Nearly everyone can identify a pond, but what, exactly, distinguishes it from a lake or a wetland? A new study co-led by Cornell University offers the first data-driven, functional definition of a pond and evidence of ponds’ distinct ecological function, which could have broad implications for science and policy.

“The lack of a universal pond definition causes a lot of confusion, from people wondering about the difference between a pond and a lake, to aquatic monitoring programs with different definitions across governmental agencies, even up to accurately modeling global carbon budgets,” said Meredith Holgerson, assistant professor of ecology and evolutionary biology and co-first author of “A functional definition to distinguish ponds from lakes and wetlands,” published June 21 in Scientific Reports.

“We wanted to evaluate how scientists and policymakers define ponds and examine whether ponds are functionally distinct from lakes and wetlands,” Holgerson said.

Their conclusion: Ponds are small and shallow waterbodies, with a maximum surface area of five hectares, a maximum depth of 5 meters and less than 30% emergent vegetation.

There are hundreds of millions or even billions of ponds globally — more than 95% of the world’s still waterbodies are small (under 10 hectares) — but the humble pond is understudied and has been largely left out of federal and state monitoring and protection programs. This is partly because their numbers make monitoring difficult but also because agencies fail to define them or distinguish them from lakes or wetlands. The neglect has implications for the accuracy of climate modeling, as ponds are high emitters of greenhouse gases, and their contribution to the global carbon budget is uncertain.

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