Warming Waters Identified as Cause of Marine Life Depletions off California
- In the mid-1970s, the abundance of marine life along the western coast of
the United States began a momentous decline, the start of a trend that
today has yet to rebound. Numbers of fish, seabirds, kelp beds, and
zooplankton-the critical base of the oceanic food web-plummeted.
A recent study led by a scientist at Scripps Institution of Oceanography
at the University of California, San Diego, has found warming ocean
temperatures as the likely driving force behind the 25-year
Scripps's John McGowan and his colleagues used data recorded by the
California Cooperative Oceanic Fisheries Investigations (CalCOFI) to
examine the mechanism behind the changes seen in the California Current,
the large current originating in the northern Pacific Ocean that passes
along the western coast of North America.
"We had seen a big change in the California Current ecosystems since the
late 1970s, and in this report we looked at the possible mechanisms
accounting for that change. We found that the most likely cause is a
change in the upper-ocean heat content," said McGowan, who published the
results in Deep Sea Research Part II, in a special
edition that focused on the California Current and CalCOFI. The paper was
coauthored by Steven Bograd and Ronald Lynn of the National Marine
Fisheries Service and Arthur Miller of Scripps.
The authors caution that similar forces impacting ecosystem populations
could emerge elsewhere, especially if ocean temperatures continue to
rise. They say their results demonstrate that significant changes in
sea-temperature balances can "greatly alter the marine community
ecosystem structure and productivity, sounding the alarm to the potential
impacts of a global warming trend." They further note that the ability to
distinguish between human-caused and climate-caused changes will be
necessary in the future in order to model marine population trends for
conservation and management decisions.
In coming to their conclusion, McGowan and his coauthors looked at two
other possible causes for the ecosystem decline, testing and ultimately
showing that those are not likely. McGowan also shows that fishing
pressure cannot be blamed solely for the decline. "The massive declines
we've seen in fish eggs and larvae population after
1976 cannot be due entirely to fishing pressure because many of the
larvae are from species that are simply not harvested, and they too have
decreased," said McGowan.
Rather, the paper places the spotlight squarely on a "regime shift" to
warmer upper-ocean temperatures. This led to a disturbance in the method
in which lower, nutrient-rich water mixes with the upper ocean.
Essentially, a thickening of the warmer water layer caused the
nutrient-rich waters to deepen, disrupting the food supply for plankton
and other sea life in the upper layers.
"After this regime shift we saw the massive changes take place, not just
in plankton but in fish, seabirds, kelp beds, and nearshore
invertebrates," said McGowan. "In the larger sense this paper confirms
and reaffirms the notion that there are large-scale
environmental changes happening on land, lakes, and in our ocean. It's
uncertain how long it's going to continue and whether it will increase in
velocity or decrease. It's fear of the unknown, but something big is
happening. I think an awful lot has to do with
global warming and that's going to continue."
The conclusions reached in the paper are one example of the value and
importance of the CalCOFI program, launched more than 50 years ago to
explore the dynamic California Current. Although initially focused on the
disappearance of the sardine off the California coast, the data collected
by the CalCOFI program-from recordings such as ocean circulation,
temperature, oxygen levels, and salinity to observations of marine
life-have become invaluable.
"There are a lot of principles of interactions that can be derived from
this magnificent 50-year data set," said McGowan. "It's been called a
'national treasure' because it's so highly interdisciplinary and so
accurate, so trustworthy."
Says Bograd: "CalCOFI is the world's longest-running multidisciplinary
field program. The accumulation of physical, chemical, and biological
data spanning more than five decades now allows us to explore the
dynamics of the California Current and its
ecosystems across a range of temporal scales. CalCOFI also has been
instrumental in training numerous students and young scientists over the
McGowan believes the value of CalCOFI will increase in the years ahead as
science and government continue to pursue questions of human-produced
versus naturally produced changes. He says that since its beginning, the
CalCOFI program has focused on distinguishing this separation.
The value of CalCOFI surfaced as far back as the 1950s, when a 1958-59 El
Nino event was identified as having a profound effect on marine
populations. That event was, as McGowan puts it, an "eye-opener" for
future El Nino events.
Volume 50 of Deep Sea Research Part II, published this fall, was devoted
to CalCOFI and the California Current. Fourteen research papers in the
issue highlight various aspects of the California Current, including
"CalCOFI: a half century of physical, chemical, and biological research
in the California Current System" by Bograd and his colleagues and
"Long-term change and stability in the California Current System: Lessons
from CalCOFI and other long-term data sets" by Ginger Rebstock.
"It seemed fitting to present a sample of research papers from CalCOFI in
a special volume, as a celebration of more than 50 years of successful
scientific endeavors," said Bograd. "Hopefully it will also reinforce the
notion that long-term sampling programs such as CalCOFI are absolutely
necessary if we are to understand how marine
ecosystems respond to climate change. As oceanographic sampling programs
go, CalCOFI is the crown jewel."
Scripps Institution of Oceanography November 7, 2003
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