MICROZOOPLANKTON COMMUNITY STRUCTURE IN THE NORTH-EASTERN ATLANTIC - TRENDS WITH LATITUDE, DEPTH AND DATE, BETWEEN MAY AND EARLY AUGUST

Microzooplankton from water samples collected from depths of between 1 0 and 50 m at four stations between 47 degrees and 60 degrees N along the 20 degrees W line of longitude during each of three cruises were c lassified into taxonomic groups and measured to estimate their abundan ce and biomass. Metazoan members of the microzooplankton were rarely f ound and their overall biomass was negligible in comparison with proti sts. The mean numbers of protistan microzooplankton cells >15 run rang ed between 3 and 11 cells ml(-1), with mean biomass values between 2 a nd 14 mu gC l(-1). Numbers and biomass increased towards midsummer and then decreased again, except at the most northerly station where the values were highest in the early August sample. The ratio of dinoflage llate to ciliate biomass tended to increase through the summer to leve ls of three or four and then decrease again, with later changes furthe r north; this ratio also tended to be higher at deeper levels than in the mixed layer. Gymnodinioid forms usually dominated dinoflagellate n umbers and biomass, but they were overshadowed by Protoperidinium and others at the two southern stations in June and July samples. Strombid ium species were the dominant ciliates, commonly providing 25-50% of t otal microzooplankton biomass, while predatory haptorid ciliates were occasionally numerous, but tintinnids never formed more than 10% of th e biomass.