Seasons and Cycles of Change
Bathymetric map of the Bering Sea. The Chukchi Sea is the
northern part of the Bering Sea, lying north and south of the Bering Strait,
the narrow passage of water between Alaska and Siberia. (Image
courtesy of NOAA Pacific Marine Environmental Laboratory)
In September of 1997, SeaWiFS began its remote-sensing mission. As the
first images were returned, one of the initial remarkable images was of a
bright blue light emanating from the Bering Sea. The SeaWiFS Project
realized quickly that they were observing from space a phenomenon that had
just been noted earlier that summer: a strangely persistent and widespread
bloom of the coccolithophorid species Emiliania huxleyi (E. hux.) (The
Science Focus! feature More than Meets the Eye also
mentions these particular phytoplankton species.)
E. hux. blooms are usually short-lived, but the bloom in the Bering
Sea had first been observed that summer, and it persisted into October. It
reappeared the following spring and summer. This phenomenon indicated that
significant changes were occurring in the Bering Sea -- changes that were
related to larger alterations in the Pacific Ocean and the global climate.
In order to determine what was happening in the Bering Sea in 1997,
researchers first had to examine the characteristics and dynamics of this
important body of water. The Bering Sea is a vital region for numerous
organisms and is extraordinarily productive. The persistent coccolithophorid
bloom affected almost every level of the Bering Sea ecosystem in some way.
The basics: The Bering Sea is a polar sea, lying north of the
Aleutian Island chain, between the state of Alaska and the northeastern section
of Siberia, and bounded to the southwest by the northern Kamchatka Peninsula.
The total area of the Bering Sea is approximately three million square
kilometers. Despite the fact that it is a polar sea, much of the Bering Sea
remains free of ice through the winter. SeaWiFS and MODIS images demonstrate
the appearance of the Bering Sea in the winter and early spring (when it can be glimpsed through
the clouds).
The above image is a SeaWiFS image of the southeastern Bering Sea acquired on February 8, 2000, showing the sea ice edge north of the Alaska Peninsula and Unimak Island.
The above image is a MODIS image of sea ice in the Bering
Strait, acquired on May 7, 2000. Click on the image to view a full-resolution
version.
The bathymetry (bottom depth) of the Bering Sea is divided into two
major regions: the shallow shelf region lying to the eastern side, which is
less than 150 meters deep, and the deep region, which reaches to more than
3500 meters. (See the map at the top of the page.) The shallow shelf region,
particularly in the northern Chukchi Sea, is vital to many of the important
biological elements of the Bering Sea ecosystem.
The polar nature of the Bering Sea means that its seasonal cycle is
simplified compared the four seasons of the temperate zone: in winter the
Bering Sea is minimally productive, due to the cold temperatures and the small
amount of daylight. As the length of daylight increases in spring, the Bering
Sea explodes in productivity, and numerous animals and birds migrate to the
region to gorge on the spring banquet of phytoplankton, zooplankton, and
fish. In fact, the Bering Sea supports about 80% of the entire seabird
population in the United States.
Likely the most noteworthy migrant to the Bering Sea is the California gray
whale, which migrates to the Bering Sea from Baja California every spring.
The gray whales feed primarily on bottom-dwelling organisms called amphipods,
and even ram into the sea bottom to scoop them up. The whales use their
baleen to extract the amphipods from the mud and seawater. Feeding in
Alaskan waters allows the whales to migrate back to Baja California and
provides the females with the nutritional resources to raise calves in their
winter habitat.
Sonar image of whale feeding pits in the Chukchi Sea. Source: USGS Fact Sheet:
Whales and Walrus: Tillers of the Seafloor
Photograph of gray whales during bottom feeding activities. Source: Erin
Carruthers
Arctic benthic amphipod Gammaracanthus loricatus. Source: Fisheries and Oceans Canada
The changes: In 1977, an event which oceanographers refer to as a
regime shift took place in the North Pacific Ocean and the Bering Sea. The
term refers to the state of the Pacific Decadal Oscillation (PDO), which
describes the climate (winds, temperatures, rainfall patterns) of the North
Pacific Region. Putting it simply, the PDO can be in either a warm or
cold phase. In 1977, there was a fairly abrupt shift from the cold phase
to the warm phase, which resulted in warmer sea surface temperatures (SST) in
the Gulf of Alaska and the Bering Sea.
One of the main effects of the warmer SST was a suppression of phytoplankton
productivity, which led to reductions in shrimp, crab, and populations of
smelt fish, such as capelin. (Other fish populations, particularly cod and
pollock, increased at the same time.) The reduced numbers of smelt may have
led to declines in the numbers of Steller's sea lions and harbor seals, which
were one of the main food items for killer whales. The declining numbers of
sea lions and seals apparently led to increased predation on sea otters by
killer whales, resulting in a drastic reduction in the population of Alaskan
sea otters of more than 70%. These low sea otter numbers allowed sea urchins
(a major part of the sea otter diet) to proliferate. Sea urchins feed on
kelp, so the kelp forests in the Gulf of Alaska and the Bering Sea were
overgrazed, which affected all of the organisms associated with the kelp,
including seabirds like puffins and kittiwakes. (Note: The Gulf of Alaska
and Bering Sea fisheries are the largest in the United States, and some
researchers also blame overfishing for the decline in marine mammal
populations. Because of the climatic regime shift, it is difficult to
conclusively determine the primary cause.)
Shift forward to 1997. Due to the combined effects of two other climate
cycles, the Arctic Oscillation (AO) and the more familiar El Niño
Southern Oscillation (ENSO), winds over the Bering Sea calmed and cloud cover
was reduced. The increased amount of sunlight caused the water temperature
to rise more than 2 degrees Centigrade above normal, while at the same time the
calm wind conditions did not stir the water column sufficiently to bring
nutrients to the surface. These conditions stymied the growth of the normal
diatom blooms in the Bering Sea and were quite amenable to the growth of E.
hux.
SeaWiFS image of the Bering Sea acquired on April 25, 1998.
The turquoise waters of a coccolithophore bloom are highly visible from
space, and they can appear in patterns that are remarkably beautiful to the
human eye. But for fish and birds, the situation is markedly different.
Because the coccolithophore bloom reflects light, primary production is
suppressed. The effects move up the food chain, reducing the populations of
zooplankton, particularly euphausiids (closely related to the krill found in
Antarctic waters). Birds accustomed to gorging on euphausiids, notably
shearwaters, starved to death in enormous numbers. Observations indicated
that the birds avoided the bright turquoise waters of the coccolithophore
bloom, perhaps because they could not see their accustomed prey in the
water. Other observations indicate that salmon migration patterns were also
altered by the presence of the bloom.
SeaWiFS image acquired on September 15, 2000. It is not
clear if the blue-white waters seen here represent living or dead cells of
E. hux, as their reflective properties do not change significantly when
the cells die.
What's happened since, and what's happening now: SeaWiFS images,
such as the one shown above, recorded the return of the turquoise waters
characteristic of coccolithophore blooms to the Bering Sea in the spring of
1998, 1999, and 2000. The bloom was clearly active in 1998, but the
situation was not as clear in 1999, and the blooms in 2000 were not as intense
or widespread, though they were seen into September. There were also
alarming reports about increased numbers of grey whale carcasses washing up on
the the shores of California and Baja California. Some researchers speculated
that the gray whales may have been dying during their southern migration due to
reduced feeding success in the Bering Sea. More research is required to
determine if the changes in the Bering Sea ecosystem that have been recently
observed are influencing the gray whale population.
SeaWiFS image acquired on June 7, 2001, showing
several blooms in the Bering Sea. The brightness and contrast of this image
have been altered to emphasize the bloom features.
In the spring of 2001, SeaWiFS saw different colors in the Bering Sea
(above): swirls of dark green and reddish green, indicating that something
other than coccolithophores was blooming. A report from John Cullen of
Dalhousie University (Halifax, Nova Scotia) described the results of a research
cruise on the NOAA R/V Ronald H. Brown, which sampled the blooms and
found the species Phaeocystis, as well as chain diatom species, and
surface chlorophyll concentrations ranging from 3 to 15 milligrams per cubic
meter of seawater (which is a high concentration). This report, and the
absence of a coccolithophore bloom, indicates that the anomalous conditions
that fostered the explosion of E. hux. may be subsiding. There are
other possible indicators that the PDO may have shifted back to a cold phase in
1998, which is one potential cause of the currently reduced rainfall in the
Pacific Northwest.
Microphotographs of chain diatoms. Chain diatom image courtesy of F.J.R. Taylor.
Phaeocystis.
Phaeocystis image from The mystery of the foam on the sea shore by Wim van Egmond
Large blooms of Phaeocystis can lead to the formation of noxious foams which can accumulate on nearby coastal areas, such as the one shown here.
Source:
European Commission - Ocean Research - The balance of living organisms
ACKNOWLEDGMENT
The National Oceanic and Atmospheric Administration's
Pacific Marine Environmental Laboratory (PMEL) "North Pacific
Ocean Theme Page" (see link below) provided an invaluable starting point for
Bering Sea information. We thank Dr. Glenn Cota of Old Dominion
University for a review of this article.
Links
Bering Sea General
Bering Sea, Arctic sea ice, climate change
Bering Sea ecosystem changes
Pacific Decadal Oscillation
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