Copepod egg production during highly productive late spring conditions: importance of freshly ingested food and lipid storage

Abstract

The reproductive activity of 2 calanoid copepods, Calanus helgolandicus and Calanoides carinatus, under post-bloom conditions was investigated during a cruise in the central Cantabrian Sea (SW Bay of Biscay) in May 2000. Several mesoscale structures, such as an upwelling of rich waters associated with the topography of one submarine canyon, were observed on the continental shelf. Consequently, a high biomass of large phytoplankton was found in coastal and mid-shelf areas. These unexpected productive conditions after the spring bloom supplied copepods with favourable food resources. Accordingly, females showed mature gonads, high percentages of spawning and high egg production rates (EPRs). A surface saline current was detected flowing parallel to the coast along the slope. A microbial food web prevailed there and, unlike the coast and mid-shelf areas, low chlorophyll concentrations and a high proportion of small phytoplankton were found. The microbial food web did not seem to be suitable to support copepod production; thus, only a few females had mature gonads and fecundity was low. The number of eggs produced increased as herbivorous feed- ing increased, but feeding on phytoplankton did not cover the carbon requirements at all stations, and a mixed diet was suggested. The relationship between EPR and diatom concentration showed a saturation response for both species, but interestingly the saturation concentration was low and fecundity values were sub-maximal. Females had low lipid levels and low C:N ratios, indicating that egg production could be entirely fuelled by freshly ingested food. The low lipid reserves of females could also help to explain the sub-maximal EPR of C. helgolandicus. Nevertheless, low lipid storage was not a constraint for C. carinatus because it is well known that this species uses only freshly ingested food to support egg production. The implications of this extra productive time at the end of spring on the annual recruitment of C. helgolandicus and C. carinatus are discussed.