Dickinson County: Silver Lake Fen

DSC_0003[1]Imagine you’re a student in my Aquatic Ecology class, on a field trip to a site not far from the town of Lake Park. As we enter the site, I implore you to “step carefully, the footing is tricky.” We slowly gather just inside, I pause dramatically after everyone quiets, and…I jump into the air.

And you find yourself bobbing up and down, as if on a waterbed mattress during a pillow fight.

Now that I have your attention (!!), I start asking questions: what do you see (watery fibrous material underfoot), smell (a stink like rotten eggs), and feel (unstable “ground,” like a floating sponge). Your senses don’t lie: this is a pile of dead plants (peat), built up over thousands of years, and floating in water. It is…a quaking fen.

The “quaking” part is obvious, but what is a “fen,” exactly? Is it like a bog? Yes: they are both wetlands accumulating peat (undecomposed organic matter) and have unusual water chemistry. In the fen, water comes out of the ground carrying lots of minerals dissolved in it, and it is usually fairly alkaline (remember chemistry? pH above 7.0). Nutrients like nitrogen are in the normal range for ecosystems. The plants are often grass-like sedges and certain wildflowers.

A bog, in contrast, has much different water chemistry, and different plants growing within. There is only one bog in Iowa…or maybe, zero? That’s a blog post for another day.

As often happens in a wetland, I marvel at how water moves through the system. Here, the soggy peat mat is perched more than 20 feet higher than the surface of the adjacent Silver Lake. A past researcher stuck a pipe into this peat mat, and water overflowed from the top of the pipe—six feet above the surface of the peat. Clearly, the water is under a great deal of pressure, presumably caused by gravity pushing down on the groundwater from nearby slopes. Geologists have noted a highly permeable “sand lens” running to the fen—water can move through this lens almost like it’s flowing in a pipe. It hits this spot and wells up…thereby supporting a unique plant community. And it has done so for thousands of years: researchers have studied the peat, noting it has accumulated slowly but surely.

“But Paul,” you say. “Aren’t plants biodegradable? Why don’t they decompose? Why do they build up year-after-year?” That takes us back to the water, and the stinky smell. Our noses are great at detecting the odor of reduced sulfur (hydrogen sulfide), and it tells us we have an environment in the peat essentially devoid of oxygen, with a buildup of toxic chemicals. In essence, this smelly water acts as a preservative. And so the peat doesn’t break down—it keeps accumulating.

This is one of my favorite places to visit in all of Iowa. If you bring a friend when you visit, steal my tradition for introducing Silver Lake Fen, and give ’em a good bounce for me!


Author: Paul Weihe

Associate Professor of Biology at Central College, traditional author (Textbook of Limnology, Cole & Weihe, 5th ed.; Waveland Press), and now...blogger!

7 thoughts on “Dickinson County: Silver Lake Fen”

    1. Although my name is on the comment about wanting a date, I’m pretty sure My Better Half wrote after I walked away from the computer. Although, the bad pun DOES sound like me 😉


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