Climate Change and Decline of Cod Population Combine to Cause Rapid North Atlantic Ecosystem Changes
26.02.2007 - Other
While some scientists have pointed to the decline of cod from overfishing as the main reason for the the large and rapid changes taking place in Northwest Atlantic shelf ecosystems, others warn that climate changes are also playing a big role.
Previously, during summer months, a wind-mixed layer of warmer, less salty water (which is less dense and lighter) floated on the ocean surface. When the air temperature cools down during autumn, temperature and density differences diminish between that surface layer and the cooler and saltier waters below, initiating a mixing process leading to a rapid deepening of the surface mixed layer.
Since the late 1980s, increasing amounts of fresh water coming from the Arctic have flowed into the North Atlantic Ocean and climate-driven shifts in Arctic wind patterns have redirected ocean currents. The combination of these processes has led to a freshening of seawater along most of the Northwest Atlantic shelf, keeping the surface mixed layer relatively shallow.
Without the fall rapid deepening of the surface mixed layer, phytoplankton (tiny free-floating plants like algae) populations have continued access to daylight needed for growth, and their numbers have stayed abundant throughout the fall. In turn, zooplankton (tiny free-floating animals like copepods), which feed on the phytoplankton, have increased in number during the fall through the early winter.
Phytoplankton and zooplankton being the base of the food chain, changes in their seasonal cycles will affect the rest of the ecosystem just as surely as the decline of Cod in the early 1990s has led to increases in bottom-living species such as snow crab and shrimp that cod feeds on.
