Harmful Algal Blooms Coming to a Lake Near Us?

There is nothing more beautiful than looking down into the water on a clear day and seeing all the way down to the bottom of the lake, seeing the ways the sand ripples, maybe catching the eye of a fish swimming by. We are fortunate to experience this kind of water clarity most of the time, but although Lake Leelanau currently has high water quality, it is not a guarantee. Harmful algal blooms (HABs) threaten our water quality when too many nutrients in the lake lead to a bloom of a type of algae that releases toxins that can harm aquatic life, humans, and pets. Surface water phosphorus levels and warming lake temperatures have increased in both North and South Lake Leelanau over the past 30 years, which, combined, create a higher probability of HAB outbreaks. Read how to identify these dangerous blooms and what you can do to prevent them from occurring in our lake.

What are Harmful Algal Blooms?

The presence of harmful algal blooms in Northern Michigan lakes has attracted a lot of attention, thanks to widely published HAB events like those on Lake Cadillac (Wexford County) and Black Lake (Cheboygan/Presque Isle County). Although harmful algal blooms are more frequent in the southern lower peninsula, they occur throughout the state (follow this link to EGLE’s interactive HAB map).

Harmful algal blooms are dense concentrations of cyanobacteria, also known as blue-green algae, that have the potential to release toxins. Cyanobacteria are a diverse group of single-celled organisms that can produce their own food from sunlight and live in almost any habitat, including being part of our lake’s natural ecosystem. When conditions are favorable, HABs can develop quickly and last for days, weeks, or longer. The toxins released during HABs can cause death of fish and fowl on nearby shorelines and produce harmful conditions to aquatic life, humans, and pets.

Although harmful algal blooms are caused by cyanobacteria, they aren’t necessarily green in color. This is because the cyanobacteria that cause HABs are actually a group of different species, and the bloom’s appearance depends on which species is dominant. HABs can range in color from bright, almost neon green to chalky light blue to even reddish or brown. Check out EGLE’s informative video on how to identify HABs and how to distinguish them from potential look-alikes, such as harmless filamentous algae.

Examples of HABs showing different colors

What Causes Harmful Algal Blooms in Lakes?

HABs occur when a lake accumulates a disproportionate amount of nutrients, such as from the use of fertilizer and poor septic practices, and water temperatures become warm enough and the surface water calm enough to become favorable for rapid cyanobacteria growth. This means most HABs occur during the warm summer/early autumn seasons. Nutrients such as phosphorus and nitrogen enter the lake as they run off from rain events or watering across lawns and impervious surfaces, or seep through the groundwater from leaking septic systems, and accumulate in lakes over time. Those nutrients provide the food for HAB growth. Furthermore, invasive zebra and quagga mussels (both recently confirmed in Lake Leelanau) can help create the right water conditions for a HAB. This is because the invasive mussels eat by filtering algae out of the lake water, but they don’t eat the cyanobacteria and release those back into the water. Because the presence of invasive mussels means fewer algae to compete with, the cyanobacteria increase in abundance. The combination of high nutrients (especially phosphorus), warm and calm waters, and the influence of invasive mussels all combine to make the perfect recipe for a HAB.

Michigan Sea Grant HAB schematic drawing

Is Lake Leelanau Next?

The Lake Leelanau Lake Association recently published their lake water quality study showing trends for nitrogen and phosphorus levels in our lake. The study, conducted by Grand Valley State University in 2025, concludes that our lake is very clean indeed! However, both North and South Lake Leelanau have seen increases in surface water phosphorus levels and warming lake temperatures in the past 30 years, which combined, create a higher probability of HAB outbreaks. Even lakes that typically have low nutrient levels, like Lake Leelanau, can still be susceptible to HABs due to slight increases in nutrients in the lake.

Black Lake, in Cheboygan/Presque Isle County, is another low-nutrient lake in Northern Michigan that has suffered from HABs in the late summer/early autumn most years since 2019. The Black Lake Association solicited a study to determine where the phosphorus in the lake is coming from, and the results showed a pulse of phosphorus entering the lake at the end of the summer. The culprit: leaking septic systems. The Black Lake Association believes that the combination of a late summer phosphorus pulse from aging septic systems, combined with the filtering out of other algae by invasive mussels, helps create the right conditions for HABs.

HAB on inland lake shoreline EGLE

What Can You Do?

We, as individuals, have little control over the warming temperatures of our lake, and it appears that this warming trend will continue. Naturalizing the shoreline with native vegetative buffers, reducing impervious surfaces, maintaining your septic system, and discontinuing fertilizing lawns and gardens or selecting fertilizer without phosphorus will reduce the impact of nutrient runoff into Lake Leelanau. Lake water is an excellent alternative fertilizer, and the price is right! Contact the Lake Leelanau Lake Association for a complimentary shoreline consultation. This is a great way to explore ideas and obtain information and resources to identify ways to help reduce nutrient exposure to our lake.

If you see suspicious-looking growth in the water that you think could be harmful cyanobacteria, contact EGLE by either calling their Environmental Assistance Center at 800-662-9278 or emailing [email protected]. Because of the potential health risk HABs pose, do not go into water that looks like it could have a HAB, and prevent children and pets from entering the water. Remember the catchphrase: "When in doubt, stay out!”