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Calanoid copepods dominate mesozooplankton biomass in the Arctic Ocean. Variations in C content, C:N ratio and stable isotope composition (δ13C, δ15N) of Calanus hyperboreus, Calanus glacialis and Metridia longa collected from January to August 2008 in the southeast Beaufort Sea provided insights into their metabolism, feeding and reproduction. Seasonal differences in the C–prosome length relationships and C:N ratios were driven by distinct spawning strategies and changes in lipid content. Relatively high copepod biomass over the study period (2.6–9.7 g C m−2) corresponded to favorable growth conditions in 2007–2008. The mean 15N enrichment of copepods (+2.8–4.7‰) relative to particulate organic nitrogen values recorded at the ice bottom and at the chlorophyll maximum indicated a primarily herbivorous diet. In all species, δ13C and δ15N decreased markedly in April, reflecting the feeding onset on ice algae, but a rapid transition to feeding on phytoplankton occurred as a pelagic bloom was triggered by the early ice melt in May. A second decline in the δ13C and δ15N of copepods was recorded in June–July, coincident with a second increase in phytoplankton production. The two isotope depletion events in copepods were both followed by a return to high values and an increase in their C:N as a consequence of previous C fixation and nitrate limitation in phytoplankton and the likely formation of body reserves/tissue. Our results illustrate that Arctic calanoids respond quickly to any increase in primary production and can cope with changes in its nature and timing. [ABSTRACT FROM AUTHOR]