Fosforo e destino dei laghi

Blooming algae may be changing lakes for the worse — and possibly forever

REUTERS/Joshua Lott
An algal bloom in Lake Erie last August shut down Toledo’s municipal water supply for three days.

A new study of noxious blue-green algae suggests that their summertime blooms may be on the rise not only because of changing climate conditions and continued fertilizer runoff, but also because these phytoplankton have a special gift for self-perpetuation: an ability to make use of phosphorus deposits buried long ago in lake-bottom sediments.

After a period of decline following cleanup efforts that began in the 1970s, concern over cyanobacteria blooms has been rising for a decade or so, driven by beach closures and other examples of their threats to groundwater quality and the overall health of lakes.

There are public-health risks as well — the toxins they produce can be harmful to people and pets as well as wildlife. Last month, Scientific American reported on investigations of a possible link between frequent algal blooms and high incidence of  amyotrophic lateral sclerosis (ALS), or Lou Gehrig’s disease.

But the new findings carry important implications for places far beyond those where bad blooms are already causing big problems – places like Lake Erie, where a bloom last August shut down Toledo’s municipal water supply for three days.

Because they suggest a scenario in which algae can drive long-term or even permanent shifts in the bloom-producing potential of a lake, they point to a higher likelihood of future problems in places where water quality has generally been higher and algal blooms less common.

Long-lasting lake changes

The study is the work of a team led by Kathryn Cottingham, a biologist at Dartmouth College, with backing from the National Science Foundation. It was published last Thursday in Ecosphere, an online journal of the Ecological Society of America.

By combining observations made at lakes in New England with reviews of databases on other lakes, the paper challenges the notion that cyanobacteria blooms are simply a response to fresh inputs of phosphorus in fertilizer runoff, even if aggravated by the slight but steady increases in water temperature associated with warming climate regimes.

Rather, it demonstrates a model in which cyanobacteria seem to have an ability to “access pools of phosphorus in sediments and bottom waters” that typically are not available to algae and other phytoplankton.

Combined with their well-known ability to capture nitrogen gas dissolved in the water column, this capacity to mine old phosphorus deposits enables the toxic algae to essentially create and perpetuate the conditions they need to bloom.

And that can lead to significant, enduring changes in how both nutrients are cycled within a lake, with impacts that are not trivial.

According to Dartmouth’s announcement of the study, the findings mean that when these algae “get a toe-hold in low-to-moderate nutrient lakes, [they] can set up positive feedback loops that amplify the effects of pollutants and climate change and make conditions even more favorable for blooms, which threaten water resources and public health worldwide.”

“We usually think of cyanobacteria as responders to human manipulations of watersheds that increase nutrient loading, but our findings show they can also be drivers of nitrogen and phosphorus cycling in lakes,” Cottingham said.

“This is important because cyanobacteria are on the increase in response to global change — both warming temperatures and land use — and may be driving nutrient cycling in more lakes in the future, especially the clear-water, low-nutrient lakes that are so important for drinking water,  fisheries and recreation.”

Lake of the Woods riddle

This is not the first study to suggest that current phosphorus inputs might not be the only driver of blue-green algae blooms.


But in the Dartmouth researchers’ modeling, the algae required no such help.

In one scenario, the algae might overwinter near a lake bottom and bring new phosphorus to the surface as the water warms. In another, they might spend winters nearer the surface, then be transported to the bottom with the turnover that follows ice-out.

Or they might be more or less permanently resident in the muck, sending phosphorus upward without moving around much at all.

The Cottingham paper says these findings suggest potential changes in management responses to the problem of algal blooms, without detailing what these might be. And it’s a tough problem: Large-scale water treatment to remove phosphorus is impractical; algicides are not only costly but cause as many problems as they solve; even the prediction of significant blooms remains a challenge.

But I don’t think you need a Ph.D. to see these findings as yet another basis for ramping up efforts to reduce phosphorus runoff into groundwater, particularly from the agricultural sources that typically are the No. 1 contributor these days.

It’s just more evidence — not that any was needed, really — that phosphorus loading is a harmful gift that truly keeps on giving.

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