Thursday, January 5, 2006

Tiny Marine Organisms Reflect Ocean Warming

Scripps Institution of Oceanography / University of California, San Diego

Sediment cores collected from the seafloor
off Southern California reveal that plankton populations in the Northeastern Pacific changed
significantly in response to a general warming trend that started in the early 1900s. As ocean
temperatures increased, subtropical and tropical species of small marine organisms called
foraminifera (forams) became more abundant. Forams that live in cooler waters decreased, especially
after the mid-1970s. These changes are unlike anything seen during the previous 1,400 years.
Oceanographer David Field discovered these dramatic changes during his Ph.D. work at Scripps
Institution of Oceanography at the University of California, San Diego. He currently works as a
postdoctoral fellow at the Monterey Bay Aquarium Research Institute (MBARI). Field and his
co-authors describe their findings in the current issue of Science magazine.



Foraminifera are small, amoeba-like organisms that live inside tiny shells ("tests") several of
which might fit on the head of pin. Most forams live near the surface of the world's oceans.
Different species of forams live in ocean waters of different temperatures. When forams die, they
sink to the seafloor, where their shells are often preserved as fossils in seafloor sediments.

Field studied fossilized forams in one- to three-meter-long sediment cores collected at the bottom
of the Santa Barbara Basin, off Southern California. In this area, dead plankton and sediments
settle onto the seafloor to form distinct annual layers similar to growth rings in a tree. At 600
meters beneath the ocean surface, seawater in the Santa Barbara Basin contains very little oxygen,
so few bottom-dwelling animals disturb the sediments and the annual layers remain relatively intact.

In conducting this study, Field worked with Timothy Baumgartner and Vicente Ferreira-Bartrina of
Centro de Investigación Científica y de Educación Superior de Ensenada, Mexico and with Christopher
Charles and Mark Ohman of Scripps Institution of Oceanography at the University of California, San
Diego.

Field and his coauthors examined yearly sediment layers that were formed up to 1,400 years ago.
Counting the different species of foraminifera in each layer, they discovered that many species of
tropical and subtropical forams became more abundant after about 1925. Although previous studies
have shown an ocean warming trend beginning at about this time, scientists have debated how much of
this warming trend was due to natural variability. In order to address this issue, scientists needed
more long-term data. Field's data set extends far enough back in time to demonstrate that the 20th
century warming trend surpassed the range of natural variability.

Many previous studies have shown that a rapid warming and a dramatic change in eastern North Pacific ecosystems occurred in the mid-1970s. At this time, species of plankton, kelp, fish and seabirds that prefer warmer waters increased and species favoring colder conditions decreased. Most scientists agree that part of the warming of the global oceans and atmosphere since the mid-1970s has been caused by human emissions of greenhouse gases. However, it has been unclear whether the ecosystem changes at this time were associated with anthropogenic warming. Field's sediment cores show that tropical and subtropical species of forams became even more abundant during this period while forams that prefer cooler waters decreased. The resulting foram community was unlike anything seen during the last 1,400 years. These long-term data indicate that the ecosystem changes since the mid-1970s are best explained by anthropogenic warming.

According to Field, "These data show that ocean warming has been affecting foram populations prior
to the late twentieth century. However, changes since the 1970s have been particularly unusual, and
show that ocean ecosystems in the northeastern Pacific have passed some threshold of natural
variability." He also points out that most scientific data about the ocean have been collected
during recent decades, after ocean temperatures and marine ecosystems had already begun to change.
As Field notes, "It's a classic case of 'shifting baselines'—conditions that scientists think of as
normal today might actually be very atypical when you look back a few hundred years."

This study provides a long-term context for many other studies of oceanographic and ecological
conditions off the California coast. It was conducted as part of the National Science Foundation's
Long-Term Ecological Research (LTER) program. The LTER program supports ecological research over
long time periods in a variety of terrestrial and marine ecosystems. The research was supported by
the Achievement Rewards for College Students-Los Angeles division, the University of California ship
funds and Coastal Initiatives, the National Oceanic and Atmospheric Administration, S. and B.
Kimmich, the National Science Foundation and the California Current Ecosystem LTER.

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Scripps Institution of Oceanography, at University of California, San Diego, is one of the oldest, largest and most important centers for global science research and education in the world. The National Research Council has ranked Scripps first in faculty quality among oceanography programs nationwide. Now in its second century of discovery, the scientific scope of the institution has grown to include biological, physical, chemical, geological, geophysical and atmospheric studies of the earth as a system. Hundreds of research programs covering a wide range of scientific areas are under way today in 65 countries. The institution has a staff of about 1,300, and annual expenditures of approximately $155 million from federal, state and private sources. Scripps operates one of the largest U.S. academic fleets with four oceanographic research ships and one research platform for worldwide exploration.

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