Award: OCE-1459096

This project addresses a fundamental question: how are coastal ocean ecosystems changing due to a warming world? We focus on the northeast U.S. coastal waters in the Gulf of Maine, located at the southern margin of the subarctic North Atlantic Ocean. This cold-water ecosystem is characterized by herring, cod, and seasonally migrating fish, mammals, and seabirds such as bluefin tuna, large whales, and Atlantic puffins. Over the past decade, the Gulf of Maine has experienced extreme warming at all depths, and the effects on the cold-water ecosystem is of widespread concern. We have identified a planktonic copepod, Calanus finmarchicus, as a key foundation species. C. finmarchicus is remarkably abundant throughout its subarctic North Atlantic range and especially in the Gulf of Maine. The life history of C. finmarchicus is adapted to the subarctic seasonal cycle of primary production. These copepods start reproduction in late winter and early spring as phytoplankton production is increasing. Developing C. finmarchicus accumulate lipids produced by phytoplankton. By its last pre-adult life stage, it has stored a large quantity of high energy fats that enable it to overwinter at depth starting in late summer. Herring, right whales, and other planktivores are tuned to the C. finmarchicus life cycle and eat large quantities of the lipid-rich stages to fuel their energy and reproductive needs. In a sense, C. finmarchicus serves as a battery, storing for many months the light energy captured by phytoplankton in the spring. Recent warming has placed the habitat for C. finmarchicus in the Gulf of Maine at risk, including its ability to overwinter. Previous modeling of C. finmarchicus habitat suggested that the species will disappear from the Gulf of Maine over the next several decades. However, following recent extreme warming, C. finmarchicus surprisingly has persisted in high numbers in the Gulf of Maine. Our research in this project explores the mechanisms by which this copepod species persists and how warming and associated changes in currents may be influencing abundance. First, we continued a program of sampling of C. finmarchicus at two fixed stations in order to document changes in abundance and timing of life cycle events. Second, we coupled sophisticated circulation models with the C. finmarchicus life cycle to test a mechanism that includes transport by currents in addition to local production as key factors in maintaining abundance. Finally, we used these results and complementary datasets to put forward a conceptual model of oceanographic pathways sustaining C. finmarchicus abundance, with implications for the foraging patterns of endangered North Atlantic right whales. Our results indicate that timing of C. finmarchicus production in the western Gulf of Maine has shifted since about 2008 in response to earlier winter phytoplankton production. Earlier phytoplankton production has yielded higher abundances of juvenile C. finmarchicus and smaller copepod species in spring in the western Gulf of Maine. Later in the year, however, late-stage C. finmarchicus abundance has declined by approximately 30% in the western Gulf of Maine and even more dramatically in the eastern Gulf of Maine. We have identified two largely distinct oceanographic pathways that together explain the recent observations. Older C. finmarchicus stages originating from outside the Gulf are entrained in the food-rich Maine Coastal Current, which transports this growing and reproducing population to the western Gulf of Maine, where it is able to overwinter in Wilkinson Basin. Overwintering individuals emerge in winter and exploit the early phytoplankton production to produce a large spring cohort. We have termed this pathway the Coastal Amplification and Transport (CAST) hypothesis, and it explains how high abundances persisted despite warming. The second pathway affects the eastern Gulf of Maine. Since about 2010, the transport pathway in through the Northeast Channel has shifted from cold Nova Scotia Current supply to warmer North Atlantic Temperate Water, which is associated with poor C. finmarchicus habitat. This shift in external supply aligns with observed changes in the Atlantic Meridional Overturning Circulation resulting from warming in the Arctic. Our research highlights that coastal ecosystem change involves currents and planktonic production in addition to warming. Effects in the Gulf of Maine depend on different oceanographic pathways that may support either persistence or decline. These effects have important implications. Where declines are highest, in the eastern Gulf of Maine since 2010, North Atlantic right whales have largely stopped feeding, affecting their reproduction and mortality rates. Our project has contributed to the development of physical-biological modeling as a tool that can help track oceanographic pathways of change. These models have practical applications, for example in the prediction of foraging distribution of the North Atlantic right whale to inform management decisions. Effective predictions based such oceanographic mechanisms can help mitigate the social and economic impacts of management decisions on the fishing industry and other services provided by coastal ecosystems. Last Modified: 06/23/2019 Submitted by: Chang-Sheng Chen