"Downwelling" could be a temporary fix for aquatic dead zones

Aquatic hypoxia can be a serious problem, producing oxygen-depleted "dead zones" in lakes or seas. New research suggests that a process known as downwelling may help keep those zones from forming – although it wouldn't be a cheap solution.

Hypoxia typically occurs when excessive amounts of nutrients from land-based agriculture or other human activities flow into large water bodies, creating algae blooms. The overly-abundant algae proceed to use up much of the dissolved oxygen in the water, causing fish and other organisms to die off. Because there's already less oxygen the deeper one goes in a lake or the ocean, the problem is particularly severe at lower depths.

Ideally, the formation of dead zones would be minimized by altering farming practises, creating better wastewater treatment facilities, and otherwise getting people to do things differently. This could be a slow and complex process, however, so researchers at Washington, DC's Carnegie Institution for Science began investigating new ways of keeping the existing pollution from causing hypoxia.

Current approaches include utilizing bottom-laid pipes to bubble air up through the water column, or using fountains to increase oxygenation at the surface. Because neither of these are highly efficient, the Carnegie scientists instead looked to downwelling – this involves pumping more-oxygenated water down from the surface, so that it can disperse oxygen into the hypoxic depths.

In tests performed at the Searsville Reservoir in Woodside, California, it was found that downwelling could increase oxygen saturation by 10 to 30 percent – reportedly enough to keep many marine organisms from dying of oxygen deprivation. The effect was only noted within a few meters of the water pipes, however. This means that an extensive network of piping, and a large amount of energy for operating the pumps, would be required to make an appreciable difference over a typical dead zone.

According to the team's calculations, using downwelling to treat the dead zones that occur annually in Chesapeake Bay would cost somewhere between US$4 million and $47 million – that figure climbs to a range of $26 million to $263 million for use on the dead zones in the Gulf of Mexico.

Nonetheless, it is believed that in the short term, it would still be considerably cheaper (and thus more doable) than making sweeping changes to agriculture, wastewater treatment, and other well-established human activities. Additionally, based on computer models, large-scale downwelling should be three to 100 times more efficient at oxygenation than the use of air-bubbling pipes, and 10,000 to a million times more efficient than fountains.

"Reducing nutrient pollution is the only way to eliminate hypoxia permanently," says Carnegie atmospheric scientist Ken Caldeira, who is leading the study along with marine scientist David Koweek. "However, our work shows that downwelling is a technological solution that could mitigate the risk of low-oxygen dead zones while nutrient management strategies are put in place."

A paper on the research was recently published in the journal Science of the Total Environment.

Source: Carnegie Institution for Science