We’d be remiss if we explored invasive species in Minnesota and never talked about invasive aquatic species. After all, Minnesota is home to over 11,000 lakes, over 6,500 rivers, and over 10 million acres of wetlands. Water is in our DNA, but unfortunately, invasive species are in our waters.

Minnesota hosts 18 recognized invasive aquatic animals, 10 aquatic plants, and 10 aquatic diseases. Many of them arrived in the state through infested watercraft or traveling through connected waterways such as the Mississippi or the Great Lakes. Invasive aquatic species can have many of the same problematic effects in lakes and rivers that terrestrial species have in forests or prairies. However, removal of aquatic invasive species can be a lot more complicated since they are underwater, and don’t always stay in one spot. Preventing the spread of aquatic invasives to new waterbodies is the best way of protecting water resources, and knowing what those species are is one way of preventing spread. So, in this post, we will highlight a snapshot of some of the most common invasive aquatic species found in Minnesota.

Zebra Mussel

First on our list in the infamous zebra mussel (Dreissena polymorpha), a species that is dreaded for many boat and lakeshore owners across the Twin Cities. Zebra mussels first arrived in the United States through the ballast water of cargo ships that was dumped into the Great Lakes. They were first confirmed in Minnesota in Duluth in 1989, and from there they have spread quite prolifically. Zebra mussels cause many problems. First, they reproduce like crazy, with females producing 100,000-500,00 eggs per year. They attach to any hard surfaces, including each other, and can quickly grow to densities of 10,000s in a single square yard. They will attach to docks, boats, and water equipment, as well as rocks along shorelines and swim areas. Their shells are particularly hard, and can cut up swimmers’ feet (I actually know someone who had to get stiches from a zebra mussel induced cut). Arguably, the biggest problem with zebra mussels is the harm they cause to lake biology. Zebra mussels feed on plankton by filtering water through their bodies. A single individual can filter a quarter of water in a single day, stripping the plankton from it. All that removed plankton results in super clear water. This may sound good, but clearer water means more sunlight reaching aquatic plants on the bottom which means increased plant growth. Increased plant growth in turn can upset the nutrient balance in the water, leading to fish kills and plant die-off. Additionally, the lack of plankton limits the food availability to other aquatic species that feed on it.

What It Looks Like: Zebra mussels have hard d-shaped, stripped shells. They can vary in size from a quarter of an inch to over one-half inches long. They attach to hard surfaces and form dense clumps. 

Regulations: It is illegal to possess, import, purchase, transport, or introduce zebra mussels in Minnesota.

What You Can Do: Always check your gear when leaving and entering waterbodies. Drain all water from your equipment after you leave—baby zebra mussels, called veligers, can survive in small pockets of water at the bottom of a boat for a long time. If you have a dock or shoreline, check it periodically for zebra mussels, or offer to participate in zebra mussel monitoring programs through local and state governments.

Carp

Invasive carp got a lot of hype a number of years ago when a series of videos went viral of silver carp leaping up into the air in hoards, landing in boats and physically hurting boaters when they hit them. There are multiple species of invasive carp present in Minnesota—common carp (Cyprinus carpio), bighead carp (Hypophthalmichthys nobilis), black carp (Mylopharyngodon piceus), grass carp (Ctenopharyngodon idella), and silver carp (Hypophthalmichthys molitrix). While different in appearance and biology, all of these fish have similar effects on Minnesota waters. They are prolific feeders who aggressively outcompete other aquatic animals for food. Each species feeds on slightly different prey, from mussels to vegetation to invertebrates, but combined, they threaten native fish by removing food sources, and threaten endangered native mussels by overconsumption of them. Both common carp and grass carp can alter water quality by releasing phosphorous through their feeding. The subsequent decline in water quality effects all levels of the natural lake community. Finally, silver carp, when startled by boat traffic, leap up to 10 feet in the air, potentially harming boaters. 

What It Looks Like: Carp are, on average, large feeder fish, but each species has its own appearance and identification markers. Check out online resources like the MN DNR for more information on each species’ specific appearance.

Regulations: All species of carp except common carp are prohibited invasive species, meaning that it is illegal to possess, import, purchase, transport, or introduce them in Minnesota. Common carp are regulated, meaning it is illegal to introduce them to the wild.

What You Can Do: Invasive carp are typically spread as juveniles through the release of live bait into waterways. Dispose of unused bait in the trash or specific live bait receptacles. Report any carp sightings or catches to the MN Dept. of Natural Resources.

Curly-Leaf Pondweed

This aquatic plant has been around for a while. First identified in Minnesota in 1910, curly-leaf pondweed (Potamogeton crispus) is believed to have been first introduced to this area way back in the 1880s. Like many of the native pondweeds, curly-leaf is a rooted, submersed plant. However, it can form dense mats that block out other vegetation, limit light penetration, and inhibit recreation. Curly-leaf pondweed does not offer the same habitat benefits of other aquatic plants, and by limiting plant diversity through competition, curly-leaf can damage water habitat for other species. It spreads through reproductive structures called turions that can get caught in gear and boats, and transported to other lakes.

What It Looks Like: Curly-leaf pondweed can be easily mistaken for other native pondweeds. It’s distinct by its lasagna-shaped leaves which have serrated edges. Leaves are arranged alternate around the stem. Turions look like small pinecones. 

Regulations: It is illegal to possess, import, purchase, transport, or introduce curly-leaf pondweed in Minnesota.

What You Can Do: Always clean all plant material, mud, and water from your boat and gear when leaving and entering a lake.

Eurasian Watermilfoil

Eurasian watermilfoil (Myriophyllum spicatum) is similar to curly-leaf pondweed in many of the impacts it has on Minnesota waters. In fact, these species are often found in lakes together. Eurasian watermilfoil was first discovered in Lake Minnetonka in the 1980s. It likely spread through boaters unknowingly transporting it to other nearby lakes. Like curly-leaf pondweed, this aquatic invasive forms dense mats which crowd out other plants, and block the surface for recreationists. Its roots survive the winter, allowing it to quickly start growing in the springtime. By outcompeting native plants, Eurasian watermilfoil also decreases biodiversity and degrades aquatic habitats.

What It Looks Like: Eurasian watermilfoil consists of four feather-like leaves arranged in whorls around a central stem. The leaves comprise of 12-21 smaller pairs of leaflets. Though it can look similar to northern watermilfoil and coontail, two native plants, it has different numbers of leaflets, and becomes limp when removed from the water. 

Regulations: It is illegal to possess, import, purchase, transport, or introduce Eurasian watermilfoil in Minnesota.

What You Can Do: Always clean all plant material, mud, and water from your boat and gear when leaving and entering a lake.

Starry Stonewort

The final species in our invasive aquatic snapshot has garnered a lot of buzz the past few years in Minnesota. Starry stonewort (Nitellopsis obtuse) is a relatively new invasive aquatic macro-algae in Minnesota, having only been discovered in a few lakes. Initially found in 2015, it is only known to be present in 13 lakes. Due to its newness, there is a large effort going into monitoring for its occurrence in other lakes, as well as attempting to remove it completely from the lakes where it’s been found. This removal can be difficult though, since starry stonewort grows in dense mats, and its single-cell structure allows it to break easily. Like the two previously listed species, starry stonewort affects water habitat through its ability to form dense mats and outcompete native vegetation. Additionally, because starry stonewort likes to collect near shorelines, it can clog boat ramps and water accesses.

What It Looks Like: Starry stonewort is a micro-algae, meaning it has no vascular structure. Though it looks like a plant, each stem is a single cell. Stems are bright green, branching, thin, and crush easily underhand. Although similar to native stoneworts, starry stonewort is distinguished by star-shaped bulbils that function as reproductive structures.

Regulations: It is illegal to possess, import, purchase, transport, or introduce Eurasian watermilfoil in Minnesota.

What You Can Do: Always clean all plant material, mud, and water from your boat and gear when leaving and entering a lake. If you find starry stonewort, or something that looks like it, report it to your local DNR invasive species specialist. You can also participate in volunteer monitoring events such as Starry Trek for discovering and tracking new occurrences of starry stonewort.