by Anna Mueller – A team at the Center for Limnology is spending the summer monitoring urban ponds in Middleton, a city of roughly 20,000 people nestled on the western end of Lake Mendota. Urban ponds are usually designed for one of two purposes – recreation, like fishing or kayaking and water retention, to slow the runoff from storms and prevent flooding.
For member’s of Grace Wilkinson’s lab here at the CFL, urban ponds offer an opportunity to explore waterways that are often understudied and valued less than their large lake neighbors, despite being a major part of our local ecosystems. There are between 200-300 urban ponds in the Madison area alone.
The ponds being monitored this summer follow a flow path that ends in Lake Mendota. Stricker’s pond flows into Tiedeman, which flows to the smaller Lake View West pond and finally empties into Lake Mendota. However, a combination of little rain, hot weather and evaporation, and the pumping of Tiedeman pond earlier in the spring has caused very low water levels and mud flats, drying up the flow between ponds.
Stricker’s pond is surrounded by a border of thick, green mud packed with fly larvae, bird feces and organic matter. Walking along the shore, you sink to the tops of your rain boots in mud. The pond has a depth of less than a meter in most places due to the low water levels. When I went out with some of the Wilkinson lab students earlier this summer, we paddled out to the middle of the pond in a canoe to retrieve the sonde, an instrument that measures conductivity, temperature, dissolved oxygen, pH, and algal matter every two minutes. The large amount of data is wirelessly downloaded via Bluetooth in batches to a computer.
The team also performed Secchi disk measurements, zooplankton tows, and profile measurements, such as chlorophyll and temperature, at different depths. To measure dissolved organic carbon, pond water was sent through a 0.45 micron filter to eliminate bacteria and algae that would consume the carbon in the water. Methane in the water is measured in an attempt to understand whether urban ponds are working as methane sinks, or are releasing methane into the atmosphere, important information as we calculate how freshwater systems impact greenhouse gas emissions.
The team’s goal is to better understand the biogeochemical cycling in the hundreds of engineered ponds in our neighborhoods and how they affect both our climate and the water quality of the larger bodies of water they eventually empty into. They are extremely nutrient-rich, almost always resembling green soup. Even if you were to cut off all incoming inputs, these ponds would continue to bloom and be soupy for maybe 100 years because unused phosphorus in the bottom sediment can get resuspended over and over again.
Although urban ponds have been engineered by humans, they are now irreversibly connected to our waterways and ecosystems.