Phenological responses of the Arctic, ubiquitous, and boreal copepod species to long-term changes in the annual seasonality of the water temperature in the White Sea

The process of climate change by which global temperatures increase and seasonal shifts occur is more pronounced at higher latitudes. These changes induce shifts in the phenology of biota, including zooplankton. Regression analysis revealed significant advance of the spring-summer water warming in the sub-Arctic White Sea (Chupa Inlet, Kandalaksha Bay), which occurs 3 weeks earlier in the last decade compared with 1961. The shift of timing of phenological events of Calanus glacialis, Pseudocalanus spp., Microsetella norvegica, Oithona similis, Acartia spp., Centropages hamatus, and Temora longicornis is particularly pronounced. These shifts were accompanied by a significant increase in Calanus and Microsetella abundance with an observed decrease in Acartia abundance. No correlation between population abundance and year-to-year changes in the species phenology was determined. We propose hypotheses to explain species adaptation to these phenological shifts in the ecosystem: (1) flexibility of life cycle of planktonic copepods, which ensures synchronization of life cycle phases with key events in the environment; and (2) copepod ability to switch to carnivory when algal sources are scarce. Interspecific interactions between ubiquitous species influenced their phenology and abundance, while early development of Microsetella caused later development of Oithona and higher abundance of the latter. We speculate that competition for food resources is a driving factor in the interactions, since the trophic niches of these species overlap.