Como Lake Gets Double Dose of Treatment
Thursday, May 14, 2020 10:25 AM

This diagram shows the density of curly-leaf pondweed in Como Lake last summer.

Capitol Region Watershed District plans to inject aluminum sulfate into Como Lake to slash phosphorus loads and improve water quality in Como Lake. It's high phosphorus levels -- three times the state standard -- that lead to algae blooms that suffocate the lake. Low oxygen leads to fish kills, strong odors, and green water.

Beginning May 18, a barge injects liquid aluminum sulfate -- more commonly known as alum -- below the surface. As alum settles through the lake, it becomes a floc. The fine particles chemically bind to phosphorus in the water and lock up sediments on the lake bottom. That keeps the phosphorus away from algae. Less phosphorus, less algae.

Alum is safe for humans, pets and wildlife. There is no need to avoid contact with the water during or after application. Alum is used frequently to treat drinking water. You an also find it in baking powder and stomach antacids.

Treatment is expected to take 3-5 days, depending on weather. Crews, storage tanks, and equipment will set up in the Duck Point parking lot beginning this Sunday, May 17. When treatment starts, white furrows will be visible in the water as the alum gets to work. The watershed district says visitors should see improvements in water clarity immediately, and see fewer algae blooms this summer. The treatment is expected to be effective for several years.

The watershed district is posting signs around the lake telling visitors what's going on, but you can find out more here.

Herbicide fights curly-leaf pondweed
Alum is the second tactic being tried this spring to re-balance the lake’s ecosystem. In April and again in early May, the watershed district also injected fluridone. That aquatic herbicide is targeted to begin eradicating the destructive presence of curly-leaf pondweed.

The invasive pondweed first was detected in Como Lake in the early 1990s. It outcompeted natives, dominates the lake’s ecosystem, and now makes up 90 percent of the lake’s plants, says Britta Belden, a project manager for the watershed district.

Curly-leaf pondweed is the main source of phosphorus, which lake sediment stores until releasing it during the low-oxygen days of summer. The biggest challenge with curly-leaf, Belden says, is that – unlike native plants -- it grows in the winter, under the ice. It dies off in late June, creating dense mats on the water’s surface and releasing a surge of phosphorus into the lake.

That surge provides perfect fuel for algae and further chokes off oxygen when levels already are low because of peak sunshine, warming temperatures, and the lake’s shallow depth. The result: algae blooms, terrible smells, and a terrible habitat for fish and other lake creatures.

“Essentially, the lake overdosed on phosphorus,” says Britta Belden, water resource project manager for the watershed district.

Hoping to rebalance ecosystem
The state Department of Natural Resources traditionally “harvested” curly-leaf pondweed from key locations in the lake every summer. But that process actually cuts loose turions – buds that help the pondweed spread, Belden says.

The fluridone treatment -- once on April 13 and once on May 5 -- was timed to attack the pondweed before it begins reproducing but when it’s too cold for native plants to start growing, Fossum says. The herbicide also stunts pondweed's growth and causes a slow die-off, in which the plants turn white or pink.

Fluridone is a common aquatic herbicide. At the low concentrations it is being used in Como – 4 parts per billion – it has no known effects on people, pets, birds, insects, fish, or other wildlife, Belden says. Nor does it accumulate, she says. That’s important, because the watershed district intends to use the herbicide every spring for up to seven years.

“Over time, as we see less curly-leaf pondweed, it should be replaced by plant populations that are native and more appropriate for the lake,” says Bob Fossum, the watershed district’s manager of monitoring and research. “Over time, we hope to reduce herbicide use to spot treatments.”

Doubling down on phosphorus fight
Until this year, the watershed district has focused on attacking the external sources of phosphorus that flow into Como Lake. It has built storm water ponds, underground filtration systems, and rain gardens; discouraged fertilizer use; and supported resident initiatives such as Adopt-a-Drain and curb cleaning. Those tactics keep grass clippings, leaves, lawn runoff, pet waste – and the phosphorus they carry – from washing down storm drains into the lake. Those efforts, which already have reduced phosphorus flow from the neighborhood by 20 percent, will continue.

Now the watershed district is focusing on internal sources of phosphorus. Lake watchers should see immediate results this summer, Belden says, but this is only year one in a long-term “adaptive management” strategy to reduce Como Lake’s phosphorus by 60 percent.

“We’re going to try things, see what works, and adapt as needed," she says. "We’ll see how the lake responds – it is a living, breathing thing.”

Originally published April 8, 2020. Updated May 14.

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