Aquatic ecologists tend to avoid the term “weeds” when referring to macrophytes – the rooted aquatic plants that many swimmers and boaters disdain. These plants provide food for waterfowl and habitat for fish, but they can also play a critical role in maintaining water clarity.

Many lakes and ponds have two stable states: weedy and clear or devoid of weeds and muddy. This leaves riparian landowners and lake managers with a choice between two undesirable endpoints when nutrient levels are intermediate.

At low nutrient levels, the rooted plants win out because water is clear and plenty of light reaches the bottom of the lake. At high nutrient levels, the algae win out and effectively shade out rooted plants – this means extremely low water clarity and sometimes harmful algal blooms. At intermediate nutrient levels, things get a bit tricky. In this case, lakes can be pushed in one direction or the other – sometimes inadvertently.

When rooted plants are destroyed, mucky bottoms get stirred up and re-suspend nutrients. Competition with algae ceases and foul blooms occur. If plant biomass is not mechanically removed, the rotting vegetation further adds to nutrient availability, turbidity, and algae growth.

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In late June, the Commission’s Science Advisory Board released a report on the decline in nutrients in the offshore waters of the lakes and the harmful impact this decline is having on fish populations.

Phosphorus is an essential nutrient that fuels life in the Great Lakes ecosystems. The amount of phosphorus determines how many living things can survive in the lake, from tiny plankton up to top predator fish. And in every lake except Erie, the phosphorus levels in the offshore region — waters more than twenty meters deep — are below the targets established under the Great Lakes Water Quality Agreement between the United States and Canada.

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During the four months of weed harvesting on the lake, town crews removed 354 boatloads — 2,656 tons — of invasive milfoil weeds from the lake. The harvesting also removed 11,000 pounds of nitrogen, 17,000 pounds of phosphorous and 5,800 pounds of potassium from the lake.

In 2015, town crews harvested 423 metric tons of weeds from the lake.

If the harvester wasn’t on the lake this year, it probably would have been unusable.

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DNR Fisheries Biologist Mike Hawkins said mechanically harvesting has the added benefit of removing the plant material — and the phosphorus it contains — from the water.

One major trade-off comes in the form of excess nutrients, specifically nitrogen and phosphorus, that runoff from fertilizers used on local farm fields and from discharges from wastewater treatment plants and factories.

In an effort to make the water safe to drink, Decatur, Illinois constructed a $7.6 million nitrate reduction facility in 2002. But, that does not mean the nutrients have gone away as runoff from farms in the lake’s watershed and wastewater discharge from heavy industry find their way into the lake and, eventually, beyond.

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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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Phosphorus in the soil flushed out of agricultural soils into wetlands and local bodies because of the erosion. The excess of nutrients harms the aquatic plant and animals.

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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

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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