Focus on phosphorus control to improve water quality

Post by Stephen Carpenter

Summer is the season for unsightly and toxic blooms in lakes, and 2016 has been a banner year, with major blooms in Florida, Wisconsin’s Lake Petenwell, and other Wisconsin lakes, including those in Madison. The blooms of cyanobacteria produce toxins that threaten human health, like the bloom in Lake Erie that shut down Toledo’s water supply in 2014.

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An algal bloom in Lake Erie captured via satellite photo. Toledo sits on the far western shore of the lake. Photo: NOAA

What’s more, many cyanobacteria float and form scums that accumulate, rot on beaches, and cause fish kills.

These severe blooms amplify the urgency behind a statement issued today by Canadian and American scientists, myself included, for governments around the world to focus on a proven solution — that is, we must control phosphorus to decrease the intensity and frequency of harmful algal blooms.

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A mid-October algal bloom in Lake Mendota, Madison, Wis. Photo: Sam Oliver

Readers of this blog know that phosphorus inputs to lakes and reservoirs, which come from agricultural and urban runoff, are the main driver of blooms, and that phosphorus reduction is the key to improving water quality. Some government agencies, however, have lost sight of this basic fact of lake management.

Recently, some scientists and managers have argued for the control of nitrogen and phosphorus at sewage treatment plants. In response, the European Union has required the removal of both nitrogen and phosphorus from sewage effluents, and in 2011 the US EPA announced that it would be “partnering” with states to control both phosphorus and nitrogen. New Zealand imposed a nitrogen-loading cap on the watershed of its largest lake, Lake Taupo, but failed to define a limit for phosphorus loading.

Our article shows evidence that, instead, we must keep the focus on controlling phosphorus.

We reviewed thirty-seven long-term, whole-lake studies conducted in nine countries in Europe and North America and found that phosphorus reduction efforts successfully reduced algal blooms in a wide range of lakes, from small ponds to Lake Superior, and within a diversity of climatic and geological settings.

Even in the sole long-term study where the input of nitrogen alone was reduced, the size of algal blooms did not decrease. Instead, cyanobacteria flourished. Cyanobacteria can grow even when we reduce nitrogen in the water column because, unlike other aquatic organisms, they can take in nitrogen from the atmosphere.

In other words, if they have enough phosphorus, these microorganisms can get their nitrogen fixes from the air, even if no nitrogen is available in the water.

Our data also show that when managers decreased both phosphorus and nitrogen inputs, the decline in algae correlated only with declining phosphorus, while excess nitrogen either accumulated in dissolved form or was converted by bacteria to nitrogen gas.

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Stephen Carpenter. Photo by Jeff Miller. Copyright UW-Madison University Communications

These conclusions are actually good news. Controlling phosphorus at wastewater treatment plants is much less costly than controlling nitrogen. Estimated costs of reducing both elements range from four to eight times higher than reducing phosphorus alone. So, by focusing on phosphorus, we will be saving ourselves a lot of money, while also assuring success.

There are still reasons to control excessive nitrogen, which comes mostly from agricultural fertilizer or manure, or from burning fossil fuels. It can cause groundwater pollution in Wisconsin and other regions, as well as add to air pollution in our cities.

But, for protecting the water quality of our lakes and reservoirs, our study shows we must focus on the phosphorus.