Ecological investigations on the zooplankton community of Balsfjorden,northern Norway: Generation cycles,and variations in body weight and body content of carbon and nitrogen related to overwintering and reproduction in the copepod Calanus finmarchicus (Gunnerus)

The generation cycles of Calanus finmarchicus (Gunnerus) are described together with the seasonal variations in length, wet wt, dry wt, carbon content, nitrogen content and $\text{C}\text{N}$ ratio in copepodite stage IV, V and stage VI males and females from Balsfjorden (69°21′N: 19°06′E), a subarctic fjord in northern Norway. C. finmarchicus overwinters in copepodite stage IV (≈ 20%) or V (≈80%) and produces one generation a year. Variations in body weight and body content of carbon and nitrogen in the different copepodite stages showed a pronounced seasonal pattern. For instance, the $\text{C}\text{N}$ ratio was lowest (4.9) in adult females during the spawning period. Copepodite stage IV and V had higher $\text{C}\text{N}$ values in summer and autumn (12 to 14) than in spring (8 to 10). Variations in length, weight and chemical composition revealed that the overwintering stock of C. finmarchicus went through two growth phases during this period. From September to January no significant changes in the measured variables were detected. During the second phase of the overwintering period, January to April, the different stages showed a profound decrease in weight and change in chemical composition. This seemed to be connected with the onset of sexual differentiation in stage V starting in January, subsequent moulting into adults and gonad maturation in these adults. These results are further discussed in relation to the different prevailing hypotheses concerning overwintering strategy in Calanus species.