We will no longer be supporting IE7 and below as a web browser effective June 1st 2020. Click here for more information.

Sign In

Lake Simcoe Region Conservation Authority

Skip Navigation LinksLake Simcoe Region Conservation Authority > Starry Stonewort


Starry Stonewort

Lake Simcoe's Newest Invader​

Aquatic plants are a natural, normal and necessary part of a healthy lake ecosystem and provide many benefits for aquatic life. In Lake Simcoe, there are 18 species of native aquatic plants and three invasive species. Unlike native species, invasive species can wreak havoc on the lake, greatly affecting species diversity and changing shallow water​ habitat.

Starry stonewort was found in Lake Simcoe in 2009 and has continued to spread. During summer, it can double in amount every two days, forming dense cloud-shaped mats that do not respond to any herbicides. In fact, if herbicides are used to kill off other aquatic plants, that only serves to give starry stonewort a greater opportunity to take over.

What is Starry Stonewort?

Starry stonewort is an aggressive macroalgae native to Eurasia. Macroalgae is a collective term used for seaweeds … and like seaweed on the coast, starry stonewort is likely to wash up on your shoreline.

Starry-stonewort-bulbi.jpgThis grass-like algae spreads rapidly and forms dense, impenetrable stands. It's named for the six-pointed, star-shaped white “bulbils" (a small seed-like structure) that are produced and deposited into the mud to allow starry stonewort to survive the winter. It develops branching stems up to two metres in length, in water depths of two to 10 metres.

Starry stonewort can outcompete other aquatic plants, harming habitat for fish and wildlife by reducing cover and food sources. Its dense mats can also impact boating, swimming, and other recreational activities in Lake Simcoe.​

Starry stonewort (left and right) growing into and over a patch of native muskgrass.

starry stonewort overgrowing native macroalgae

A shallow water plant community, dominated by Eurasian watermilfoil (another invasive plant), still retains species diversity and a "forest-like" structure with habitat and sh​elter for smaller fish.


When starry stonewort takes hold, it causes a loss of aquatic plant diversity, shallow water habitat and fish shelter. It forms a "wall" pushing smaller fish offshore towards predators.


Where was it found?

Starry stonewort was first documented in 2009 in a sample collected for benthic invertebrates. Since then, it has been documented multiple times in Lake Simcoe.  ​

Starry Stonewort (2009 - 2018)​​

Starry-stonewort-map.PNG starry-stonewort-samples.PNG

How did it get into Lake Simcoe?​

​Starry stonewort arrived in the St. Lawrence River in 1974 and likely made it's way to Lake Simcoe through the Trent-Severn Waterway. 

Fragments of starry stonewort can get tangled in trailers, motors, anchors and inside watercrafts (boats, canoes and kayaks). Small bulbils can also stick to anchors, ropes, fabric and footwear. These small fragments are enough to start new growth and allow the algae to spread quickly.

Since 2009, starry stonewort has spread to all areas of Lake Simcoe and by 2018 it made up 67% of the total amount of plants in the lake. The Conservation Authority monitors the amount and locations of starry stonewort every year and in 2020 the amount suddenly decreased. This population collapse continued in 2021 but the amount of starry stonewort increased again in 2022. Our lake research team is investigating what caused these population changes and if it holds a clue for managing the amount and spread of this invasive species.

Rare female starry stonewort discovered in Lake Simcoe

An interesting fact about starry stonewort is that the macroalgae has separate male and female plants. Since 1974, all starry stonewort found in North America have been male, indicating that the invader spread by fragmentation, or with the star-shaped bulbils. 

In 2022, our lake research team, in collaboration with researchers at Toronto Metropolitan University and Ontario Tech discovered female starry stonewort plants in Lake Simcoe and Lake Scugog. This exciting discovery was recently published in the scientific journal Botany​ (read with a subscription). We are continuing our research to understand why and how, female starry stonewort suddenly appeared in our lakes.

Left photo: starry stonewort male with red-orange antheridia. Right photo: Starry stonewort female with white spherical oogonia​.

Starry stonewort male with red-orange antheridia Starry stonewort female with white spherical oogonia.What can you do?

Starry stonewort is difficult to remove once it has taken hold because harvesting and herbicide have little to no impact.

If starry stonewort gets tangled in your props, don't disentangle and throw it back into the water – this contributes to its spread. Instead, dispose of it away from the water, on land, and ensure that all boats, motors, trailers and fishing gear are cleaned, drained and dried, before moving to a new location.

If you spot starry stonewort or any other invasive species, please contact the Invading Species Hotline, 1-800-563-7711.

What are we doing?​

​​To better understand what types of aquatic plants live in Lake Simcoe, we monitor where plants are found (and in what amounts) and record the number of species as part of our Lake monitoring program. This year, we will move from a five-year plant survey to an annual survey that will help us identify changes to aquatic plants, forecast weed growth, and better detect new invasive species.

We are one of the only conservation authorities in Ontario able to monitor a complete lake ecosystem. Lake monitoring tracks things like phosphorus levels, dissolved oxygen levels, other parameters like chlorides, heavy metals, suspended solids, fish, aquatic plants, macroinvertebrates, and water clarity. 

These measurements provide a snapshot of water quality and, over the long-term, provide an understanding of emerging trends, issues and patterns, such as the rise and fall of the invasive zebra mussel​, which has now been out-competed by its cousin, the quagga mussel.