Environmental niche overlap in sibling planktonic species Calanus finmarchicus and C. glacialis in Arctic fjords.

Knowledge of environmental preferences of the key planktonic species, such as copepods in the Arctic, is crucial to understand ecosystem function and its future under climate change. Here, we assessed the environmental conditions influencing the development stages of Atlantic and Arctic , and we quantified the extent to which their niches overlap by incorporating multiple environmental data. We based our analysis on a 3-year seasonal collection of zooplankton by sediment traps, located on moorings in two contrasting Svalbard fjords: the Arctic Rijpfjorden and the Atlantic-influenced Kongsfjorden. Despite large differences in water temperature between the fjords, local realized ecological niches of the sibling species overlapped almost perfectly. The exception was the earliest copepodites of in Rijpfjorden, which probably utilized the local ice algal bloom in spring. However, during periods with no sea ice, like in Kongsfjorden, the siblings of both species showed high synchronization in the population structure. Interestingly, differences in temperature preferences of and were much higher between the studied fjords than between the species. Our analysis confirmed the high plasticity of copepods and their abilities to adapt to highly variable environmental settings, not only on an interannual basis but also in a climate warming context, indicating some resilience in the community.