Study shows big rains eventually bring big algal blooms

Center for Limnology systems engineer Mark Gahler, right, co-author of a new study on the relationship between large storms and algal blooms, and colleague Jonathon Thom collect Lake Mendota data from instruments on board David Buoy. Credit: Paul Shcramm, UW-Madison

Algal blooms are a growing problem in regions of the world with many lakes. The floating green foam is not only a nuisance to anyone wishing to enjoy the water, it can also become toxic and a public health hazard.

The main driver behind these flowers is phosphorus, an element commonly used in agriculture to fertilize crops, and which can leach into lakes from land—especially during heavy rains. A new study from the University of Wisconsin-Madison shows how soon after a storm phosphorus “loading” triggers algal explosions, but also details the many other factors that weigh on when and if the lake reaches a tipping point.

“The fact that you just had a big storm doesn’t mean you’re going to get a big one now [algae] bloom. The buds are much more complicated,” says Steve Carpenter, lead author of a report published in the magazine Proceedings of the National Academy of Sciences.

Carpenter, director emeritus at the UW-Madison Center for Limnology and professor emeritus of integrative biology, led a team of researchers on a deep dive into long-term datasets collected from Lake Mendota, Madison’s largest and longest-studied body of water.

Scientists knew that just a handful of the strongest precipitation storms delivered about three-quarters of the phosphorus into the lake each year. In other lakes, previous surveys had documented large algal blooms after large storms.

But in Lake Mendota, these dynamics are more complex. While phosphorus is undoubtedly the key ingredient for an algal bloom, Lake Mendota can simmer for quite a while before producing one. In fact, the report found that the average lag time from a major storm to a major bloom is 15 days, with some lags between a heavy, phosphorus-yielding rain event and a notable algal bloom lasting up to two months.

Studie zeigt, dass große Regenfälle große Algenblüten bringen … schließlich

(A) Correlation exponents γ ± 2 SEs for simultaneous daily observations of precipitation, runoff, P-load and phycocyanin. (B) Autocorrelation function decay, weight = τ-γ, versus delay τ (days) for precipitation, runoff, P-load and phycocyanin. The line colors are the same as the bars in Panel A. Credit: Proceedings of the National Academy of Sciences (2022). DOI: 10.1073/pnas.2214343119

So what’s up? It turns out that phosphorus is only part of the algal bloom recipe. Carpenter identifies three other key factors at play: calm winds, warm surface waters, and low levels of tiny crustaceans called zooplankton.

Much like the annual “greening” that occurs on land as winter turns into spring, lakes experience an explosion of growth, particularly algae, as their waters warm. When it’s windy, these algae get mixed into the water column, but when the wind dies down, they can float to the surface and form a bloom. And all of this can only happen when populations of algae-eating zooplankton are small. When zooplankton numbers are high, they can eat enough algae to keep a bloom at bay.

Carpenter points to periods in the Lake Mendota dataset when these variables kept the phosphorus-rich lake in check. High levels of zooplankton from the 1980s through the early 2000s kept the water much clearer on average. Additionally, a cool, gray, cloudy summer in 1993 kept surface water temperatures lower, resulting in remarkably few blooms, despite the lake’s phosphorus loading from heavy spring and summer rains.

The interaction of these variables will make it difficult to get to a point where we can predict algal blooms with any degree of accuracy, says Carpenter. They are also beyond human control. That leaves the researchers with a clear option for people who want to tend to move water quality in a better direction and prevent future algal blooms: turn off the phosphorus tap.

A drastic reduction in phosphorus use on land will result in less of it ending up in our waters. While it will take time for the phosphorus already in a lake to circulate through the system, natural processes will eventually reduce the load. It’s not a quick fix, says Carpenter, but it’s the only option on the table right now.

“The sun will shine, the wind will blow, grazers will sway with the food web and species invasions,” he says. “The only thing you can really control is you can keep phosphorus down.”

More information:
Stephen R. Carpenter et al, Long-distance dependency and extremes of precipitation, phosphorus pollution, and cyanobacteria, Proceedings of the National Academy of Sciences (2022). DOI: 10.1073/pnas.2214343119

Provided by the University of Wisconsin-Madison

Citation: Study Finds Big Rains Eventually Bring Big Algae Blooms (2022, November 28), retrieved November 28, 2022 from

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