The role of the copepod Oithona spp. in North Sea plankton communities

Copepod production was investigated in the North Sea, with special emphasis on the trophodynamic role of the cyclopoid copepod Oithona similis. During a cruise in May-June 1992, no significant correlation was found between the specific egg production rate (SEP) of Oithona spp. and available food. However, O. similis SEP was positively correlated to the protozooplankton, in contrast to the co-occurring calanoid Paracalanus parvus SEP which was significantly correlated to the chlorophyll a concentration. In the shallow Dogger Bank region Oithona spp. contributed 50 and 70% of the total copepod biomass and production, respectively. In the northern regions dominated by Calanus spp. Oithona's contribution to biomass and production decreased to about 10 and 20%, respectively. These regional differences were confirmed by analysis of seasonal data, in which Oithona spp. contributed 40 and 13% of the annual copepod production in Dogger Bank and the northern regions, respectively. Biomass and production of Oithona spp. show little temporal and spatial variation as compared to calanoids. When calanoid populations are low or in shallow environments where eggs of the free- spawning calanoid are lost to the benthos, Oithona is an important component of the food web.