Are copepods important grazers of the spring phytoplankton bloom in the western Irish Sea?

The spring phytoplankton bloom and copepod grazing were studied at a coastal and offshore station in the western Irish Sea during 1997. Maximum chlorophyll standing stocks of 132.8 mg m -2 inshore and 199.4 mg m -2 offshore were measured in late April. At that time, mean water column temperatures were 10 and 8°C at the coastal and offshore station, respectively. Spring bloom production at the coastal station was estimated as 31.2 g C m -2 and was dominated by the diatom Guinardia delicatula. Offshore, production was 28.2 g C m -2 and the bloom was composed of small ( 10 µm) phytoflagellates and the silicoflagellate Dictyocha speculum. Maximum copepod abundance (189 and 544 3 103 individuals m-2, inshore and offshore, respectively) coincided with the spring bloom. Pseudocalanus and Temora ingestion rates were derived from measurements of gut pigment fluorescence, and were found to vary during the course of the spring bloom as a result of changes in gut content. Grazing by late copepodite and adult Pseudocalanus and Temora was variable inshore, but overall accounted for 17% of bloom production. Offshore, 22% of bloom production was grazed with maximum grazing (76% of daily production) occurring at the end of the bloom. Large copepod species were not major grazers of the spring bloom. Greater utilization of spring bloom production by copepods in the western Irish Sea compared to regions of the North Sea is attributed to differences in population size at the time of the bloom.