Seasonal dynamics of reproductive parameters of the calanoid copepods Calanus helgolandicus and Calanoides carinatus in the Cantabrian Sea (SW Bay of Biscay)

Abstract

The seasonal reproductive dynamics of the calanoid copepods Calanus helgolandicus and Calanoides carinatus and the main driving factors that control their reproductive activity were evaluated through a three-year study performed at two shelf stations in the Cantabrian Sea (SW Bay of Biscay). As a general pattern, the seasonal dynamics showed two peaks of reproduction for both species: the highest peak was associated with the spring phytoplankton bloom, whereas a second and less intense peak coincided with the autumn bloom. Between these two blooms, copepod recruitment showed low rates, and the lowest reproductive activity was recorded in winter. Gonads started to mature in pre-bloom conditions, probably fuelled by stored lipids and concurrent feeding, but the highest RIs (Reproductive Index) and EPRs (egg production rates) were found during the spring blooms. Accordingly feeding seems to be crucial for the main recruitment event for both species. Temperature, Chlorophyll-a (Chl-a) concentration and diatom abundance explained part of the variability found in reproductive activity, with the phytoplankton biomass as the most relevant environmental factor. Temperature probably had an indirect effect on fecundity; temperature and female size were inversely correlated, and large females produced larger clutch sizes. The food assemblage seemed to be more adequate for egg hatching success (HS) than for EPR, because HS was high most of the time, but EPRs were often lower than maximum production rates, pointing to a food limitation. However, the nauplius production rates compared well with those in other areas with higher phytoplankton productivity. Interestingly, a trade-off between number of eggs produced and their viability was found; small clutch sizes with high viability were produced during inadequate food conditions. This strategy maximizes naupliar survival under short food resources by investing in egg quality rather than in quantity. The annual cycle and reproductive dynamics of C. carinatus, traditionally described for African upwelling systems, were compared with that of C. helgolandicus, a resident species in the Bay of Biscay. The high similarity between the species suggests that C. carinatus has developed a stable population in the Cantabrian Sea, which denotes a high plasticity of this species.