RiverKeeper Blog: Getting out Ahead of Invasive Species

By Rachel Calabro, Save The Bay RiverKeeper

If you've ever driven by a pond that's covered with thousands of little triangular leaves so thick you think you could walk across them, you've seen the Water Chestnut - an invasive species that's wreaking havoc for our native plants and fish.

This summer, Save The Bay has been working with local residents in Rehoboth, Massachusetts to remove invasive water chestnut from Shad Factory Pond. Water Chestnut is an annual plant that is rooted in the pond bottom and has floating leaves and small white flowers. Large black nuts form under the surface and have very sharp barbs that can stick to animals and can float downstream. The nuts stay viable in the sediment for up to 12 years, and each nut can produce 10-15 plants, so you can see how hard it is to eliminate this plant once it gets established in a waterbody.

Thick mats of floating water chestnut leaves can take over in ponds and slow moving rivers by shading out other plants and reducing oxygen in the water. It spreads rapidly and displaces native species. The most common method for removal is to hand-pull the plants in mid-summer before the nuts fall. This hand-pulling is hard work, but over several years can effectively limit the spread of the plant. When the problem gets too big for volunteers, mechanical harvesters are often used to pull plants on a large scale.

Water Chestnut is of particular concern on Shad Factory Pond because the Palmer River is an important fish run for herring and shad. Save The Bay is beginning a study this summer in partnership with the Massachusetts Division of Marine Fisheries to study spawning habitat in the pond. We are looking at dissolved oxygen, water clarity, pH, nutrients and spawning substrate to see if there is enough suitable habitat for fish. In addition to the water chestnut, other invasive plants including milfoil and fanwort along with native pond lilies and other submerged plants are limiting the available oxygen and harming spawning substrate.

Central Pond in East Providence, part of the Ten Mile River, also has a large infestation of water chestnut. This is of concern because fish passage was recently restored to this system as well. Harmful algae blooms have also been an issue on this pond, and invasive species can make the problem worse. As we try to re-introduce fish to river systems around the Bay, we also need to be concerned with what they will find when they arrive.