Bad news for bottom dwellers

Deep sea communities are suffering from long-term food shortages

May 21, 1999
Web posted at: 11:50 AM EDT

Bottom Sea dwellers in the deep eastern North Pacific are suffering from a long-term food shortage, according to a seven-year study conducted between 1989 and 1996.

Scientists at the Scripps Institution of Oceanography at the University of California, San Diego, say a likely culprit for the decline in food supply is a documented increase in ocean temperature that spans the same time period.

Using the amount of sinking particulate organic carbon as a measure of food supply, and respiration by marine organisms living in seafloor sediment as a measure of food demand, scientists at Scripps tracked the adequacy of the food supply to organisms that live on the ocean bottom. They report in the May 14 issue of the journal Science that the ratio of food supply to food demand decreased by nearly 50 percent between 1989 and 1996.

Those bottom dwelling organisms affected by the declining food supply include a wide range of species from bacteria and protozoa, to worms and crustaceans.

The entire diet for these organisms, which live 13,000 feet deep, ultimately comes from the ocean surface in the form of feces, carcasses and other non-living matter.

"If the food deficit continues, it is going to change the configuration of the deep-sea communities of organisms," said Kenneth Smith, a research biologist in the Scripps Marine Biology Research Division and co-author of the report in the Journal Science. "No organism can survive long periods of time without food. There's going to have to be some adjustments. In the long term, either the animal will have to adapt to the shortage or a more opportunistic species will take over."

Smith and Ronald Kaufmann, a former Scripps post-doctoral researcher, said the drop in food supply may be related to an increase in sea surface temperatures over the same period that may have triggered a decline in the population of zooplankton, tiny creatures that live mostly in the surface waters of the ocean and form a vital link near the base of the food chain. All sorts of animals make up zooplankton, from the larvae of large fish and invertebrates to fully-grown worms and crustaceans.The decline in zooplankton may in turn have led to a reduction in the amount of food exported from surface waters to the deep ocean.

"Deep-sea communities rely on food produced in the lighted, surface layer of the ocean in order to survive," said Kaufmann, now an assistant professor in the Marine and Environmental Studies Program at the University of San Diego. "A long-term reduction in surface productivity could severely impact the amount of food delivered to the deep ocean and profoundly impact geochemical cycling."

Scientists say rising ocean temperatures may be the reason zooplankton are declining. Scripps scientists John McGowan, Daniel Cayan and LeRoy Dorman reported in Science last year that a climatic "regime shift" occurred over the eastern Pacific beginning in 1977 and caused average sea surface temperatures to increase by about 2 degrees Fahrenheit. This long-term change in the California Current, a 200-mile-wide swath of southward flowing water off the western coast of North America, has continued to persist to the present time. The scientists further documented that the incidence of warm water events, such as El Ninos, have increased significantly in the last two decades along the West Coast.

The evidence supporting a possible correlation between increasing sea surface temperatures and a detrimental effect on marine life is growing.

McGowan and Scripps scientist Dean Roemmich reported in Science in 1995 that zooplankton had declined by 70 percent between 1951 and 1993 at the same time as sea surface temperatures rose. Other studies have indicated an accelerating decline in the catch of commercial pelagic fishes, including Pacific and Chinook salmon since the beginning of the warm regime shift in 1977.

Researchers have also noted a decline in kelp along the West Coast, a decrease in the abundance of some species of sea birds, and a general increased frequency of southern species moving north.

To date, however, little has been known about the direct effect of rising sea surface temperatures on organisms that dwell on the ocean bottom thousands of feet below the surface, said Smith, who plans to continue his research.

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