Visitors to the La Crosse River marsh this year have been witness to the steady spread of green carpet-like mats over the expanse of previously open water. Over the past few weeks, they’ve also smelled the gases produced by decaying vegetation as the water heats up and some of this floating plant mass begins to decompose.
The recent flooding of the La Crosse River and the marsh has washed much of this growth downstream so that it’s less apparent now; however, it’s still natural to question: What is this plant mass doing to the marsh? Why is there more growth this year? Is this normal? Will it be like this every year?
The vegetation we are seeing is a combination of different floating aquatic plants and algae. The filamentous algae are linked single-cell organisms in long strands or mats that extend deep into the water and also cover the surface when in large concentrations. The other floating plant is duckweed, which is a-few-cells-thick floating plant living on the surface of the water. It reproduces by forming buds, and it can double its mass every 16 to 24 hours under the ideal growing conditions of heat, sunlight and high water nutrients. Duckweed is an important food source for waterfowl and other animals, because it makes and stores starch, and it pulls nutrients (nitrogen and phosphorous) from the water as it grows. During fall migration, marsh visitors can see waterfowl feeding on duckweed in marsh backwaters as they stop here on their southward journey.
However, vegetation creates problems for water quality when it covers more than 20 percent of the surface (especially combined in a mat with algae). Excessive duckweed and algae mats block sunlight from getting to deeper water and then inhibit growth of other beneficial plants such as wild rice and other aquatic plants. Although both duckweed and algae put oxygen into the air when growing, the mass of vegetation inhibits surface oxygenation of the water while the decaying biomass also consumes oxygen. This can lower water oxygen levels to cause fish kills in late summer and fall. (This year’s flooding has opened up surface vegetation cover enough to reduce that risk.) The decaying biomass eventually sinks to the bottom during winter and adds to the accumulation of organic material, which ultimately fills the marsh and converts it to a bog with little or no open water. (A gradual process called eutrophication.)
To put it simply, some duckweed and algae growth can be beneficial for a marsh system by providing food, oxygen and some shade and cover. However, excessive growth inhibits plant diversity, reduces fish and wildlife habitat and hastens eutrophication of the marsh.
So why did we see more duckweed and algae this summer? We think that this summer’s rainy, hot weather stimulated more growth due to higher temperatures, while heavy rain has washed more nutrients than usual into the marsh from fertilized land surfaces. Both nitrogen and phosphorus are an annual problem when rains and erosion flush them into rivers and marshes in large amounts.
“Non-point” sources of nutrients are agricultural fertilizer run-off and animal manure along the La Crosse river upstream. Nearby non-point nutrient sources enter the La Crosse marsh from fertilized golf courses and a number of sports fields, as well as excessive fertilizer from households lawns and gardens; all of it drains into the marsh by streams and storm drains. Phosphorus nutrient contamination stays in marsh mud and organic matter, and builds up every year to continue to release phosphorus into the water in ever greater amounts. Nitrogen washes out more readily without such build-up problems. However, both nitrogen and phosphorus create an annual overgrowth problem when rains and erosion flush them into rivers and marshes in large amounts.
Excess nutrients in marsh and river water can be limited by careful and moderate use of fertilizer, and by avoidance of phosphate-based fertilizers. Unless everyone in the La Crosse River watershed works to reduce nutrient run-off, we can expect worsening problems with filamentous algae and excessive duckweed.
Fish and wildlife habitat will suffer as the marsh environment gradually degrades, shortening the marsh’s life as the place we know and appreciate. We can all make a positive difference if we remember where excess fertilizer goes when we use it. Avoiding phosphorus fertilizers and restricting overall use of other fertilizers will reduce late summer algae/duckweed overgrowth in the marsh and protect the vitality of the marsh we love to visit.