Aquatic Plant Management in the District
Healthy native aquatic vegetation is important to lakes because of their ability to maintain water clarity and good habitat. Plant uptake nutrients from sediment and store in their tissues which limit algae growth. They produce oxygen as a byproduct of photosynthesis, which oxygenates the water column, allow to a more robust fishery. Plant root structure stabilizes lake sediments and prevent them from being disturbed and mixing into the water, reducing water clarity and resuspending nutrients. Emergent plants help dissipate water energy and prevent shoreline erosion. The plant communities also provide habitat for invertebrates, waterfowl, loon, and cover for fish. Aquatic plants provide food and shelter for a variety of animals, some more obvious that other, but all are essential to a balanced ecosystem. A native and diverse plant community is important for the maintaining the good water quality and ecosystem balance.
The District uses specific treatment techniques and chemicals that target invasive plant communities. The areas that are treated have become dominated with the invasive plant which have choked out natives. Removal of the invasive species allows room for a diverse native plant community to return.
Invasive Species Management Projects
The District manages invasive and nuisance levels of Flowering Rush utilizing chemical application. Past research with Dr. John Madsen and Dr. Grey Turnage, Mississippi State University, has shown that applying the chemical Diquat subsurface twice a year gives significant reductions in below ground biomass of the plant. With this approach, massive reduction in plant densities and area have been achieved.
After successful reduction in plant growth, the District began to investigate if reduced applications could maintain the plant level at low population densities. In 2015, adaptive management protocols were established to test the effect of reducing treatment frequency from two annual treatments to one (and none in areas of very low density). The results were positive and indicated that reducing to one annual treatment did not allow for increases in plant biomass. The newly established protocols is the current management method. Below are some of the annual reports of the research.
2016 Final Report
2015 Final Report
2014 Final Report
In 2015, the District began a pilot study to determine the effects of applying the subsurface Diquat in a mixed stand of Flowering Rush and native Hardstem Bulrush. After two year of treating a 5 acres test plot, it became apparent that the chemical application had no damaging effects on the Bulrush and showed significant reductions in Flowering Rush. Native plants including Pond Lily and Hardstem Bulrush have begun to migrate into areas that were previously infested with Flowering Rush. The area was expanded to 23 acres in 2017 and to the entire infestation within the bulush in 2018, 46 acres.
Hardstem Bulrush Pilot Study
The work and research that was done in the Pelican River Watershed District was used in an paper that was published in the Journal of Aquatic Plant Management, link below.
Management of Flowering Rush in the Detroit Lake, MN
Historically, the PRWD obtained permits from the MN DNR each year to harvest curly-leaf pondweed to help control the population and density. No eradication techniques had been found, however, harvesting allowed for recreational activities to continue. In 2016, the District began chemical treatment on the most dense patches on Lakes Detroit, Curfman, Sallie and Melissa. The chemical treatment was successful and is the current management method.
In 2018, ice-off was later than usual and in mid-May unseasonably high temperatures were experienced. This made conducting early season CLP treatment a challenge. Fortunately, the District was able to delineate the CLP and treat before the water temperatures became too warm. Upon surveying the treatment areas in Detroit, Sallie, Melissa and Muskrat, it was found that of the 95 points surveyed, only 10 were found that had any amount of CLP growth, and these plants suffered significant damage. There is a location on the southwest side of Lake Sallie where growth was still detected. Staff will review alternative treatment options for this area for the future.
Zebra mussels were first confirmed in Lake Melissa in 2014 and were found in Lake Sallie and Detroit Lake in 2016. There were no new infestations in 2017, however, Floyd Lake was designated as infested in 2018.
Early in 2018, District staff contacted the University of Minnesota AIS research staff to develop protocols to effectively monitor and observe changes in lake ecology due to zebra mussel infestations. It was determined zooplankton studies would be the best indicator for changes in algae composition and distribution shifts (nutrient levels) and food web changes which can affect fisheries. A program similar to current MN DNR monitoring studies was recommended and implemented.
At multiple locations on Sallie, Melissa and Detroit, PRWD coordinates zooplankton testing with the DNR and added phytoplnakton composition and biomass sampling, per the U of M research recommendation. PRWD ships samples to MN DNR, who performs the analysis and drafts final reports.
Decontamination of boats and equipment is critical in stopping the spread of the invasive mussels. Equipment should be left out over the winter to be sure there are no surviving mussels if you plan to sell or move equipment to a different lake.
The image below is from a District lake in 2018 and shows how zebra mussels cover branches and plants in a lake system.