Scale-dependent effects of climate on two copepod species, Calanus glacialis and Pseudocalanus minutus, in an Arctic-boreal sea

Abstract

Climate-driven changes in the phenology and composition of plankton affect ecosystem structure and function, but knowledge about such changes is limited by the scarcity of highquality, high-resolution, long-term monitoring data. Using a high-resolution observation series from the White Sea, spanning <50 yr, we explored how water temperature and salinity influenced 2 key copepod species, Calanus glacialis and Pseudocalanus minutus. The results of the analysis depended critically on the temporal and life-stage resolution of the analysis. Copepod biomass was negatively correlated with salinity, but not correlated with temperature, when using annually aggregated data. However, salinity showed very small effects at a monthly resolution, failing to support a causal effect of salinity. On the other hand, temperature did show effects: in warm years, the biomass of C. glacialis increased earlier in spring and declined earlier in autumn. Analysis of stage-resolved data revealed a new level of complexity. The increase of biomass in spring at warmer temperatures mainly consisted of young life stages, whereas the decrease in autumn was mainly caused by reductions in older life stages. Temperature affected the phenology of several life stages of P. minutus, but not its total biomass, implying that climate effects on different life stages cancelled each other out. We argue that such climate-driven fluctuations in zooplankton phenology and age structure are likely to influence the role of the zooplankton as predators, competitors and prey, but that these effects of climate could remain unnoticed when using the coarser resolution of many sampling programs. © Inter-Research 2012.