in the Western Irish sea, April 1980 to November 1981

Abstract

There has been no change in the overall species composition of the western Irish Sea zooplankton during the last 80 years. Although Pseudocalanus elongatus and Acartia clausi were always the most abundant species encountered in the survey area, Calanus finmarchicus has been the main contributor to the total zooplankton biomass. The marked decline in numbers of this copepod in 1981 was almost totally responsible for the large decrease in zooplankton biomass observed during that year. The highest zooplankton standing stocks were found to occur above the thermocline in the stratified region between April and June. In the mixed isothermal region zooplankton standing stocks were much lower with a maximum at least one month later than on the stratified side of the front. The frontal region was found to have little influence on the distribution of the total zooplankton. On the isolated occasions when higher zooplankton concentrations were observed within the frontal region they were always found to be associated with localized high densities of surface chlorophyll and due to increased copepod production rather than mechanical aggregation.