Physiology and population structure of Calanus finmarchicus (Copepoda: Calanoida) during a Lagrangian tracer release experiment in the North Atlantic

A population of Calanus finmarchicus was followed in the open sea for a period of 10 days at the end of June 1996 using a tracer release method. Gut fluorescence, egg production, carbon content and stage specific abundance, together with phytoplankton concentration and composition, were measured during this period. Chlorophyll levels were less than 1 μg l^-1 and the phytoplankton population was dominated by coccolithophorids. Gut fluorescence decreased during a period with strong winds, whereas egg production remained constant. Estimated phytoplankton ingestion was too low to cover egg production requirements. However, the microzooplankton concentration seemed to be high enough to complement phytoplankton ingestion, enabling egg production carbon requirements to be covered. C/N ratios of adult females and stage CV were low, indicating low levels of lipid storage. The population was dominated by stage CIV, but shifted to a dominance of younger stages during the study. Mortality calculated from population structure indicates that higher death rates occurred at stage CV.