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 per iod of 10 days at the end of June 1996 using a tracer release method. Gut f luorescence, egg production, carbon content and stage specific abundance, t ogether with phytoplankton concentration and composition, were measured dur ing this period. Chlorophyll levels were less than 1 mu g l(-1) and the phy toplankton population was dominated by coccolithophorids. Gut fluorescence decreased during a period with strong winds, whereas egg production remaine d constant. Estimated phytoplankton ingestion was too low to cover egg prod uction requirements. However, the microzooplankton concentration seemed to be high enough to complement phytoplankton ingestion, enabling egg producti on carbon requirements to be covered. CM ratios of adult females and stage CV were low, indicating low levels of lipid storage. The population was dom inated by stage CIV, but shifted to a dominance of younger stages during th e study. Mortality calculated from population structure indicates that high er death rates occurred at stage CV.