A similar version of this piece was originally posted on the UW-Madison Center for Limnology’s blog.
In case you missed the news the last couple of days, around 400,000 residents of the city of Toledo, Ohio were advised to completely avoid the city’s drinking water thanks to a bloom of cyanobacteria (often called blue-green algae) called mycrocystis. The bloom occurred in Lake Erie, from where Toledo gets its water supply. Mycrocystis produces a toxin called mycrocystin, which is a neurotoxin that can be harmful, or even fatal.
Yesterday, water tests came back negative for the toxin, and the city lifted its drinking water advisory, but it left folks all over the country wondering if such a scenario is possible where they live.
While this scenario sounds a bit like the beginning of one of the Yahara 2070 scenarios, Abandonment and Renewal, Toledo and Madison have a key difference between them: where their drinking water comes from. Nonetheless, cyanobacteria are also common in the Madison lakes, which prompted some questions from the Yahara Watershed community to scientists at the Center for Limnology, including WSC PI and CFL director Steve Carpenter.
So, could Madison ever experience something like the Toledo scare? Carpenter answers a few pressing questions:
Could what is happening in Toledo happen in one or more of the Madison lakes?
Steve: It’s exceedingly unlikely. We don’t drink the lakes in Madison. Our drinking water comes from groundwater, while Toledo extracts its drinking water from Lake Erie.
Is a mycrocystis or other toxic algae event primarily related to an excess of phosphorous, or are elevated levels of nitrogen also part of the cause?
Steve: High phosphorus (P) makes it possible to have blooms of the kinds of cyanobacteria that make toxins. If a lake has high P, it will have lots of algae. But it is harder to predict which kinds of algae. From week to week, the dominant kind of algae can depend on short-term shifts in the availability of P, nitrogen, iron, possibly other nutrients, as well as on the consumption of algae by fish and zooplankton, called “grazing.”
What else could contribute to an event like this? Can we restrict corrective action to limiting nutrients like phosphorus and nitrogen?
Steve: Usually the big blooms are driven by high nutrients, warm temperatures, low densities of grazers, and physical conditions (such as low wind) that allow the cyanobacteria to aggregate near the water’s surface. Generally, the thinking is that we can’t do anything about temperature, wind etc. Nutrients and grazers can be managed.
Among the nutrients, there have been suggestions that we could mitigate cyanobacteria by mitigating nitrogen. There are a few oddball lakes in geological formations [that are] rich in P where this might be true. But for the great majority of lakes, I have never seen any evidence that it would work. Toxin-producing cyanobacteria need trace amounts of iron, but it is pretty hard to get iron concentrations low enough to limit toxin production. The Yahara lakes have very low iron and plenty of toxic cyanobacteria.
The most important step for reducing toxic cyanobacteria is to reduce P inputs. Grazers can help keep the concentrations of cyanobacteria within limits, but I do not think that grazers alone can eliminate toxic cyanobacteria. The Madison lakes have had a terrific grazer community since the late 1980s and we still get lots of toxic blooms.
Do we know what levels of microcystin are dangerous to human health? Is there a standard for testing?
Steve: The World Health Organization’s “safe level” standard is 1 microgram per liter. The U.S. has no standard. Some background is here.
Although I am not a toxicologist, my impression is that the environmental toxicology of cyanbacteria toxins is in an early phase of development. The chemical analysis techniques have made enormous strides in the past 10-20 years. Only in the last few years have we had good capability to measure the toxins in water. The more people look, the more they find.
We know the stuff is incredibly deadly, but we don’t know as much about how fast or effectively it can move from ingested water and algae into our bloodstreams.
The Madison lakes get toxic blooms from time to time, and it is hard to tell by eye if any particular bloom is toxic. My standard advice to folks who call the CFL and ask if the water is safe is: Can you see lots of green particles visible to the eye in the water? If so, then don’t go swim in it.