Nutritional regulation of egg production of Calanus finmarchicus in the North Atlantic
Ship-board experiments in the North Atlantic were used to study how food quality influences the egg production of Calanus finmarchicus feeding on natural planktonic diets. Food quality was expressed in terms of carbon (C), nitrogen (N), and the essential fatty acids eicosapentaenoic acid (EPA; 20:5(n-3)) and docosahexaenoic acid (DHA; 22:6(n-3)). Five consecutive 24 hr bottle incubations were conducted in April and July/August 2002 under in situ conditions to determine egg production rates and the ingested quantities of C, N, EPA and DHA. Biomass contributions towards growth were determined and the biochemical composition of the eggs was examined. In order to accurately determine ingestion rates, a method to account for microzooplankton grazing in particle removal experiments was developed. Balanced physiological budgets were compiled for C. finmarchicus in both seasons. The input terms of these budgets consisted of ingestion and the use of biomass, and the outputs were growth, respiration, excretion and egestion. Respiration and excretion were not determined experimentally, and were therefore determined by mass balance and compared to literature-derived values. In April, close agreement between literature- and mass balance-derived rates of respiration and excretion demonstrated that the experimentally determined components of the budget were accurate. Ingestion rates were low, and > 80 % of the C utilised was derived internally from somatic biomass. The absence of storage fatty acids and the low C:N ratio (~ 4 µg µg-1) of the biomass lost from the females indicated that these animals had been catabolising structural protein and were close to exhaustion. This suggests that when food is scarce, C. finmarchicus adopts a semelparous reproductive strategy. In July/August, the observed growth exceeded the estimated ingestion rates. This shortfall was possibly provided by cannibalising eggs. Assuming that EPA and DHA were used with high efficiency (0.9), the stoichiometric analysis predicted that these compounds were non-limiting in April. Using typical maximum growth efficiencies for C (< 0.6) and N (0.4), the former was predicted to be limiting because the biomass utilised was rich in N, EPA and DHA relative to the demand for C.