SOUTH KINGSTOWN, R.I. — Intentionally introducing chemical herbicides into waterbodies doesn’t sound like a typical summertime activity, but for state environmental officials it may be the only way to preserve the health of the state’s freshwater rivers and lakes.

Over the past month, the Rhode Island Department of Environmental Management — via a third-party, licensed contractor — has begun treating two lakes that are both teeming with aquatic invasive species.

The first waterbody, Indian Lake in South Kingstown, is being treated for hydrilla, an invasive weed that was first spotted in the 260-acre pond last September. It has proven to be a remarkably resilient, needing only a 1-inch fragment to invade.

The second pond, Meshanticut Lake, a 12-acre waterbody in a state park in Cranston, is being treated for sacred lotus — the only waterbody in the state with this invasive species — fanwort, and millfoil.

Each species is highly aggressive and extremely competent at crowding out native species of fish, insects, and other aquatic plants that call freshwater lakes and ponds here home. They are also difficult to remove entirely; last year DEM collaborated with students from the University of Rhode Island to pull 93 bags of sacred lotus from Meshanticut Lake, but that wasn’t enough to curb the spread of the aquatic invasive in the pond.

“Herbicides are the best management practice,” said Katie DeGoosh-DiMarzio, an environmental scientist in DEM’s Office of Water Resources. “Mostly because these invasive species are so quick to fragment into little pieces, and get chopped up by boat motors, and the chemical when it’s in the water can get all those little fragments.”

Mechanical harvesting — physically removing the invasive plant from the water — often isn’t as effective as using chemicals, according to DeGoosh-DiMarzio. Most aquatic invasives spread by fragmenting, and any small part of the weed can repopulate a waterbody.

It’s a dark superpower that has allowed aquatic invasives to spread throughout rivers and ponds in New England. The newest arrival, hydrilla, was first sighted in the Connecticut River in 2016. Environmental surveys of the river, in 2019 and 2020, reported the invasive had spread to occupy 774 acres of the river, ranging from Agawam, Mass., to Long Island Sound.

Indian Lake was the first waterbody in Rhode Island to be infected with hydrilla, and thanks to a federal grant, it’s also the first one to be treated for the invasive weed. DeGoosh-DiMarzio said the lake is receiving a course of a chemical called fluridone, which, after getting absorbed by hydrilla, attacks the roots and reproductive system of the plant, ultimately killing it off.

A state-hired contractor will visit every two weeks this summer to ensure the concentration of the chemical is enough to kill off the hydrilla. The upside? Hydrilla is susceptible to a very low level of the chemical — about 3 parts per billion — and state environmental officials don’t anticipate any unintended consequences on any other plant or animal life in the pond.

Left untreated, invasives like hydrilla literally push out other organisms that call the rivers and ponds home, both by crowding out competing plants, but also by growing so large they block sunlight from reaching the water, thus lowering the levels of dissolved oxygen, a key indicator for fish and wildlife.

“Because there’s so much biomass and so many plants,” said DeGoosh-DiMarzio, “if it all decomposes at the same time, it would just suck up all the dissolved oxygen in the water and that could be problematic for fish.”

It’s a growing problem in Rhode Island. DEM reported last winter that 112 ponds and lakes, and 29 river segments in the state, contain at least one species of invasive plant, with many of them hosting multiple species at once, such as Meshanicut Lake.

For those who enjoy recreational water activities — kayakers, swimmers, anglers, and boaters — aquatic invasives will quite literally get in the way of those pursuits.

Meshanticut Lake, meanwhile, is getting a treatment of three different chemicals for three different invasives: a chemical called diquat for millfoil, imazamox for fanwort, and flumioxazin for sacred lotus. Unlike the chemical treating hydrilla, these herbicides are sprayed on the leaves of the invasives and prevent them from achieving photosynthesis, ultimately killing them.

But complete elimination of the weeds is almost impossible. Even with repeated, successive chemical treatments, aquatic invasives have shown to be resilient. For state environmental officials, the goal is containment, to prevent them from choking out a pond and spreading to new ones. In Meshanticut, the root structures will remain, so the invasives will certainly return next year.

To that end, DEM has been sending some of its summer interns to Indian Lake to ensure boaters putting boats in and out of the water have cleaned vessels of any invasive fragments. DeGoosh-DiMarzio said a more permanent cleaning station, also funded by a federal grant, would be installed at the department’s boat ramp and fishing access by the end of the season.

“We’ll definitely be coming back to Meshanticut,” DeGoosh-DiMarzio said. “For Indian Lake, you usually need at least four or five years of treatment in succession to get rid of it. Hopefully, if we can get that many treatments, we’ll have a better chance of really getting rid of it.”