Though not enough information is available to draw a strong conclusion, all of these anomalies appear to stem in part from a lack of nutrients and warmer temperatures throughout the surface of the Bering Sea. The NOAA report (Stabeno et al. 1998) further suggests El Niño could be blamed for the depletion of nutrients and rise in temperatures in 1997, but not in 1998. Two years ago across the eastern half of the Pacific, the winds normally blowing from east to west subsided in classic El Nino fashion. These winds keep the warm water in the western Pacific away from the east, and when they stopped, much of the warm western water drifted to the coasts of Northern America. The warm waters were nutrient poor and did not allow the colder, nutrient-rich waters along the coast to rise to the oceans surface. Many plants and animals suffered all along the Pacific coast of North and South America.
In 1998, the El Nino subsided, and colder waters were allowed to come to the top again. But in the Alaskan waters, the previous years pattern was repeated, stated the NOAA report. The diatoms exhausted the Bering Sea shelf of the nutrients by late spring. Over the summer, surface waters warmed, skies cleared, and few nutrients came in from the edge of the continental shelf or from underlying waters. These changes allowed the coccolithophores to gain a foothold and grow in mass.
Though no one in the scientific community knows why these conditions are persisting, several theories have been proposed. Phyllis Stabeno, a physical oceanographer at NOAA in Seattle, said the lack of nutrients may be a result of recent changes in the slope current, which runs northward along the edge of the continental shelf.
Stabeno said, "The current carries nutrients past the Bering Sea. For these nutrients to get onto the shelf, there need to be cross-shelf flows to move the water from the slope and onto the shelf." These cross-shelf flows are formed when eddies and other imperfections in the current redirect some of its water towards the continental shelf.
She explained that over the past several years, the speed of the slope current has more than doubled. This may have caused the current to become a more effective carrier of nutrients. And if nutrients are no longer being thrown onto the shelf from the current, scientists could have the reason for the anomalies in the Bering Sea.
Global warming could be the reason some of these normally balanced mechanisms in the Bering Sea have gone haywire. This would certainly explain the warmer temperatures on the Bering Seas surface. As for the slope current, an increased melting of the Arctic ice caps could theoretically speed up the current.
The only way for this theory to be proven is if there is a prolonged and increasingly severe change in the Bering Sea. For now, it looks as if there will be another large coccolithophore bloom again this year. Stabeno said she has seen the latest aerial photographs of the Bering Sea, and there are coccolithophores poking out along the edges of the partially thawed ice.
|This satellite image of the Bering Sea was aquired on July 20, 1998. The mottled water just off the coast of Alaska remains relatively free of coccolithophoresinstead it is colored brown by silt. (SeaWiFS Image courtesy Norman Kuring, SeaWiFS project)|