A. Paytan et al., PNAS Early Edition (2009)

Polluted. Colored areas identify toxic copper hot spots, with blue representing toxic conditions for about 3 to 4 months of the year and green for about 6 to 7 months, while the red circles highlight areas susceptible to increased copper toxicity from human sources.

Death From Above for Plankton

Nutrient-rich particles sifting out of the atmosphere can be a boon for phytoplankton, the tiny floating algae that support ocean food webs. But new research suggests that some particles are poisoning phytoplankton, potentially disrupting marine ecosystems and altering the amount of greenhouse gases withdrawn from the atmosphere.

These atmospheric particles, or aerosols, can be natural or humanmade and include mineral-rich dust, soot, organic molecules, and sea salt crystals. Previous studies have emphasized the benefits of aerosol deposition in the ocean. This delivers high concentrations of phosphate, nitrogen, and iron that stimulate growth of phytoplankton. But aerosols don't contain only nutrients. They're a dry cocktail of substances that also include pollutants responsible for acid rain. Until now, scientists have overlooked their potential harm to the ocean.

Oceanographer Adina Paytan of the University of California, Santa Cruz, and colleagues wanted to get a wider view of aerosols' effects on ocean productivity, so they decided to determine how several components of aerosols, not just the nutrients, affected phytoplankton communities in the Red Sea. The team collected aerosols blowing in from Europe and Africa and mixed them with water samples from the Red Sea.

Phytoplankton exposed to the European samples thrived, but the ones mixed with the African aerosols took a nose-dive. The researchers noticed that the aerosols derived from Africa had nearly three times as much copper as the European samples. Further experiments confirmed that the copper levels in African aerosols are lethal to two groups of phytoplankton, as the team reports online this week in the Proceedings of the National Academy of Sciences.

To get a global view of the possible impact, Paytan and her team modeled the amount of copper deposition in the oceans. They identified two hot spots for human-induced copper deposition: the Bay of Bengal just east of India, and areas in the Pacific Ocean near southeast Asia and China. Paytan says that phytoplankton in these areas may be more sensitive to the toxic element because they probably aren't accustomed to the current levels, which are globally 50% higher than before the industrial revolution.

"This is a useful heads-up to a potential new problem," says marine and atmospheric chemist Alex Baker of the University of East Anglia in the U.K. Bill Landing, an oceanographer at Florida State University in Tallahassee, says that although extrapolating their findings to the world's oceans may be a bit premature, the study is a good first step. "They do a good job of pointing out the potential impacts, and most of them aren't good," he says.

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