Geoscience Reference
In-Depth Information
sumptions are seriously shortsighted. Historical records now show that many highly valued marine anim-
als—great whales, sea turtles, seabirds, and large fishes like tuna—were severely depleted before the midcen-
tury and that many reached their low point decades to more than a century ago. Historical trends of some 256
records reviewed by Lotze and Worm followed similar trajectories: slow changes over millennia, followed by
rapid depletion over the last 150 to 300 years, with some recovery in the 20th century, particularly for marine
mammals and birds.
Although exploitation and habitat loss have been the main culprits in the historical declines in population
observed in marine species, climate variation has played a large role in long-term fluctuations. Cod catches in
Newfoundland from 1505 to 2004 indicate that climate-related declines occurred from 1800 to 1900 during the
Little Ice Age, large declines in the 1960s due to over-fishing, and the collapse in the late 1980s perhaps
caused by both. In this age of global warming, climate change may become more critical to predicting the fu-
ture of marine ecosystems.
Overfishing was the main cause of the dramatic ecosystem shift observed in the Northwest Atlantic in the
early 1990s, from one dominated by fish-eating species such as cod to plankton-consuming ones like herring.
Climate change, however, also seems to have played a significant role. In the late 1980s and early 1990s, cli-
mate change over the Arctic Ocean began to boost the outflow of low-salinity water from the Canadian Arctic
Archipelago into the Labrador Sea. The freshwater had its source in the increased melting of permafrost, snow,
and ice in the Arctic, as well as increased precipitation in the last three decades. This pulse of low salinity wa-
ter passed Georges Bank and reached the Mid-Atlantic Bight by 1991, causing a freshening of the seawater
along the entire Northwest Atlantic Continental Shelf.
As a result, a larger and deeper than usual layer of freshwater now sat atop the seawater, which is less buoy-
ant than freshwater. These layers are normally mixed to a depth of 25 to 200 meters (80 to 650 feet) in summer
by wind turbulence. But in autumn, as the air temperature cools, the temperature and density differences
between the surface layer and the cooler, saltier layer below break down, increasing the mixing of the layers.
With the influx of freshwater due to Arctic climate change, however, the surface layer is remaining buoyant,
with major biological consequences. Usually, as the mixed layer deepens during autumn, phytoplankton num-
bers decrease as they spend less time near the surface, where they are exposed to sunlight necessary for their
growth. Now phytoplankton in the surface layer remain abundant, fueling the growth of zooplankton popula-
tions, which feed on the tiny plants. The inflated zooplankton numbers are probably another reason that her-
ring became much more abundant in the 1990s.
The freshening of the waters in the Gulf of Maine has also altered phytoplankton and zooplankton as-
semblages in the gulf, with a general change from large zooplankton to smaller zooplankton. These changes
may require animals higher in the food chain to forage for longer periods of time to meet their energetic needs.
Calanus finmarchicus determines the arrival time of migrating North Atlantic right whales in the spring and
their reproductive success. Changes in the magnitude and the timing of the peak abundance of the tiny prey
species may alter migration, behavior, and, ultimately, numbers of this critically endangered whale.
According to the 2010 State of the Gulf of Maine Report, the higher silicate concentration found in the
fresher Labrador Current waters is expected to favor diatom production, which is thought to increase overall
ecosystem productivity, since diatoms are larger than dinoflagellates.
Atmospheric warming in concert with melting sea ice is changing the physical oceanography as well as the
ocean chemistry of the Northeast U.S. Continental Shelf, including the Mid-Atlantic Bight and the Gulf of
Maine. Climate largely determines an ecosystem's structure, function, and productivity, and surface sea tem-
peratures have been rising in the region for the last forty years and are predicted to rise another 2° to 4°C (36°
to 39°F) by 2080.
Temperature is critical to the growth, development, and survival for cold-blooded species found in coastal
and marine waters. It is also key to where organisms are found and when and where they make their migra-
