Growing up in New England, the end of long, dreary winters was marked by the chorus of spring peepers at a small pond next to my house. As the weather warmed, the trill of American toads announced summer’s arrival. Ponds and lakes are common features in residential landscapes, and are often beloved by the people who live there. But when the landscape changes from forest to suburbs, how do ponds respond? How healthy are the suburban ponds that support our frogs and toads? I had the chance to explore this question towards the end of my PhD at Yale University.
My collaborators and I studied 18 ponds in a suburban Connecticut town that spanned a land-use spectrum from completely forested to highly residential. We wanted to know how suburbanization altered the physical, chemical, and biological characteristics of ponds. We found that by converting forests to yards, pavement, and houses, the ponds changed: there was less forest canopy overhead, making the ponds warmer, which promoted higher algae growth. We expected to see more nutrients (e.g., nitrogen and phosphorus) in suburban ponds, but this was not supported by our water samples. When we delved deeper, however, we found a troubling story. As it turns out, suburban ponds are subject to wastewater nutrient inputs from residential septic systems.
Septic systems are designed to leach nutrients, such as nitrogen, into the surrounding soils, which can enter our waterways. We found that small ponds in highly residential neighborhoods receive this wastewater, which is then taken up by plants and becomes food for insects and frogs in the ponds. By using nitrogen stable isotopes, we were able to differentiate between natural sources of nitrogen (e.g., nitrogen from plants and soils) versus wastewater nitrogen. In the most suburban ponds, up to 70% of a wood frog’s nitrogen came from septic wastewater! We also found that the frog’s diet shifted from more leaf litter in forested ponds to more algae in suburban ponds.
These two major findings indicate that suburbanization alters nutrients and food web pathways in ponds. While we know frogs are now getting their nitrogen from human waste and are eating algae instead of leaves, we don’t yet know if this will have long-term negative consequences. But it is certainly unappetizing to think that our idyllic backyard ponds are actually full of septic wastewater!
Check out the study recently published in the Canadian Journal of Fisheries and Aquatic Sciences.