Changing Currents 

Turn the Bering Sea a Different Shade of Blue  

by John Weier
March 30, 1999

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During the past two summers (1997-98), a type of one-celled microscopic plant changed the color of the Bering Sea from its natural deep blue to a shimmering aquamarine in a matter of weeks. These plants, known as coccolithophores, produce and then shed hubcap-shaped, limestone (calcite) scales called coccoliths. Like all phytoplankton, the coccolithophores contain chlorophyll, are unicellular and have the tendency to multiply rapidly near the surface of the ocean. Yet, in large numbers, coccolithophores dump tiny white calcite plates by the bucketful into the surrounding waters and completely change its hue.   bering sea
Scanning Electron Microscope picture of Emiliania huxleyi - a widespread species of coccolithophore. Each plate ("hubcap") is a separate coccolith, which will remain in the ocean after the organism itself has died. (Micrograph courtesy Jeremy R. Young, The Natural History Museum, London)
 
The Bering Sea coccolithophores present a unique problem for researchers because a massive bloom of the organisms has never before been observed there. Many believe their arrival in the Bering Sea is yet another sign of a larger transformation in the surrounding ecosystem. The bloom's presence coincides with lower salmon counts along the coasts, a redistribution of microscopic animals in the surrounding ocean, and the massive deaths of shearwaters–hook billed, surface-feeding seabirds related to the albatross. Together, these events may spell major trouble for the Bering Sea ecosystem and the fisheries that ply these waters.

An Eerie Calm off the Coast of Alaska
The coccolithophore blooms formed primarily above the continental shelf to the west of Alaska. This underwater shelf is roughly the size of California and extends several hundred miles off the Alaskan coast, where it drops off to the floor of the Pacific. At their peak, the scaly plants covered practically the entire area. The only place where coccolithophores didn’t appear was in a 50-mile-wide band off the coast of Alaska.

 
Map of the Bering Sea

The size of this bloom over the last two years took many scientists by surprise. William Balch, a marine biologist at Bigelow Laboratory in Maine, has spent most of his career studying coccolithophores and their effect on the environment. He said, "The Bering Sea blooms happened at the wrong place at the wrong time."

Given the stormy history of the Bering Sea, the coccolithophores should not be so abundant. Balch explained that most one-celled marine plants (phytoplankton) do not do well in still, lukewarm water. Marine plant "fertilizers," usually in the form of nitrates, come from deep, cold layers of the ocean and are brought to the surface by strong currents and inclement weather. When the water is calm and warm, the plants do not get these nutrients, so they cannot grow, Balch said.

The coccolithophores are the exception. "They favor temperate conditions where the nutrients have been stripped away," said Balch. Under normal conditions, the coccolithophores do not compete well with other microscopic plants in nutrient-rich seas. When the competition is diminished by a low food source, the scaly plants have a chance. They usually end up proliferating in areas where the temperature is moderate, the sun is usually out, the water is calm, and the nutrient levels are low.

The problem is that the waters in the Bering Sea do not fit this description. For years they were unsettled and turbid. Diatoms, another single-celled plant that produces a glass (silicate) outer covering, were known to dominate the upper layers of the region. Currents and storms brought plenty of nutrients from the deep waters in the Bering Sea onto the shelf.

 
The coccolithophore bloom appeared over the continental shelf in the Bering Sea, represented in this map by the light blue off the west coast of Alaska.
 
Yet over the past two years the coccolithophores have done exceedingly well, and the diatom population has dropped. The concentration of the coccoliths detached from coccolithophores reached nearly 6 million per milliliter over the past two summers. Balch said, "There is some evidence from sediments that coccolithophores have existed in the Bering Sea in the past for brief periods. But recent blooms are unprecedented."

According to a National Atmospheric and Oceanic Administration (NOAA) report of the Fisheries-Oceanography Coordinated Investigations (FOCI) International Workshop on Recent Conditions in the Bering Sea (Stabeno et al. 1998), the environmental conditions there have changed over the last two years. Over the summer months, researchers found fewer nutrients than normal in the upper layers of the Bering Sea and warmer temperatures on the surface of the water.

The report also pointed out a slew of other anomalies in addition to the coccoliths that occurred over the past two years in the Bering Sea. Most notably, the shearwaters began dying, an abnormally low number of salmon returned to the rivers emptying into the Bering Sea, and the distribution of microscopic animals (zooplankton) changed along the coastal waters (NOAA 1998). Common sense suggests that the armor-coated plants must be wreaking havoc on the animal life there. But most researchers have come to the conclusion that the presence of the coccolithophores is just another manifestation of a larger phenomenon. The coccoliths’ impact on the animal life in and around Alaska is actually thought to be of secondary importance.

next A Blinding Sight for Some Seabirds

The data used in this study are available in one or more of NASA's Earth Science Data Centers.

 
April 25, 1998
Alaska and the Bering Sea from SeaWiFS on April 25, 1998. The bright aquamarine water is caused by the huge numbers of coccolithophores. This bloom was present in 1997 and 1998, and appears to be re-occuring in 1999. (Image courtesy Norman Kuring, SeaWiFS Project)
 

A Blinding Sight for Some Seabirds
One of the few examples of how the coccolithophores may have affected animal life directly can be found in the remote Pribilof Islands, about 500 miles from the coast of Alaska. The United States Fish and Wildlife Service conducted a study in the Pribilof Islands to determine what has been harming the seabird populations there. George Hunt, a seabird biologist at the University of California at Irvine, has been assisting the government agency with the interpretation of its data.

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pribilof islands
 
He said that many species of seabirds breed on these islands and feed in the surrounding waters. Some species of the birds, such as the kittiwake, get their food from the water’s surface. With wings outstretched they hover above the coastal waters. When they see their prey, they glide down and pick up the prey with their beaks. Other species, such as the Alaskan murre, dive as much as 180 meters into the ocean, grab their prey and reemerge at the surface.

During the two large coccolithophore blooms, the Pribilof Islands were surrounded by coccoliths. The white scales reflected most incoming sunlight and in doing so cast a shadow on the depths below, Hunt said. Both the murre and kittiwakes foraged for food in these waters. The researchers studied the two birds as they hunted during the coccolithophore blooms. "For the birds chasing food under the water, the coccoliths made it harder to find prey. In contrast, the surface feeders’ prey was easier to find with the coccoliths present, because the coccoliths highlighted the prey for them," said Hunt. Unexpectedly, the birds that fed off the surface had more offspring than usual, and the birds that dove for their prey had fewer offspring than usual.

next Big Trouble in the Bering
back Changing Currents Turn the Bering Sea a New Shade of Blue

  This true color image (a detail of the April 25, 1998 image) from the SeaWiFS instrument shows the bright reflection caused by billions of coccoliths. The Pribilof islands (in the lower left of this picture) are a breeding ground for migratory birds. The bird populations there began to change with the appearance of the coccolithophores. (Image courtesy Norman Kuring, SeaWiFS project)
 

Big Trouble in the Bering

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shearwaterAs 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.
 
euphasiid
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)
Bear catching salmonSalmon 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.

next Final Thoughts
back A Blinding Sight for Some Seabirds

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
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 ocean’s surface. Many plants and animals suffered all along the Pacific coast of North and South America.
 
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coccoliths in the bering sea

In 1998, the El Nino subsided, and colder waters were allowed to come to the top again. But in the Alaskan waters, the previous year’s 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 Sea’s 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.

Works Cited

  1. P. J. Stabeno, N. Mantua, J.E. Overland, S. A. Macklin, and G. Weller, 1998: Draft Report of the FOCI International Workshop on Recent Conditions in the Bering Sea, Climate and Upper Ocean Physics pp. 9-14.
  2. J. M. Napp, K. O. Coyle. T. E. Whitledge, D. E. Varela, M. V. Flint, N. Shiga, 1998: Draft Report of the FOCI International Workshop on Recent Conditions in the Bering Sea, Nutrients and Lower Trophic Level Response, pp. 15-21.
  3. R. D. Brodeur, G. H. Kruse, P. A. Livingston, G. Walters, J. Ianelli, G. L. Swartzman, M. Stepanenko, and T. Wyllie-Echeverria, 1998: Draft Report of the FOCI International Workshop on Recent Conditions in the Bering Sea, pp. 22-26
  4. NOAA, 1998: Draft Report of the FOCI International Workshop on Recent Conditions in the Bering Sea (S.A. Macklin ed.), Introduction, pp. 2-5.

back Big Trouble in the Bering

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 coccolithophores—instead it is colored brown by silt. (SeaWiFS Image courtesy Norman Kuring, SeaWiFS project)