Do pollutants bar amphibians from stormwater ponds?

Posted on December 18, 2015

Because of the complex array of ecologically threatening processes in urban environments, our understanding of the role that artificial ponds play in allowing aquatic populations to persist in towns and cities is quite limited. Together with green roofs, swales and stream buffers, stormwater ponds are key instruments in runoff management, but by retaining sediment as well as rainwater they tend to accumulate and concentrate pollutants, which may render the ponds inhospitable to aquatic life. To assess whether aquatic pollutants, or other factors such as the length of time that ponds hold water, or the amount of nearby vegetation, limit the distribution of amphibians, researchers carried out a large-scale field survey of 63 stormwater ponds in Maryland, U.S.A. The presence of amphibians (wood frogs, Rana sylvatica and American toads, Bufo americanus) was deduced from calls, egg masses and larval surveys in the breeding season. Pollution data were based on water samples and sediment cores, and GIS technology was used to measure forest cover within 500 m of a pond. Analysis showed that the breeding success of wood frogs was positively related to pond retention time, and was significantly higher in ponds with a chloride concentration below 250 mg/L. In contrast, toad breeding was sensitive only to the length of the retention period. Therefore, stormwater pollutants (in particular road salts) seemed to be a limiting factor for frogs but not toads. Some of the results suggested that frogs actively avoided ponds with high salt loads, and the researchers recommend that managers exclude such ponds when developing plans to increase aquatic connectivity. Trace metals were detected at relatively high concentrations in some stormwater ponds but had no obvious impact on amphibian distributions, possibly because metal toxicity is mediated by factors such as the grain size and organic content of sediment, and by interactions with other contaminants.

Reference: Gallagher, M.T. et al. 2014. The role of pollutant accumulation in determining the use of stormwater ponds by amphibians. Wetlands Ecology and Management 22, 551–564.