Usually, nutrients are a good thing. But when humans introduce too many into an ecosystem, it throws off nature’s balance.
An abundance of nitrogen and phosphorous from agricultural runoff created the infamous “dead zone” in the Gulf of Mexico — a massive expanse of water almost completely devoid of oxygen and aquatic life — but the same conditions may be causing problems in the upper Mississippi River, according to a recently published study from the U.S. Geological Survey, the Wisconsin Department of Natural Resources and the U.S. Army Corps of Engineers.
“There’s been some public outcry to start covering this issue,” said Shawn Giblin, a DNR scientist in La Crosse who co-authored the study. “You hear about the Gulf of Mexico, but this study helps people realize this is a local issue too.”
Over the past several years, the backwaters of the upper Mississippi River have seen an increase in duckweed and algal blooms, growing into thick, green mats on the surface of the water. The free-floating flora is a nuisance for anglers, boaters and swimmers alike, but the overgrowth is problematic for the aquatic ecosystem as well.
Layers of scum can block out light, preventing photosynthesis in other species growing below the surface. Decomposing plants use up the water’s dissolved oxygen, choking out fish and forcing them into the main channel.
“There’s definitely reduced habitat available,” Giblin said.
Previous studies have shown the relationship between elevated nutrients and unwanted plant growth, but this study helped determine various “thresholds” for growth among variables like water velocity, nutrient concentrations in the water and the plants and the presence of other aquatic plants in the area, USGS scientist and study co-author Jeff Houser said.
“What tends to matter most is physical environment,” he said.
Of the areas sampled between 2009 and 2011, lakes and backwaters most directly connected to the river contained the highest nutrient concentration, suggesting transfer from water currents and sediment. Slow-moving water and rooted aquatic vegetation helped promote the duckweed and algae growth, as well.
Efforts to reduce excess nutrients — particularly phosphorous — through adaptive land management are underway in Wisconsin, but more land use improvements are needed for the river to return to healthy levels, Houser said.
“It’s not going to change overnight,” he said.
“You hear about the Gulf of Mexico, but this study helps people realize this is a local issue too.” Shawn Giblin, DNR scientist who co-authored the study