Strange Stirrings in the Arctic Drive Ecosystem Changes Far Downstream
Changes in Arctic climate during the late 1980s pumped large amounts of low-salinity water into the shelf ecosystems of the NW Atlantic. The subsequent freshening triggered a regime shift during the 1990s in ecosystems stretching from Newfoundland to the Middle Atlantic Bight. Observed changes in the abundances of phytoplankton, zooplankton, and fish populations in these ecosystems were previously attributed to the overfishing of cod and its cascading effects down the food chain. Our findings suggest that, with or without the collapse of cod, a bottom-up, climate-driven regime shift would have taken place in the NW Atlantic during the 1990s. It is becoming increasingly clear that NW Atlantic shelf ecosystems are being tested by climate forcing from the bottom up and overfishing from the top down. Predicting the fate of these ecosystems will be one of oceanography's grand challenges for the 21st century.
During the early 1990s, dramatic changes were observed in marine ecosystems of the Northwest Atlantic continental shelf stretching from Newfoundland to the Middle Atlantic Bight. The cod fishery collapsed in the Canadian Maritimes and New England, while herring, snow crab, and shrimp fisheries expanded and prospered. A large copepod species, Calanus finmarchicus, that dominates zooplankton production during the spring and early summer months declined in abundance, while smaller species that dominate during the late summer and autumn months thrived. And even the base of the marine food chain was not immune to these changes as phytoplankton production increased throughout the autumn and sometimes right into the winter.
Ocean scientists call these large, rapid changes in marine ecosystems regime shifts. Regime shifts can result from natural variability in the ocean environment, human activities, such as overfishing and anthropogenic climate change, or various combinations of the two. Canadian fisheries scientists initially attributed the regime shift observed on the Northwest Atlantic shelf to the overfishing of cod. They suggested that by removing cod from the ecosystem, their prey, such as herring, snow crab, and shrimp, were released from predation pressure and this enabled them to increase in abundance. As populations of these cod prey increased in abundance, their prey would subsequently decline in abundance and these kinds of predator-prey interactions would continue to cascade down the entire food chain. If true, then observations from the Northwest Atlantic shelf would provide the first example of a large marine ecosystem undergoing a regime shift in response to the cascading effects of overfishing.
However, as is so often the case with nature, the story is not quite that simple. While the overfishing of cod undoubtedly had important impacts on their prey, it is not clear how far these impacts cascaded down the food chain. And, at the same time that the cod fishery collapsed during the early 1990s, the ocean environment in the Northwest Atlantic was undergoing dramatic changes of its own in response to climate forcing from the Arctic.
During the late 1980s, unprecedented changes were observed in the Arctic atmosphere, ocean, and cryosphere. In 1987, atmospheric pressure began to drop precipitously in the central Arctic, reaching an historical low in 1989. This drop in pressure was associated with the emergence of a strong counterclockwise circulation in the Arctic wind patterns. As these counterclockwise winds strengthened, the clockwise circulation of the Arctic Ocean's Beaufort Gyre began to weaken. Under favorable wind conditions, the Beaufort Gyre acts like a reservoir for freshwater in the Arctic Ocean. However, under the wind conditions of the late 1980s, the Beaufort Gyre began to spin down, opening up a floodgate for low-salinity water passing through the Canadian Archipelago and Fram Strait and into the North Atlantic.
NW Atlantic shelf ecosystems shifted rapidly as they became notably fresher during the 1990s relative to the 1980s. Looking at the observed ecosystem changes from the bottom up, this freshening would have enhanced stratification and resulted in greater phytoplankton abundance during autumn, when phytoplankton production is typically light-limited. This, in turn, would have provided more food to support the increase in small copepods described previously. The trends for the large copepod species, Calanus finmarchicus, is difficult to explain at first since its younger stages similarly increased in abundance while its larger stages actually declined. Size-selective predation by herring, which also became much more abundant during the 1990s, may be behind these paradoxical observations.
Commercially harvested fish and crustacean populations also have undergone large changes in the NW Atlantic since 1990. Cod stocks collapsed during the early 1990s, and although overfishing was the predominant cause of this collapse, the cold, Arctic-derived waters in the northern portion of their range have hampered the subsequent recovery of cod despite a decade-long fishing moratorium in the Canadian Maritime Provinces. Other species of fish and crustaceans have increased in abundance during this period, and for certain species, such as snow crab and shrimp, the suggested release from top-down predation appears likely. However, with or without the collapse of cod, a bottom-up, climate-driven regime shift would have taken place in the NW Atlantic during the 1990s. The resiliency of NW Atlantic shelf ecosystems is being tested by anthropogenic climate forcing from the bottom up and predator overexploitation from the top down. Predicting the fate of these ecosystems will be one of oceanography's grand challenges for the 21st century.
*Greene, C.H., and A.J. Pershing. 2007. Climate drives sea change. Science 315:1084-1085.