Big Trouble in the Bering |
 
| ||
 As to the other widespread
anomalies mentioned, the coccolithophores are not the primary cause. The study
done on the Pribilof Islands actually provides some evidence that the
coccolithophores are not responsible for the late summer deaths of the
shearwaters up and down Alaska's coast. Like the kittiwakes, shearwaters feed
off the surface. Even if the shearwaters venture past their coastal domain to
look for food in the coccolith-infested waters, they should not have a hard time
finding their target. "It’s not clear the coccoliths are doing
anything to the shearwater population," said Hunt. | Shearwater in flight. The deaths of many Bering Sea seabirds in recent years may be related to the same climatic changes that allow coccolithophores to bloom. | ||
| Another study funded by the National Science Foundation is still underway in part to
determine the cause of the shearwater deaths. So far the researchers involved
have learned that the widespread death of shearwater birds occurred in areas
both affected by, and unaffected by, the coccolith bloom, said Hunt. They
hypothesize that the most probable explanation for shearwater deaths is the
widespread warming of the surface waters in the area. "This has changed
the location of the birds’ prey," Hunt said. "In addition, along
the southeastern Bering Sea shelf, a krill-like zooplankton known as a
euphausiid appears to have decreased in number by at least an order of magnitude
(multiple of ten)." These euphausiids are one of the primary sources of
food for the shearwaters in coastal waters. |  Euphasiids (a species of krill about 2cm from end to end) feed on large diatoms, instead of the smaller armored coccolithophores. With the displacement of diatoms by coccolithophores, euphasiid populations have been declining rapidly. (Photograph copyright Steven Haddock) | ||
 Salmon and other predatory
fish also devour the euphausiids. So the drop in the population of these large
zooplankton near the coastal areas, may be part of the reason the salmon failed
to return to the rivers of western Alaska, said Hunt. According to the NOAA
report (Brodeur et al. 1998), the salmon populations over the past two summers
fell 53 percent below what was expected, putting many fisheries out of business.
In addition, more salmon are being found off the coast of Siberia and on
Alaska’s southern shores outside the Bering Sea. The salmon that do return
to western Alaska vary in age and strength. In earlier years with low turnout,
the healthier, larger salmon would normally make up the majority of the
returning fish.
The disappearance of the zooplankton probably isn’t the only reason the fish counts are down and the birds are dying, but the disappearance contributes to the problem. Steve Zeeman, a biological oceanographer at the University of Maine and a colleague of William Balch, explained that the disappearance of the zooplankton has to do with size. The diatoms that usually dominate the shelf are quite a bit larger than the coccolithophores. Since larger microscopic animals like euphausiids survive by feeding on larger one-celled plants like diatoms, the absence of the plant will cause a drop in the animal population. "And we then see a decline of wildlife along the food chain," said Zeeman. His conjecture is supported by the NOAA report (Napp et al. 1998), which states that the short-term change in zooplankton coincides with the change in the Bering Sea’s phytoplankton. |
Salmon, seabirds, and some marine animals eat euphausiids. As the krill populations crashed, animals higher up in the food chain also suffered. This bear grabbing lunch from an Alaskan stream represents the wide ranging impact of a microscopic plant. In addition to its effects on wildlife, environmental change also impacts human communities that rely on natural resources. (Photograph copyright Lon Overacker) |
Final Thoughts
A Blinding Sight for Some Seabirds
