Aquatic Plant Harvesting and Phosphorus Reduction

By Bill Bartodziej



Figure 1: A mechanical paddle wheel harvester working on Casey Lake

Finding the Right Solution

Shallow lakes in our watershed are important neighborhood resources. Managing these systems, however, can be very challenging with substantial phosphorus inputs from storm water and disturbance by invasive species such as the common carp. Because of phosphorus enrichment (eutrophication), shallow lakes are typically either dominated by algae or aquatic plants. It’s rare to find an urban shallow lake “in the middle” with a little bit of both. Over the years, we have learned that citizens really don’t care much for lakes in either extreme state. If we had our way, the general public and watershed managers would like to see moderate plant growth, good water quality, and sustainable game fish populations in our urban shallow lakes. Our overriding goals are to have healthy aquatic
ecosystems that look nice and provide a multitude of recreational opportunities.

In our watershed, we have applied phosphorus control measures, common carp control, and aquatic plant management to improve shallow lake systems. For controlling nuisance plant growth, mechanical harvesting (Figure 1) is often favored over herbicide use in shallow lakes due to the risks of oxygen depletion and nutrient release from decaying plant material. Through harvesting, we are able to curb the effects of excess plant growth – recreation becomes easier and aesthetics improve. 

Another potential benefit of harvesting is the removal of phosphorus that is contained in the plant tissues. Compared to watershed Best Management Practices (BMPs), such as rain gardens, pervious pavement, and green roofs, it’s possible that aquatic plant harvesting is a relatively economical way to remove phosphorus from shallow lakes. This tool may end up being a “win-win” where we improve recreation and aesthetics, and also effectively reduce phosphorus. So beginning last year, we started to critically assess aquatic plant harvesting in Casey Lake, North St. Paul. We addressed questions like: “How much phosphorus are we pulling out of the lake?” and “How does this cost compare to other BMPs?” Below is a synopsis of what we found.


Casey Lake

Casey Lake is at the headwaters of the Phalen Chain of Lakes. On the east shore is one of North St. Paul’s premier parks, with ball fields, picnic areas, a walking path and lake access. Casey is 12 acres with an average depth of 2 feet and a maximum depth of 4 feet. Its watershed (235 acres) is fully developed with a dominance of residential suburban housing and 27% impervious surface area.

Figure 2: When carp were abundant in Casey Lake, algal blooms
like this one were common.

In 2009, Casey Lake was identified as a productive common carp nursery area. The carp population was extremely high (total number of carp ≈ 12,000) with biomass estimated at 450 lbs/ac. Water quality was poor, algae dominated the system, and rooted aquatic plants were absent (Figure 2). In Midwestern shallow lake systems, carp biomass over 90 lbs/ac is considered the threshold where serious ecological damage becomes evident. In addition to having adverse effects on Casey’s water quality and ecology, substantial numbers of carp could travel downstream into the Phalen Chain of Lakes.

As part of a comprehensive carp management plan, Casey Lake’s water level was drawn down in the winter of 2012-13. In the spring of 2013, netting and electrofishing surveys suggested a complete kill of the fish community. Subsequently, the Minnesota Department of Natural Resources stocked bluegill sunfish (Lepomis macrochirus), green sunfish (Lepomis cyanellus), and largemouth bass (Micropterus salmoides) to both restore the fishery and to reduce the chances of carp re-establishing in Casey. North St. Paul installed an aeration system to increase the survival of game fish over the winter months.


Without Carp - 2013

After the carp eradication, Casey switched from an algae-dominated state to one blanketed by aquatic plants. In 2013, water clarity increased and rooted aquatic plants became established, covering a majority of the lake bottom (Figure 3). Plants did not reach the surface and were at moderate levels where recreation and aesthetics were not affected.

Figure 3: After carp were eliminated, water clarity significantly
increased in Casey Lake.

Harvesting Last Year

However, in 2014, nuisance levels of rooted aquatic plants and filamentous algae first became apparent in June (Figure 4). Because recreation was virtually impossible with the plant growth, we employed a private contractor to conduct mechanical harvesting. Two harvesting efforts began on July 11th and August 20th. A majority of the lake surface area was harvested during each effort. 

Figure 4: Nuisance algal mats appeared in June, 2014.

Through weighing representative trailer loads and keeping track of the total number of loads, we estimate that 115,000 pounds (wet weight) of plant material was taken out of Casey in 2014. By analyzing plant samples for phosphorus, we estimate that 36 pounds of total phosphorus was removed with harvesting. These values are similar to other plant-phosphorus levels reported in the scientific literature. We found that the cost of phosphorus removal through harvesting was $300 per pound.  

Harvesting This Year

Fortunately, this year, we did not have the extensive algal mats and surfaced plant mats cause problems in May and June. However, abundant algal mats and plants in July prompted the watershed to take action. Extensive surfaced mats impeded fishing from the newly installed dock and access point in the park (Figure 5). The watershed employed a private contractor in early August to begin harvesting. A total of 151,000 pounds (wet weight) of plant material was harvested in a one week period. We estimate that 47 pounds of phosphorus was removed at a cost of $160 per pound. An impressive 30’ mound of plant material is currently drying at the North St. Paul Public Works yard (Figure 6). This material will be brought to the Ramsey County Corrections Greenhouse facility and used in a compost mixture.        

Figure 5: July 2015 - Before harvesting

  

August 2015 - After harvesting

Cost Comparisons

If we look at harvesting over a two-year period for Casey, our average cost of phosphorus removed per pound is around $230. If we compare this number to standard watershed BMPs, this approach is considerably cheaper. To remove a pound of phosphorus in storm water by using a rain garden, a cost of $9,000 per pound can be expected. And pervious pavers are on the high end at $23,000 per pound. It should be noted that watershed BMPs have multiple benefits, such as reductions of storm water volume, suspended solids, and heavy metals. These tools are used because of the many benefits they provide in storm water management, not just phosphorus reduction.
 

Figure 6: A 30-foot mound of harvested aquatic plant material will be used in a compost mixture to grow beneficial plants.

 

Implications for Management

Harvesting has been an effective tool in managing the aquatic plant community on Casey Lake. In addition to improving recreation and aesthetics, taking out a considerable amount of plant biomass is an economical approach to remove phosphorus from the lake ecosystem. So far, this method does seem like a “win-win”, providing multiple benefits at a reasonable cost.

We found that harvesting in Casey Lake will substantially reduce the load of phosphorus making its way to the lake bottom in the fall and early winter through decomposition. Over time, we may see benefits such as reduced rooted aquatic plant growth and possibly less phosphorus being released from the lake bottom substrates and becoming available in the water column. Another benefit to harvesting is that removing this plant biomass also reduces the risk of oxygen depletion in the winter. Decaying plant material in the winter uses a lot of oxygen, especially in shallow lake systems. We need to keep oxygen levels pretty high in order to get the bluegill and bass populations through the winter.

We know all too well that urban shallow lakes like Casey are very challenging to manage. We are fortunate to have strong partnerships with the U of MN, North St. Paul, and DNR that provide a suite of management resources. Through continued data collection, assessment, and collaboration we look forward to learning more about how aquatic plant harvesting can be used to reach our watershed and lake management goals.