For the past 20 years, biologist Pete McIntyre has traveled to Africa’s Lake Tanganyika, Earth’s second-largest freshwater lake by volume, to study freshwater snails found nowhere else in the world. McIntyre, a professor with the University of Wisconsin-Madison, explains why these snails are important and what they tell us about pollution in the lake.
A version of this article originally appeared in the August/September 2016 issue of Nature Conservancy magazine. Read more at nature.org/magazine. — NCM
Q: First off, why study snails?
The snails of Lake Tanganyika are very diverse by freshwater snail standards. They’re interesting biologically, but we can also use them as sentinels for change in the environment. Unlike fish, they’re stuck at the bottom of the lake. So if some aspect of the environment changes, they’re also stuck with whatever bad things happen to the system.
Q: What sorts of things have they shown you?
We demonstrated that you get a dwarfism phenomenon due to siltation near villages. Outside Mahale Mountain National Park [on the eastern shore of the lake], you’ve got thin trees or no trees, and inside the park you’ve got pretty spectacular forests. That [differentiation] basically extends out into the water, and we see what we now understand as a predicable shift: the snails get smaller.
We also collected snails around villages where people are washing their clothes or cleaning their fish. What we tried to do there is use the snail tissue as a sentinel for nutrient pollution.
Q: How does that work?
Nutrients coming into the water get taken up by the algae, the snails eat the algae, and we analyze the snail tissue. Lo and behold, we got a beautiful relationship between the size of the village and the amount of human-derived nitrogen stored in snail tissues.
Q: And that pollution can be harmful to more than just snails, right?
Right, but in fact the silt eroding from the denuded land is probably worse for the snails. The combination of silt and pollution also hits the fish community hard. And remember that this is the same place where people draw their water. All kinds of bad things can happen in that situation, such as diarrheal diseases or liver flukes.
Q: Your team has helped nonprofits in the area, including The Nature Conservancy, the Jane Goodall Institute and Pathfinder International. How do you work with these groups?
We’re providing a bio-indicator tool that can help measure the efficacy of their efforts to reduce pollution — how is this changing the degradation of this amazing freshwater ecosystem? Both as a biologist and as a human being, I’ve got compelling reasons to want the condition of Tanganyika to improve. Our partners are doing the hard work of on-the-ground interventions, and the snails can help us understand what actually works.