Persistence of Calanus finmarchicus in the western Gulf of Maine during recent extreme warming

The planktonic copepod, Calanus finmarchicus, resides at the southern edge of its subarctic range in the Gulf of Maine (GoM). Here we investigate the population response of C. finmarchicus to record warming in the GoM in 2012. Demographic data from two time series stations and a plankton survey conducted in early autumn 2012 show that C. finmarchicus did not produce an autumn generation as predicted, despite warm summer and overwintering temperatures. On the contrary, we observed high abundances of the overwintering stage CV in the western GoM and a new cohort in early spring 2013 likely originating from egg production during a winter phytoplankton bloom. This spring cohort was the most abundant ever recorded in the 8-year time series. To account for these observations, we hypothesize that production of females originating from the eastern GoM and Scotian Shelf, combined with growth of copepodid stages in the Maine Coastal Current and local egg production, are the primary sources of supply maintaining high abundances in Wilkinson Basin, the primary repository of C. finmarchicus in the western GoM. Predicting fluctuations in abundance or circumstances for disappearance of C. finmarchicus in the northwest Atlantic will need models that address the roles of local production and advection.