Surface aggregations of Calanus finmarchicus during the polar night

While marine ecosystems in polar areas were thought to be mostly inactive during the polar night, recent observations in the Arctic highlight that dynamic biological interactions occur across all trophic levels. One particularly interesting observation made repeatedly is the occurrence of Calanus finmarchicus, a key species at the base of the food web, in the upper part of the water column in early January. This contrasts with the confirmed life cycle of this copepod, which predicts descent to overwintering depths in autumn and ascent at the end of the winter. Here, we compiled 6 years of data from the polar night to explore this unusual behaviour and to investigate the underlying mechanisms that may cause these surface aggregations. Highest abundance of C. finmarchicus in surface waters was closely related to Polar Surface Water in the vicinity of the sea ice edge. A total of six hypotheses are assessed to explain this distribution pattern, which includes passive transport and active behaviour as a way to improve survival chances or food access. In the context of climate change and sea ice decrease, the presence of lipid-rich copepods in cold surface waters could be a key parameter in driving the spatial distributions of top predators in the Arctic.

Automated summary

Recent observations in the Arctic have challenged the previous belief that marine ecosystems in polar areas are mostly inactive during the polar night. The presence of Calanus finmarchicus in the upper part of the water column in early January, contrary to its predicted behavior, has prompted further investigation into the underlying mechanisms and potential impacts. This study found a close relationship between the abundance of C. finmarchicus in surface waters and Polar Surface Water near the sea ice edge, suggesting the importance of lipid-rich copepods in driving the spatial distributions of top predators in the Arctic.