Regional and temporal variation of Oithona spp. biomass, stage structure and productivity in the Irminger Sea, North Atlantic

Oithona spp. standing stock and production is considered relatively stable in space and time as a result of continuous breeding, low metabolism, reduced predation mortality and the ability of these small cyclopoids to exploit microbial food webs more efficiently than larger copepods. However, through a review of the published literature, we show that Oithona spp. biomass can vary widely both over the year and with latitude. Thus, the present study set out to investigate the basin scale variability in biomass, stage structure and reproduction of Oithona spp. in relation to changes in hydrographic, physico-chemical and biological parameters encountered during three cruises conducted between April and November 2002 in the Irminger Sea, North Atlantic. Here we found that Oithona spp. biomass varied significantly with temperature and with dinoflagellates biomass concentration. On the other hand, Oithona similis egg production rates increased with both ciliates and dinoflagellates concentrations, rather than with temperature. The inverse relationship we found between Oithona spp. naupliar recruitment with Calanus spp. and fish larvae abundance suggests that predation pressure may contribute to control the spatial variation in the stage structure and biomass of Oithona spp. and that the nauplii of this genus may serve as a food source for other planktonic organisms prior to the spring phytoplankton bloom.