The Mississippi River escarpment is a bluff on the eastern side of the river that runs throughout much of Mississippi. The escarpment is characterized by its deposit of Loess soils, windblown dust, and silt, which originate from much farther north and west. Bedrock was ground into powder by glacial activity during the Pleistocene Ice Age, and when the glaciers receded, this soil was washed down the mighty Mississippi River. When it was deposited on the southern floodplains, it dried back into a fine powder, and strong westerly winds picked it up and deposited it on the eastern side of the river in thick layers.
These bluffs of fine soils supported healthy forests and persisted relatively untouched for about 1.8 million years before modern agriculture arrived in Mississippi. The soil was deep and nutrient-rich, so what better place to farm? But when the trees were cleared and the soils were turned, erosion on a massive scale ensued. Today, this region is characterized by small, steep valleys that are old erosion channels.
Near Natchez, Mississippi, a deep, 11-year-old lake covers 32 acres of one escarpment valley. This lake was pristine and productive for the first 8 years, albeit underfished and bass crowded. Then, in 2019, the owners of a neighboring property within the lake's watershed decided to harvest about 600 acres of old-age hardwoods. This clearcutting once again exposed the powdery Loess soils to erosion. For the next year or so, the lake was the color of chocolate milk.
Several forms of phosphorus will bind with soil particles and can be stored for long periods. When soils move, they carry phosphorus with them on their journey. Phosphorus is an essential and often the limiting nutrient in water. As you might imagine, the chocolatey waters of the lake brought an excess of this valuable nutrient, with negative results. By early 2021, the turbidity had settled, and the water had cleared up, but left behind was an unpalatable salad of duckweed that thrived on the newly added phosphorus.
I was contacted in early 2022 for advice. I was initially surprised, as most of my dealings with duckweed are in small, sheltered ponds where wind action is limited. This lake was quite large and did not fit my stereotypical scenario, and the problem was rather severe, with coverage exceeding 80% most days. I soon learned that the lake was long, narrow, and unexpectedly sheltered for its size, and this characteristic reduced wind on the water's surface. Without wind to blow the duckweed to shore, it spread rapidly and covered the lake.
My go-to chemical for duckweed in small ponds is fluridone. It is highly effective and slow-acting; the duckweed dies slowly, and oxygen-related fish kills are not a concern. Treating larger ponds and lakes like this one is typically cost-prohibitive, as even the no-frills generic brands of fluridone are extremely pricey. In addition to being 32 surface acres, this lake was excessively deep, so we are talking well north of $20K in herbicide. But I was working with clients who wouldn't have to sell their firstborn child to afford it, so I recommended fluridone.
I often tell people to whom I recommend this treatment not to cuss me for at least 30 days, and preferably 60 days, as results take time. You will be disappointed if you expect to see a clean pond in a week. The chemical must stay above the recommended concentration for at least 45 days, sometimes up to 90.
The lake owners applied the herbicide on July 9th. They used a motorboat and injected diluted fluridone a few inches below the water by inserting the wand off the side of the boat. The boat was piloted in a zig-zag pattern to achieve thorough coverage. The remaining mixing was done via natural dispersal due to diffusion and limited wind action.
At 19 days after treatment, the duckweed still covered most of the lake but had turned from a healthy lime-green color to a dying brown. This was encouraging, as fluridone is a very slow-acting systemic herbicide.
At 30 days, coverage was slightly reduced, and the remaining duckweed was becoming translucent. At even the slightest disturbance, some would dissolve and sink away. A slight breeze had largely blown the remaining plants into the coves and along the shore, and the main lake was becoming open again.
At 60 days, the main lake was clear, and the finger coves still contained dying rafts of duckweed. The infestation was no longer preventing access or completely blocking sunlight anywhere in the lake, and there was evidence that the herbicide was still active and at a high enough concentration to prevent regrowth. By 90 days, the lake was practically duckweed-free.
And there's more good news. The adjacent land that caused the initial water quality problem and nutrient enrichment had been sold, and the new neighbors are much better stewards of the land. Their property has been revegetated, and additional erosion is no longer a concern—no more chocolate milk for this lake.
Unfortunately, the phosphorus is still in there. Unlike nitrogen, phosphorus is not nearly as transitory and can be stored in the lake bottom for decades only to rear its ugly head again. When the duckweed died, it took the phosphorus within it to the bottom, and it likely was stored there in the sediment. The nutrient could cycle back into the water column next spring when the pond mixes. If the duckweed returns, we may need to consider a different approach, like using a harvester to remove the duckweed and its phosphorus from the lake to prevent it from recycling further.
The lake's immense watershed size may end up being a benefit. There are about 40 acres of watershed for each surface acre of water. This ratio is at least four times recommended for lakes and ponds in this region. Thus, water moves through the lake at least four times faster than recommended. Because fluridone must stay at recommended concentrations for 45 days, we treated the lake during the dry summer when flushing was less likely. However, too much flow during the rainy periods may help flush out much of the excess nutrient load over time.
Another option for long-term prevention is to achieve a state shift in the lake. A state shift is when the dominant plant community shifts from one form to another. Usually, we hope to move away from a problem species to a beneficial one, like duckweed to phytoplankton. Phytoplankton are the base of the food web for fish, and having a healthy phytoplankton bloom will tie up nutrients that are not available for other species. Phytoplankton also will shade out the bottom of the lake, preventing the growth of rooted vascular plants.
On the other hand, a state shift could occur to something more noxious. We have seen some preliminary growth of filamentous algae, but at this point it is minimal. There are also significant patches of bur marigold around the lower lake. The latter is not a problem and provides some fish habitat, but we need to keep a watchful eye on the former.
Only time will tell the fate of this Mississippi escarpment lake. I hope the fluridone will keep duckweed at bay until spring, and then the pond will green up with plankton and get back on track. In the meantime, it is time to start culling the small bass.
Dr. Wes Neal, Extension Professor at Mississippi State, serves as State Extension Fisheries Specialist and is passionate about educating the public on small lake and pond management. He is an avid researcher on farm pond management and sport fish genetics. Wes is the lead editor of Small Impoundment Management in North America, the only textbook on the subject. He loves to hunt and fish, wes.neal@msstate.edu
Reprinted with permission from Pond Boss Magazine
