Horizontal and Vertical Spatial Symptoms of Cyanobacteria Blooms

Abstract

Every summer in early July Lake Menomin turns a lovely shade of green. Waves wash up the boat launches and color the cement. Residents who live nearby experience sewer-type smells and are forced to close their windows. Can I eat fish from the lake? Is it safe to swim or water ski? These situations and questions plague residents of Menomonie and many others who live near a compromised water body.

The well-known green scum that floats at the water's surface is due to the overgrowth of cyanobacteria, commonly known as blue-green algae. Cyanobacteria is present in all lakes but can proliferate and form “cyanobacterial harmful algal blooms” (cHABs) under high nutrient conditions. The nutrients cyanobacteria need to thrive – nitrogen and phosphorus - naturally occur in the environment, but their use in farming, mainly in fertilizers designed to increase crop yields, and their subsequent runoff from the soil during rain events has introduced more nutrients than normal into lakes and streams. Other sources, like household lawn fertilizers, vehicles, and wastewater, are contributors, but because the primary land use in this area happens to be agriculture, field runoff is the source of most of the nutrient load in lakes and streams.

Not only are these blooms visually unappealing, but they are also an environmental and health hazard. The overproduction of cyanobacteria shade out native plant species and use up oxygen fish need to survive, leading to die-offs. Additionally, cyanobacteria produce a variety of toxins – liver, brain, cell, skin - that are a threat to wildlife, dogs, and cause skin irritation and gastrointestinal discomfort in humans. This has implications for recreation, boaters, and fishers. The local economy takes a hit as a result of this with less tourism and decreasing value of lake-front properties.

Because of these growing health concerns, I spent my summer studying where cyanobacteria and microcystin, a liver toxin, is most concentrated in Lake Menomin. To do this I sampled twenty-six points throughout the lake two times in July. I collected data at each site, such as the total phytoplankton biomass (both algae and cyanobacteria) and cyanobacteria specific biomass by measuring the pigments chlorophyll and phycocyanin respectively. I also collected a water sample at each site to measure the microcystin concentration.

The toxin microcystin is present in Lake Menomin. Microcystin concentrations varied across the lake and tended to be highest in the bays, where warm, stagnant, and shallow water is common, and cyanobacteria thrive. This, unfortunately, is where beaches are located. Although the bays of the lake showed the highest microcystin levels, the middle of the lake also reported high levels leaving boaters, fishers, and skiers at risk as well. Microcystin concentrations throughout the lake generally increased throughout July, exceeding EPA drinking water limits but staying below recreational limits. It’s possible the EPA recreational limit would be exceeded by the peak of cyanobacteria growth in mid to late August, but samples have yet to be taken to analyze this.

Bays had high microcystin levels not only because of their water characteristics, but also because they caught wind-blown cHABs. Because cHAB blooms occur on the surface of the water, they can be pushed around the lake by water currents influenced by strong sustained winds. Both days I sampled the dominant wind was northwestern and cHABs were blown into the eastern bay of Lake Menomin concentrating the microcystin toxin in this area. Environmental factors are important in determining which areas of the lake are safest to recreate in.

Determining the key signs of cHAB formation is also important because the risk can be assessed before it is present, allowing public officials and the public to prepare. Looking at community dynamics of chlorophyll and phycocyanin is a good indicator of bloom formation. As the summer progresses, the surface phytoplankton community, consisting of both algae and cyanobacteria, shifts to greater concentrations of cyanobacteria indicating increased bloom formation. Looking at the communities at depth and the surface can show when cHABs are developing and starting to cause greater water quality concerns.

So, what do we do with these findings? My hope is that this project will help the public and community know the real health risks of cHABs, incentivizing the public and decision-makers to improve nutrient pollution in Lake Menomin. These maps are an effective visual to understand the danger of one of the toxins produced by cyanobacteria. This allows the public to have a better grasp on the risks of recreation and public officials to understand nutrient pollution as a public health issue. There are currently many county conservationists, department of natural resource officers (DNR), and federal officials currently working to reduce nutrient pollution by preventing soil loss on agricultural fields. Their work has been valuable in not only improving water quality but also farmer costs. My hope is that my work will give further meaning and importance to the work currently being done.