From about 1680 until 1870, French-Canadian fur traders traveled in birch bark canoes, exploring lakes and rivers in the remote wilderness along what is now the border between the U.S. and Canada, trading European goods for the pelts of mink, beaver and other animals gathered by the local Ojibwe people; Voyageurs National Park in Minnesota commemorates the traders’ historic routes. Today, these rocky shores and interconnected waterways retain a secluded and wild character.

But a mystery lurks just beneath the water’s surface.

Anatoxin-a, an algal toxin also known (chillingly) as Very Fast Death Factor; and saxitoxin, another algal toxin used by CIA pilots during the Cold War in emergency suicide pills, have been detected in some of the region’s waterways. Exposure to even small doses of these natural poisons can lead to muscle weakness, loss of coordination, convulsions, and death by respiratory paralysis. Fortunately, human death and illness from freshwater algal toxins are exceedingly rare and there have been no reported human or animal deaths or illnesses due to any of these substances in the park. Still the presence of algal toxins in this rather pristine environment is a bit of an enigma. There is very little agriculture, very few people saturating lawns with fertilizer, and just not many nutrient sources to feed the algae that produce them.

In contrast, just a few hundred miles southwest of the park, where I live, in North Dakota, nutrient sources are abundant. Livestock outnumber humans and the sky seems to go on forever above vast treeless fields of wheat and sunflowers. Cattle here have died as a result of anatoxin-a poisoning after drinking water from the mucky green farm ponds where algae flourish. I work with a group of scientists from the U.S. Geological Survey, the National Park Service and other agencies, that are trying to solve the mystery: if toxins are present in both environments, why do they cause such havoc on a ranch in North Dakota but not, so far as we know, in this northern wilderness?

The toxins are produced by blue-green algae, more commonly referred to by scientists as cyanobacteria. Just like plants, these tiny organisms conduct photosynthesis by using sunlight and carbon dioxide to produce oxygen. They can also, under some circumstances, release potentially harmful toxins into the water. An important part our research is aimed at determining when that production occurs and what causes it.

We know that algae are essential parts of the food chain and that the amount of oxygen produced globally by these microscopic critters rivals that produced by trees. In fact, billions of years ago, long before there were any trees, cyanobacteria transformed Earth’s atmosphere by pumping out vast amounts of the oxygen that we humans and other organisms would need to survive. In other words, we don’t want to get rid of these algae. What we don’t know is why only a small percentage of algal blooms turn toxic. The mechanisms are complex, and other researchers have found that different bodies of water may have different triggers for toxin production. For example, in some water, salinity may play a role.

Our work in Voyageurs National Park began by collecting and analyzing water samples throughout the summer. Algae have a well-known pattern of succession in northern lakes, where a particular species will dominate for a few weeks, followed by another species, and then another. And our data showed this wax and wane of species too, but there was an unexpected twist.

Within the same cyanobacterial species, some will contain a gene that allows toxin production, whereas others do not. By analyzing their DNA, we found that those capable of producing toxins peaked in number before those that did not. But more importantly, the toxic strains were present before visible blooms appeared on the water’s surface. This has important implications for recreational water use—swimmers and boaters (and perhaps farmers watering livestock) may not be able to rely on the appearance of a pea soup-like substance as a warning to stay away.

As a result, we decided to explore more practical tools to help park rangers or your average farmer know if a park’s waterways are okay for swimming and fishing or if ponds are safe for watering livestock. Existing methods, like laboratory analyses, tend to be expensive and time-consuming. Rapid-assessment test strips, or dipsticks, are one alternative. These dipsticks work in much the same way as pregnancy tests, except they indicate danger rather than an upcoming bundle of joy. If the water contains a toxin over the level set by the World Health Organization, the test will read positive even if there is no visible algae.

So scientists will continue to explore and study these waters in boats loaded with sample bottles and dipsticks, just as the voyageurs of yesteryear may well have paddled their heavily laden canoes through algal mats to trade guns and whiskey for fur. One hopes that more answers will be just around the corner.