Annual Public Lectures Series > Charles H. Greene
Scientists and Journalists:
Getting the Point Across
June 11-15, 2007
A Sea Change for Ecosystems in the North Atlantic
Charles H. Greene, PhD, Ocean Resources and Ecosystems Program, Cornell University
Summary of comments from June 12
A research project conducted at a fortuitous time can reveal larger processes driving fluctuations in the oceans, Charles Greene said in his lecture, "A Sea Change for Ecosystems in the North Atlantic" on June 12, 2007, at the URI Graduate School of Oceanography. Greene is a professor in the Ocean Resources and Ecosystems Program at Cornell University.
His 1997-1999 study of the copepod Calanus finchmarchicus, a zooplankton that dominates in the North Atlantic, contained an unexpected result: the 1998 count of Calanus was roughly 90 percent smaller than that of the year before or after, and such a huge drop in population of the copepod had never before been documented.
His ensuing research into the causes of this dramatic change suggested that pressure systems as far away as Iceland and the Azores could affect North Atlantic aquatic communities, from the tiny zooplankton to the enormous right whale.
According to Greene, Calanus in the deep basins of the Gulf of Maine are the key to understanding Calanus in the North Atlantic region. These copepods, mere millimeters in size, descend in a resting state to the deep waters during winter months and rise to be dispersed by currents in the warmer months. The Gulf of Maine basins are transition zones, with the warm, salty Atlantic Temperate Slopewater meeting the cold, relatively fresh Labrador Subarctic Slopewater.
The currents are driven by a naturally fluctuating climate regime known as the North Atlantic Oscillation (NAO). Positive NAO systems occur when there is a large pressure difference between the high-pressure system near the Azores and the low-pressure system near Iceland; negative systems are caused under the opposite conditions. Greene pointed out that during the early part of the 20th Century, positive NAO systems dominated, while during the 1950s and 1960s, negative systems were more common. Although scientists can detect these changes, they are unable to define what causes the differences between pressure systems.
Greene has also explored a potential link between Calanus and its predator, the right whale. Because a drop in right whale calving was observed following a Calanus drop, he suggested that low Calanus availability could be linked to a reduced chance for successful pregnancy in right whales.
Time-lags between NAO systems, regional slopewater temperatures, Calanus abundance, and right whale calving rates all suggest complicated connections that, while not always direct, were potential links nonetheless.
Greene also noted that another team of scientists had suggested a trophic cascade was really at work. They hypothesized that herring populations increased because of Calanus population declines. That, they said, could be due to increased predation by herring, when cod, which prey on herring, were largely removed from the region, releasing herring populations from predation pressure.
Greene explained that his hypothesis is a regional climate-driven, bottom-up model, while theirs was a predator-driven, top-down view, and suggested that both could be true to varying degrees. "I know this is complicated," he said, "but that's the way the world is."
As Greene emphasized to the nearly-full auditorium, his lecture was about the effects of a regional climatic system on the biological community, and the potential impact of global climate change on the North Atlantic Oscillation is not predictable at this point. Pointing to a continuous plankton recorder brought over from the National Marine Fisheries Service Lab, also at the URI Narragansett Bay Campus, Greene applauded the long-term monitoring the Lab carries out, explaining that long-term studies are essential to our understanding of climate change.
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December 4, 2007