How We Use Our Land Shapes Streams’ Big Role in Greenhouse Gas Emissions

Streams carry a lot of organic matter, particularly near farms and during floods. Dorn Creek off Highway M. Photo: Sam Blackburn

by Sam Blackburn – If you were to ask someone where climate change-causing greenhouse gases come from, they’d probably say cars, air travel, or manufacturing. Someone from Wisconsin might even say dairy farms. It’s unlikely, however, that anyone would point a finger at streams and rivers.

Yet streams and rivers release a lot of greenhouse gases. Though not as large a contributor as cars, manufacturing, or farms, streams and rivers are an underappreciated source of carbon dioxide and methane emissions. They can release an amount of gas equivalent to 80% of what the Earth’s terrestrial plants take in each year.

When it rains, runoff from the surrounding landscape brings organic matter, like leaves and cow manure, into a stream. That gives the organisms living in streams, like microbes and aquatic insects, plenty of food and a byproduct of this feast is the production of carbon dioxide and methane. As these gases accumulate in streams, they are slowly released into the atmosphere via diffusion.

Sam Blackburn.

Here in the area surrounding Madison, Wisconsin, where I’ve been monitoring streams for the past couple years, our streams have higher emissions than streams in other parts of the United States.

A lot of this has to do with the area’s predominant land use, which is agriculture. Those iconic Wisconsin dairy farms and the corn and soybean fields that use their manure as fertilizer end up supplying a whole lot of organic matter for runoff to carry into our streams. This runoff has other consequences for our water bodies as well; for example, fertilizer used on farm fields ends up fueling algae blooms in our lakes.

Most of what we know about carbon dioxide and methane emissions from streams comes from observations taken during baseflow (“normal”) conditions. However, as you may have heard and read about in this blog, climate change is leading to more frequent and more severe floods than we’ve had in the past. The record-breaking flood last August was one example, and we had large floods the two previous summers as well.

Last summer, I tried to figure out what happens to stream emissions after floods. I measured emissions, stream habitat characteristics, and water chemistry at ten different sites on four stream systems near Madison: Pheasant Branch Creek, Dorn Creek, Black Earth Creek, and Badger Mill Creek/Sugar River. I then compared pre- and post-flood observations for the large August flood as well as smaller floods in May, June, and October.

My student stream technician, Quinn, watches our equipment near Highway 14 in Cross Plains. Photo: Sam Blackburn

I measured greenhouse gas emissions with a crazy looking contraption that was basically a floating chamber hooked up to a portable greenhouse gas analyzer. It allowed me to measure the change in concentration of greenhouse gases in the chamber as gases are released from the stream and, from that, I could calculate stream carbon dioxide and methane emissions.

What I saw was increases in organic matter in streams for several weeks after floods and increases in emissions of carbon dioxide and methane to match.

The record-breaking flood in August made it difficult to take samples. This is the bike path under Airport road in Middleton. Photo: Sam Blackburn

However, not every flood led to increases in organic matter and so not every flood increased the amount of carbon dioxide and methane coming out of our streams. The amount of organic matter in runoff varies based on the time of year. For example, there’s more fertilizer on fields and less rooted crops growing in the spring and fall, which means rain more readily washes soils into streams.

What my work is showing is that how we manage the land around streams matters. If we can keep excess organic matter out of streams to begin with, we can lessen their contributions to greenhouse gas emissions.

Without adequate runoff management, like cover crops and riparian buffer strips, agricultural runoff fuels greenhouse gas emissions in streams and has other negative consequences including algae blooms in our lakes. Extreme rain events and floods like we had last summer are becoming more common and will only amplify the effects of mismanaging our land, so it is even more important to be deliberate in how we govern the land around our streams.

Sam Blackburn is a masters student in Emily Stanley’s lab at the Center for Limnology