Nobody agrees on where a pond ends and a lake begins
You might think the difference between a pond and a lake is obvious, but scientists have never settled it. Definitions range from 2 hectares to 8, or rest on waves, light or rooted plants. Meanwhile, ponds that dry out every year turn out to be some of the safest nurseries for frogs.
A pond is a small body of still water collected in a hollow, whether natural or dug by people. One working guide treats it as less than 5 hectares in area and 5 metres deep, with under 30 percent covered by emergent plants, which helps separate pond ecology from that of lakes and wetlands. Freshwater biologists once proposed subtler tests: water where light reaches the bottom, where rooted plants can grow throughout, or where there is no wave action on the shore. These proved hard to measure, so many researchers now rely on size alone.
Even then the thresholds differ. Some parts of the United States set the line at 10 acres, about 4 hectares. Minnesota, famous as the land of 10,000 lakes, also points to waves reaching the shore as the mark of a true lake. The international Ramsar wetland convention caps ponds at 8 hectares, while the British charity now called Freshwater Habitats Trust defines them as between 1 square metre and 2 hectares, holding water at least four months a year.
Ponds form in many ways: as cut-off river channels on floodplains, through glaciers, in peatlands and coastal dunes, or behind beaver dams. Kettle holes, vernal pools and prairie potholes fill with rain, runoff or groundwater. They are usually fresh, sometimes brackish; salty pools connected to the sea count as marine. Their size often swings with the seasons, and many are created by spring floods.
Life in a pond depends on depth, light, nutrients, salinity and which fish are present. Food webs rest on floating algae and aquatic plants, supporting snails, beetles, frogs, turtles and otters, with herons or even alligators at the top. Because fish eat amphibian larvae, pools that dry up annually, killing the fish, give amphibians a safe place to breed. People use ponds for irrigation, livestock, flood control and trapping pollutants.
Source: Pond
