The Twin Lakes (North and South) of Calhoun County are a good example of the old question, “what’s in a name?” As a Limnologist–someone who studies water bodies on the continents: ponds, lakes, rivers, streams…and of course, wetlands!—I am sometimes asked how to differentiate between these systems. People want to know for example, when does a pond have sufficient surface area, or depth (or both?) to properly be called a lake?
For better or worse, there IS NO “official” distinction between these terms. (Really!) Suggestions involving the photic zone depth, or stratification, or minimum area…they are all attempts to be helpful, but are nevertheless always arbitrary. Here, someone decided that (likely in consideration of surface area), the Twin Lakes are each truly “lakes,” and of course the name reflects that. According to the Iowa DNR, North Twin Lake is deeper (ranging in parts to 12 feet/3.7 meters), while South Twin Lake is mostly between four and five feet (1.2-1.5 m) deep. South Lake is about a third larger in surface area, however (600 vs 453 acres/243 vs 183 hectares). In this part of the world, those are large enough to be considered lakes, and who am I to say otherwise?
Whatever name or category we choose, these are certainly shallow, productive systems (i.e., lots of algae or pondweed growing). That productivity will support a food chain and provide habitat for fish. But too much productivity makes murky water which is less appealing for swimming/boating/other recreation. A common question asked of any limnologist is “how do we reduce the algae/weeds in our pond/lake?” Of course, the answer is “stop fertilizing the pond/lake.” If you don’t want so much plant growth, don’t put silts/sediments, or growth-enhancing chemicals such as nitrogen or phosphorus into the water. Those chemicals increase the productivity of the green photosynthetic organisms—that’s why we consider them “fertilizer.” A nutrient and erosion control strategy is exactly the prescription for Twin Lakes proposed by experts from Iowa State University.
“But Paul,” you say. “Where do wetlands enter in?” Regular readers know that defining, delineating, and characterizing ecosystems are part-and-parcel of the wetland business, more so than other ecosystems. So, we are simply expanding our questions and checklists for “how to define a wetland” to use in ponds/lakes. Fair enough.
More importantly, wetlands lie between the land and water. Ecologically, wetlands have characteristics intermediate between terrestrial and aquatic systems. Hydrologically, wetlands often catch runoff, located in a spot where they physically intercept water flowing overland towards a water body (like a lake!). Near the south shore of South Twin Lake, down-slope from farm fields and US highway 20, lies a classic “cattail marsh.” This wetland contains a large area of cattail (Typha) plants, with muskrats (Ondatra zibethicus) and a few willow (Salix) trees. The wetland surely catches runoff heading toward the lake, removing silts and nutrients. Wetlands really do “clean the water,” so visitors to the lakes and lakeshore residents benefit from the work done by the wetlands.
Over the coming weeks, we’ll visit another couple wetlands designed and managed to protect or improve water quality. Let’s think about the challenge of water quality of the specific lake or stream near those wetlands; also keep in mind that the challenge of water quality in Iowa, in particular a form of nitrogen in our waters, is bigger than any particular water body. We have to think bigger, and face some contentious issues. As always, I’ll do my best to explain it all in a straightforward manner…and I’ll have pretty pictures, too! I hope you join me for the journey.