Collapse of a Food Chain

By Kathy A. Svitil|Saturday, July 01, 1995
RELATED TAGS: ECOSYSTEMS, OCEAN
Fifty years ago, the waters off southern California teemed with life. In addition to the large marine mammals--whales, dolphins, and seals- -the ocean was filled with the microscopic animals called zooplankton and with the fish that eat them, such as anchovies, mackerel, and sardines. Seabirds, feeding on both the zooplankton and the small fish, flew above the ocean waters by the hundreds of thousands.

Today those same waters are much poorer. The fish stocks have declined dramatically; the seabird populations have declined or even crashed. The reason, say biological oceanographer John McGowan and physical oceanographer Dean Roemmich of the Scripps Institution of Oceanography, is that the zooplankton population has dropped a precipitous 80 percent since 1951. And the reason for that is that the surface waters of the Pacific off California have gotten slightly warmer.

McGowan and Roemmich don’t know if the temperature increase is linked to global warming or simply a regional change--perhaps even a reversible one. Either way, the result has been biological devastation. It is an important discovery and a model for what can happen during global change, McGowan says. We’ve found that a small temperature change resulted in a very large biological change. That’s something new, something that we hadn’t had a feel for before.

McGowan and Roemmich tracked the zooplankton and water temperature changes using data from a series of 222 ocean research cruises, called the California Cooperative Fisheries Investigations, that have been run since 1951. The survey’s original purpose was to study the Pacific sardine, a commercially valuable fish whose numbers had dwindled in the 1940s. (That particular decline, it turns out, was caused at least in part by overfishing.) Over time the research cruises evolved into a large-scale investigation of the physics, biology, and ecology of the California current, which generally runs north to south down the coast.

The cruises are still run four times a year, along a series of lines that extend from the coast to 300 miles offshore, between San Diego and a point just north of Santa Barbara. At each of hundreds of stations, the researchers measure the temperature of the water, its chlorophyll content--a measure of the abundance of phytoplankton, or microscopic green plants--and its content of the nitrogen and phosphorus that phytoplankton need to grow. At each station they also determine the abundance of zooplankton by dragging a fine net alongside the ship and then measuring the volume of their catch.

For the first few decades, the cruises revealed relatively little, at least as far as the zooplankton were concerned. Although a trend might have been present, we couldn’t detect one, because we didn’t have enough numbers, McGowan says. But by the 1980s, it became apparent to McGowan and Roemmich that something was going on. There was a major decline in the zooplankton population, McGowan says. Just as obvious was the gradual rise in sea-surface temperature. There’s so much oscillation in the data--the lines are real wiggly--so we can’t say anything other than that this has taken place over the entire 43 years. But if you look at a graph, it’s clear that there has been this trend, both in population and in temperature.

McGowan and Roemmich don’t think the warming they saw--just 2 to 3 degrees Fahrenheit over the 43 years of testing--was a direct cause of the zooplankton decline. They believe instead that the warming caused a decline in phytoplankton--which are what zooplankton eat--by reducing the amount of nutrients available to the plants.

Ordinarily the supply of nutrients in the sunlit surface waters, where phytoplankton live, is constantly replenished by nutrient-rich water welling up from the deep. The water wells up to replace surface water that has been pushed out to sea by winds blowing off the California coast. The amount of water that comes up is purely determined by the strength of the offshore wind, explains Roemmich, but the depth the water comes from is determined by the strength of the density stratification of the ocean.

The depth is critically important: the water gets progressively richer in nutrients toward the ocean bottom because that is where dead plankton and other ocean debris accumulate and get broken down into nutrients again by bacteria. But as the sea surface has warmed off California, the ocean has become much more stratified--the surface water has become much less dense and more buoyant than the cooler water below-- and that, says McGowan, means it is much more difficult for the water to mix vertically. The effect is to have the upwelled water come from shallower depths. The water welling up off southern California, he estimates, is now coming from some 150 feet higher up in the water column than it used to. As a result, it is a lot less rich in nutrients.

Fewer nutrients mean fewer phytoplankton--and thus fewer zooplankton, fish, and seabirds. As the zooplankton population has dropped by 80 percent since the 1950s, the commercial catches of sardines, mackerel, and anchovies have dropped by more than 30 percent. Seabirds have fared even worse. McGowan and his colleague Richard Veit, a biologist from the University of Washington, have found that the number of sooty shearwaters, formerly the most abundant oceanic seabird off the California coast, has declined by an astonishing 90 percent just since 1986. The populations of most other seabirds have declined as well, McGowan says, except for ones like seagulls that live at least in part off the land.

McGowan and Roemmich don’t know if the temperature increase and zooplankton decline they’ve detected have occurred anywhere else in the Pacific; the data to answer that question don’t exist. The gradual nature of the changes since the 1950s, they say, rules out any connection to El Niño, the notorious periodic warming of the equatorial Pacific. Although El Niño’s effects have certainly been felt off California, it operates on a shorter cycle that would be detectable in the temperature and zooplankton data if it were there. On the other hand, McGowan says, the California current is not isolated from the Pacific, so one might expect that there is something going on, on a larger scale.

What’s going on could be a natural cycle that lasts longer than the El Niño cycle. The atmospheric people here at Scripps--the meteorologists and the climatologists--are telling us that there is a decadal scale fluctuation in all of the Pacific Ocean, Roemmich says, and that the eastern Pacific is now returning to a cooler state that characterized the early 1970s. If we went along with that scenario, we’d say, okay, if there is cooling, we ought to see the zooplankton coming back up in the next few years.

But another scenario, the two researchers think, is equally plausible and more ominous: the decline in sea life they’ve observed could be an effect of man-made global warming. We don’t know if it is a natural cycle or man-caused, McGowan says. If it is a natural cycle, then sooner or later it will reverse itself, like all other cycles. If it is a man- caused thing, it will only get worse. And if the rate of change continues as it has, it will be a disaster ecologically.
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