Last year, high, turbid waters and a layer of sediment carried into the lake by flooding and rainfall reduced weed growth, explained Cushing. With clear water and low lake levels this summer, those familiar with the bay were expecting more weed growth than they’ve seen, but Cushing believes the sediment may also be helping to suppress weed growth this summer.

SAAWA removes 400 to 500 tons of weeds from the lake each summer with the help of an Aquarius Systems Aquatic Weed Harvester. The weeds contain a lot of water, Cushing explained and once they dry out, 40 to 50 tons of organic matter remains. Removal of the weeds prevents the weeds from dying and decaying in the water where they would release phosphorus and nitrogen, the very nutrients that feed algae blooms.

Currently the weed growth is outstripping our ability to pull them out of the lake, but we are convinced there are benefits to removing as much growth as possible, before it decomposes, spoiling water quality, increasing phosphorus as well as producing conditions for blue green algae growth.

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When you’re in the middle of the Shiawassee National Wildlife Refuge, you don’t realize how massive it is. It’s 10,000 acres of marshes and bogs, forest and farmland. To put the size in perspective, Manhattan is roughly 15,000 acres.

Brendan Carson is a researcher from Loyola University Chicago. He’s working on a project at the wildlife refuge centered around harvesting cattails. Carson says the project is designed to let the cattails grow all summer, absorbing the nutrients like a sponge, before the harvesting takes place.

Carson says harvesting invasive plants is more cost effective than spraying them with chemicals.

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Operational Considerations for your lake.

Expected Results from a Weed Harvesting Program.

Most aquatic plant harvesting systems will cut and remove submersed plants to a depth of five or six feet. As this biomass is removed from the lake, the water is immediately ready for use and there are no restrictions on use of the area that might be experienced with herbicide or some biological control treatments.

Removal of this biomass prevents its eventual decay and settling to the bottom, helping to reduce sedimentation in the lake. There is some nutrient removal with harvesting too, as the nitrogen and phosphorous that is bound up in the plant exits the water body. Harvesting is usually not lethal, leaving behind an oxygen and possibly habitat producing plant, which may be desirable in some situations.

Many harvester managers have witnessed a reduction in nuisance plant growth after a few years of repeated harvesting. The plants showed signs of stress and came back thinner, lower growing, and more scattered than before. In some cases desirable plants were able to out compete the nuisance plant growth.

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This year, a different approach was taken. Starting in June, contractors sprayed the lake with a compound called Phoslock, which helps remove phosphorous from the water. Then an aquatic vegetation harvester trawled across the south end of the lake, chopping down underwater weeds and hauling them to shore. The goal – to remove as much phosphorous from the lake as possible.

Sediment in the lake – from decomposing weeds, lawn fertilizer, stormwater runoff – produces excess phosphorous, which acts as fuel for algae blooms. Relatively few outflow points on the lake result in a slow turnover rate of water and poor nutrient cycling, according to a 2011 Kitsap Public Health District report.

The harvester removes plant life and opens the water up to circulation, allowing for the water to mix and more sunlight to reach the deepest parts of the lake

There are other ways to remove invasive plant species that may not require chemical spraying. James Douglass, an environmental scientist at Florida Gulf Coast University says the other methods could have several benefits and chemical spraying is actually the worst way to solve the problem.

Douglass agrees that other methods create a win-win for everyone. “It takes that rotting material out of the water and actually uses some of the materials from those aquatic weeds as fertilizer and return nutrients to the land where they’re needed instead of having them cause algae blooms in the water,” he said. An aquatic weed harvester not only opens waterways for navigation, but it is very effective in removing excess plant material.

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One of the most obvious management activities of the Lake Hopatcong Commission is the weed harvesting program. Some may view this as a simple “mowing” of the Lake. However, much more is accomplished than simply opening boating lanes and improving swimming. The harvested weeds represent a significant amount of biomass, and associated nutrients, that if not removed would otherwise add to the siltation of the Lake and contribute to its future eutrophication.

In its first three years of operation, because of the concerted, coordinated efforts of the Lake Hopatcong Commission, over nine million pounds of weeds were removed from the Lake. This could not have been accomplished without the state-of-art harvesting equipment and the dedicated staff of the Lake Hopatcong Commission. From a water quality perspective, the removal of this much weed mass is very important. The phosphorus associated with these weeds is roughly 1,200 pounds.

Although this may not sound like a lot, a single pound of phosphorus can support over 1,100 pounds of algae. Thus, through the weed harvesting efforts of the Lake Hopatcong Commission in New Jersey, phosphorus, that otherwise would be recycled through the Lake and fuel future weed and algae growth, is removed. This has a direct positive benefit to the long-term water quality of the Lake and the control of the Lake’s eutrophication. As such, the weed harvesting program contributes in more than the obvious way in the revitalization of the lake and the improvement of its water quality.

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Aquatic plants provide several ecosystem services such as habitat, food, cover and shading, temperature moderation, and nutrient uptake and sequestration. The overall approach to mechanical weed harvesting was conservative and the plant mass removed from the lake was about 14% of the peak mass that would be present without harvesting.

Harvesting removed 24% of the total phosphorus (TP) captured by aquatic plants and this accounted for 4% of the TP load derived from external sources. In addition, the cost of TP removal by aquatic plant harvesting is quite economical when compared to phosphorus management practices that take place in upland watershed areas. Data from this study suggest that harvesting certainly presents cost-effective opportunities for TP removal and has the potential to factor into dynamic and creative watershed management approaches.

Reprinted with permission from NALMS. The original article published in Lakeline, Volume 40, No. 4 Winter 2020.

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The Friends of Georgica Pond Foundation, a group of pondfront property owners, know that removing the plant material is particularly helpful in lowering phosphorous levels in the pond. By doing so, “harmful algal blooms and ecological crisis” can be prevented. There is a definite correlation between operation of the weed harvester and improved water quality.

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As part of a summer-long effort to cut a channel in Black Lake (New York), the Black Lake Association used an aquatic weed harvester to cut and remove invasive Eurasian watermilfoil.  On one September afternoon, the Black Lake Association harvested roughly 6,000 pounds of invasive wet weeds in less than two hours.

The harvested vegetation was taken to a local dairy farm to be used as fertilizer.

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– Immediate relief from nuisance plants that interfere with navigation and recreation
– Immediate use of the water for swimming or irrigation
– There is nothing foreign introduced to the environment when using mechanical control
– Biomass is removed from the water and along with it the nitrogen, phosphorus and potassium that are bound up in the vegetation
– Harvesting does not kill the vegetation, but instead gives it a hair-cut, leaving behind living plants that continue to provide oxygen and habitat for the other organisms

– The living plants left behind continue to absorb the polluting nutrients that are used for growth
– Harvesting vegetation from the lake greatly reduces the tons of biomass that will eventually die and collapse at the end of the season. This biomass will decompose, consume oxygen, and release nutrients back into the water column to grow more weeds and algae in the future.
– Harvesting the biomass also reduces the build-up of muck and sediment on the bottom that occurs from the rotting weeds
– The Harvester is a tool to be used to improve the fishery. For example “Cruising Lanes” can be formed into certain weeds beds to provide access for fishermen, and for predator fish too; the predator fish will graze the edges of trimmed weeds beds to find prey.
– Over time plants become stressed and grow more sparse and shorter in height in frequently harvested sites
– Lake weeds make a great compost and soil conditioner. The decomposed vegetation compares to cow manure in terms nutrient content. Also, it contains no seeds that can grow on land.

– Harvesting is a highly selective process. Exactly where you steer the machine is exactly where the plants will be cut.
– Aquatic Plant Harvesters are available in many sizes with a variety of features to be customized to your application
– A Harvester is an essential tool in the lake manager’s tool box. It can be made to cut shallow or deep, narrow or wide, fast or slow, depending on what is needed.
– A Harvester compliments other lake management strategies, including herbicide use. For example, the canopy of vegetation can be harvested BEFORE a chemical treatment is done. This technique may save money because it enables the lake manager to use less herbicide. And it will reduce the negative environmental impacts of using chemicals by reducing the tonnage of biomass that settles to the bottom to decompose