SCIENCE FILE: An exploration of issues...
At the very bottom of the sea, it is dark, cold, and a little gritty. Life is sparse. Some oceanographers say that the sea floor existence is grim. And now it is grimmer yet--sea floor dwellers may be going hungry.
The denizens of the deep ocean floor are suffering from a long-lasting--and worsening--food shortage, according to a recent study of the California coast by researchers at Scripps Institution of Oceanography. This decline in food supply, they suggest, may be due to increases in the temperature of the ocean’s surface.
No one is really sure what caused the temperature increase--global warming, the natural cycling of the ocean, sunspots, or some still unknown mechanism. But its influence underlies more intense El Nino weather patterns throughout California, the disappearance of squid from Ventura and San Pedro as they seek the haven of cooler waters, and the deaths in 1997 of thousands of seals and sea lions on San Miguel Island.
Now, the waning supply of food sinking to the ocean floor joins the list.
The ocean center researchers measured how much food reached the ocean floor from the surface and compared it to the demand--how many organisms wanted to eat that food. Aside from seasonal variations--high in the summer and low in the winter--demand stayed essentially the same over the seven years of the study. The food supply, on the other hand, decreased by almost half over the years.
“Basically, there wasn’t enough food coming down to support the amount of metabolism that we observed. There was a deficit,” said coauthor Ronald Kaufmann, now an assistant professor at the University of San Diego.
Disruption of Food Chain Seen
Last year, another group from Scripps reported a gradual 2-degree increase in ocean surface temperature, affecting not just the well-studied California current, but the entire coast from the tropics to the Gulf of Alaska. The impact near the surface was dramatic.
“The surface water had warmed up significantly,” said oceanographer John McGowan, who researched the surface temperature changes. “And there was a decrease in plankton, sea birds, fish--it affected everything. The food web had been disrupted.”
Somewhat surprisingly, decreasing amounts of food didn’t seem to result in catastrophe for the bacteria, worms, and various animals that populate the deep. Perhaps, Kaufmann said, the organisms were conserving their resources.
“We didn’t measure growth or reproduction; maybe there’s reduced growth or no reproduction,” he said. “Maybe [this was] the minimal metabolism needed to stay alive.”
Scripps oceanographer Kenneth Smith, who conducted the study with Kaufmann, offers another explanation. Instead of cutting down on growth or reproduction, maybe the bottom dwellers are looking elsewhere for food. Perhaps, he says, the organisms are subsisting on cellulose or other materials that no one else can digest.
“When food becomes at a premium . . . before it gets out of the upper ocean, everyone’s had a crack at it, so the only stuff that gets to the bottom isn’t very good,” said Fred Sayles, a researcher at Woods Hole Institution of Oceanography. “It’s like asparagus at a kid’s party. The ice cream will get eaten, and the asparagus will be left over.”
Food that was less nutritious or harder to digest might already have settled from the surface to the sea floor, Smith suggests. If bottom dwellers were forced to rely increasingly on unpleasant leftovers, those that were able to use such food would have an advantage. Those that couldn’t would be dependent on what appears to be a dwindling food supply.
Generally, food for the creatures at the bottom of the sea comes from the surface. Organisms called phytoplankton live on the surface of the ocean; when they die, or are eaten, the remains and waste slowly sink to the bottom. This and other similar material is thought to provide most of the sustenance to the depths.
A Corresponding Relationship
As much as survival at the bottom of the ocean depends on what happens at the surface, scientists also think that the health of the sea’s surface relies on resources from the depths. Phytoplankton get their nutrients from below, and the supply depends on winds stirring up the ocean. As the temperature at the top gets hotter, the waters don’t mix as well and the needed nutrients stay trapped below.
If the plankton are malnourished, the food supply for the entire food chain is endangered.
To find out how much food actually reached the ocean floor, the researchers set up cones to catch pellets and other bits of organic matter as they fell and funneled them into cups. Each cup closed when it had collected 10 days’ worth of food.
Demand for food was measured by monitoring how much oxygen was inhaled by all the sea floor dwellers in a small area. After monitoring food supply and demand, the researchers concluded that the amount of food available at the depths was slowly decreasing. Smith and Kaufmann recently published their findings in the journal Science.
It is unclear whether the shift represents a trend--the way things will be from now on--or is just part of a larger cycle, where some years there is more food and some years there is less.
“The interesting thing is, in the long term, we don’t know what this means. Seven years is a long time from the scientific point of view, but short from the viewpoint of the ocean--it’s just a snapshot,” Kaufmann said. “One of the problems now is that nobody really knows what the [ocean’s] time course is.”
According to McGowan, there is evidence of past trends of increasing ocean temperatures, which lasted even longer than the 20-year period he studied. Notably, he says, this period of warming is the most rapid--the temperature has increased more over a shorter period of time. This, he says, “is cause for concern.”
Long-term consequences of continued increasing temperature and decreasing food supply are hard to specifically predict, the researchers say. Kaufmann believes that we would see an increase in El Nino-like conditions. The sea floor would be impacted as well.
“What we envision, if this continues, is a change in community [in the ocean],” Smith said. “The makeup of the community--the dominance of one organism over another--is going to change.”
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The Web of Sea Life
Scientists studying an area off the Central California coast from 1989 to 1996 found a pronounced decline in the food supply for creatures living at the bottom of the ocean. Using particle traps placed at approximately 165 and 1,970 feet above the ocean floor, the scientists found that sinking organic matter from plants and animals dwelling above that level had diminished considerably during that period. The scientists, from Scripps Institution of Oceanography in San Diego, believe that warmer ocean surface temperatures may have resulted in decreasing numbers of plankton, sea birds and fish, which in turn resulted in the dwindling food supply for the bottom-dwellers.
Researched by NONA YATES/Los Angeles Times
