Pacific plankton outdo land pollution.

Pacific plankton outdo land pollution

Sulfate pollution generated on land can ride the wind for great distances, eventually dropping into remote reaches of the ocean. Yet long-term measurements on Pacific islands reveal that these well-traveled particles are far less numerous than natural sulfate compounds in the air over the ocean.

This new finding bolsters a recent theory -- now attracting considerable scientific attention -- that tiny ocean organisms called plankton exert a powerful influence on Earth's climate.

In the June 29 NATURE, researchers describe the sulfate study and a similar study on nitrate. These particles, which form the principal components of acid rain over continents, reach the atmosphere through fossil-fuel combustion and other industrial and natural processses. As part of an international experiment in the early 1980s, Joseph M.Prospero and Dennis L. Savoie of the University of Miami analyzed weekly air samples at 13 island stations for seven years in the North Pacific and five years in the South Pacific.

Data from the network show that the sulfate and nitrate spread unevenly over the oceans, say Prospero and Savoie. Nitrate concentrations were lowest at American Samoa and other stations in the central South Pacific while reaching their highest levels in the North Pacific. Using the southern figures as a measure of "background" nitrate concentrations, the researchers calculated that the North Pacific stations received three times the background amount.

This leads Prospero and Savoie to conclude that 40 to 70 percent of the nitrate over the central North Pacific comes from continental sources. Noting that nitrate amounts swung with the arrival of Asian dust from continental storms, they say this continental nitrate originated in Asia.

Because both sea salt and plankton yield large amounts of natural sulfur compounds, the researchers had to isolate these sources to measure the continental sulfate contribution. In doing so, they showed that biological sulfate greatly weighed the continental sulfate at the remote stations, even though industry emits more sulfur than do ocean organisms. At Midway, where continental sulfate levels were particularly high, biological sulfate was four times as abundant.

This funding supports the idea that plankton help regulate the climate by emitting key sulfur compounds that convert to sulfate. According to the theory, these sulfates create nuclei for cloud particles and increase cloud reflectivity, limiting the amount of sunlight reaching the ocean (SN: 12/5/87, p.362).

Last year, Stephen E. Schwartz of the Brookhaven National Laboratory in Upton, N.Y., argued that biological sulfate has little climate effect (SN: 12/10/88, p.375). He noted that while industry is concentrated in the Northern Hemisphere, the great amounts of industrial sulfate have produced no noticeable effect on that hemisphere's clouds or climate. The data from the island stations counter Schwartz's argument by showing that marine concentrations of industrial sulfate do not rival biological-sulfate levels. Therefore, Prospero says, the industrial sulfur should not exert as much control over cloud reflectivity as Schwartz supposed.