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A dangerous creature lurks in the waters surrounding the state of Michigan. To the west, the gentle waters of Lake Michigan wash onto the white beaches of the Sleeping Bear Dunes National Lake Shore. Along with them, the Patriarch, a 21-year-old common loon and a local celebrity, washes ashore, paralyzed.

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And he isn't alone. 1,400 other loons were amassed dead on the lake shore. From what? Avian botulism.

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Botulism is caused by a neurotoxic bacteria that attacks the nervous system of birds. Even though it's naturally occurring, outbreaks have been abnormally frequent since the late 1990s, and these changes have been connected to a certain culprit lurking in the water's depths.

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Meanwhile, the opposite coast of Michigan is haunted by the dangerous waters of Lake Huron-Saginaw Bay.

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Every year, recreational contact advisories warn beachgoers of seasonal eutrophication, also known as algae blooms, of the neurotoxic blue-green algae or cyanobacteria. These algae blooms produce cyanotoxins, which are toxic to humans and other creatures.

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More deaths by neurotoxins.

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Although they were effectively controlled in this region in the 1980s, these algae blooms have been returning with increased frequency since the late 1990s. Sound familiar?

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Both cases of increased neurotoxins in these two bodies of water can be attributed to a tiny creature haunting the lake floors.

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I'm Maya. And I'm Harsimrin. And welcome to our podcast where we dive into the complexities of this prolific pest.

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Zebra mussels. Discovered in the late 1980s in Lake St. Clair between Michigan and Ontario, the zebra mussel is a small mollusk with a zigzag pattern across the shell. As an invasive species, it is native to the Black and Caspian Sea of Western Asia and Eastern Europe.

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Since their introduction, they have had devastating ecological impacts in the Great Lakes region, as they are able to colonize the lake floors in massive quantities.

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Apparently, as of 2023, there are an estimated 750 trillion mussels in the Great Lakes system. They're considered a form of biological pollutant due to their permanency as populations and because of their reproductive and competitive capabilities that make them extremely difficult to mitigate once they're introduced.

SPEAKER_05 3:58

That's crazy. Hearing about the potential of zebra mussels to spread makes me wonder about native mussel species. I bet they have different reproductive methods.

SPEAKER_02 4:07

Yeah, actually, native mussels simply don't reproduce as fast as zebra mussels do. Native freshwater mussels are part of the Union Day family, while zebra mussels are part of the Dracindae family. They're both bivalves, meaning they're soft-bodied organisms that live between two shells. But mussels native to the United States uniquely rely on a host fish to spread their muscle larva.

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Female native mussels release eggs called glochyta, which attach to host fish species and establish a parasitic relationship to them, relying on their hosts for nutrients. Then, once they grow large enough, they fall back onto the lake floor.

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And unlike native mussels, zebra mussels are able to reproduce at much quicker rates because of their physiology. They do this through a method called spawning, which typically occurs during the spring and summer. They release large numbers of egg and sperm into the water, and fertilization happens externally. One female will produce an average of a hundred thousand adult muscles in a lifetime, which can result in them producing up to a million eggs every year.

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Insane. And in these quantities, they mat over hard surfaces, completely covering them with tiny muscles. Isn't that crazy? Yeah, it is. The first muscle life stage, larva, is microscopic. And then the next stage, villagers, have tiny hair-like fins which help them swim freely through the water. Then, as baby muscles, they settle onto the ground and use bisyl threads to chemically and mechanically attach themselves to virtually any hard surfaces. That includes those native muscle species. Zebra mussels are much smaller than the average American mussel and can therefore grow directly onto them, which physically inhibits their ability to move.

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Also, zebra mussels are able to withstand many different types of environments, allowing them to survive and proliferate in almost any body of water. They can also survive outside of water for several days and cause issues by hitching rides on boats into new freshwater systems when boats have not been properly cleaned. Being bivalves, they filter feed by selectively filtering zooplankton, phytoplankton, and nutrients from the water.

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As filter feeders, one mussel can filter around one liter of water per day. By doing that, they are filtering out nutrients in the water that native species are dependent on to survive. This poses a significant threat to the well-being and survival of native species because it essentially gets rid of their food, decreasing their chance of survival in environments where zebra mussels are present.

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A recent example that relates to this is actually from 2021, when zebra mussels were found hiding in mossballs that were being sold as aquarium plants, and this was first reported in a pet coast store in Seattle.

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Mossballs are a species of green algae that forms into a ball with a diameter of two to five inches. They're typically found in pet stores or in home aquariums. During this incident, zebra mussels were discovered in mothballs across 46 states in the country. Due to their reproductive abilities, with them being in aquariums across the country, the possibility that infested water could infect native freshwater ecosystems increases dramatically. To prevent this from happening, the U.S. Fish and Wildlife Service, the Federal Aquatic Nuisance Task Force, and Regional Aquatic Invasive Species Management Groups all put out national alerts to let people know that if they purchased mossballs around this time, they needed to not use them and properly dispose of them. Zebra mussels also have cascading ecological impacts.

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Because zebra mussels are filtering through the water, they're actually increasing the water clarity that lets sunlight reach farther into the lake. This means that more sunlight is hitting the bottom of the lake and more algae is growing, which might seem like a good thing because there's more plant growth, but this algae growth actually contributes to the spread of illnesses like avian botulism. In general, freshwater mussels are known to be contaminated populations due to the pollutants we pour into trapped freshwater systems.

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As filter feeders, zebra mussels can also be thought of as accumulating bacteria in their bodies as they filter water. So far, it might seem like zebra mussels are always causing harm to their environments, but there are actually some positive aspects to them being filter feeders.

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I recently read this article by Maria Alexandria Bigou et al. that was titled Assessing Microbial Contamination and Antibiotic Resistant Bacteria Using Zebra Mussels, and it was really interesting because it talked about how zebra mussels could be beneficial to humans due to their abilities as filter feeders. During their study, they found that zebra mussels can filter bacteria and retain maximum levels of it for up to two hours after exposure. In fact, it was also found that 33% of the bacteria that was isolated from muscles was resistant to multiple antibiotics.

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This shows the resistant that is present in surface waters and highlights the need for safe water management. This is a big deal because zebra mussels are really good at detecting resistant microbes, which means they can be used as an easily accessible, low-cost way to assess the quality of sanitary water across the United States. This is a really unique example at showing how the biology of zebra mussels can be used strategically in a way that can benefit the environment.

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It's super interesting to see how this results in them becoming these bodies of contaminants and pollutants of sorts and them being utilized by researchers.

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Another example of bacterial accumulation in zebra mussels connects back to our original story about the patriarch. Outbreaks have infrequently occurred since the 1960s, although their rates have intensified since the 1990s, with the most recent in 2012 killing the famous local loon. Since 2006, 4,300 birds have died in sleeping bear alone. And almost every bird that has died from botulism has had gobies in its stomach.

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According to Kino et al. zebra mussels and loon population geographies, there's strong evidence that botulism accumulates up the food web from zebra mussels and is responsible for these increased avian outbreaks. Botulism is naturally occurring, but outbreaks rely on conditions to be warm, the water to be oxygen-starved and full of dead plant matter. Exactly what was occurring in the waters at Sleeping Bear National Lakeshore.

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By creating conditions that allowed for more sunlight to reach the bottom, more algae grows and then dies, and this excess dead plant matter depletes oxygen levels while simultaneously increasing nutrients in the water, creating the perfect environment for the botulism bacteria to thrive. In this ecosystem, round gobies eat zebra mussels, and they are then eaten by loons who are native to the region. Alongside this journey of the food web, the concentration of botulism that originates in the zebra mussels increases in toxicity in a process called bioaccumulation.

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The twofold effect in this case exemplifies the secondary and even tertiary ecological impact that zebra mussels can have at their currently unmanageable state.

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Do you know about this?

SPEAKER_02 12:05

No, go on.

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Well, tuna can weigh over 1,000 pounds, meaning they obviously need a lot to sustain themselves. They're also apex predators in their environment, meaning they eat lots of smaller prey, who, in the ocean, have fairly low amounts of methylmercury, although it accumulates and concentrates in tuna over their lifetimes. And this is the reason why the Food and Drug Administration recommends that adults don't eat more than three servings of tuna a week, because it can accumulate in our bodies as well.

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Interestingly, in aquatic systems, the neurotoxic form of mercury, methylmercury, is transformed into such by microbes. When dense mats of zebra mussels cover the lake floor, it reduces the ability for sediments to be dispersed in addition to increasing waste, which are favorable for these microbes to create toxic methylmercury.

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A study on fish populations by the Minnesota Aquatic Invasive Species Research Center looks at zebra mussels and mercury bioaccumulation in native fish species of Cass Lake Minnesota. The researchers sampled different forms of carbon from fish, macroinvertebrae, and zooplankton tissue and compared them to mercury concentration values in the area to determine where the organisms were eating.

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These results showed that important local species called walleye and perch in lakes with zebra mussels had 72 to 157% higher mercury concentrations in their tissues. In addition to ecological changes, these changes in fish also affect food sources of neighboring populations. In particular, the Red Lake Band of Chippewa Indians of north-central Minnesota. This group operates the only commercial walleye fishery in Minnesota and it threatens their livelihoods along with other fishermen who rely on the equilibrium of their native unadulterated ecosystem.

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Managing zebra mussels requires community members to aid against their uncontrollable spread. In an effort to combat them through community action, the Texas Parks and Wildlife Department made this song on ways that the public contributes to the solution.

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That's why OU so clean, drain, dry your boat.

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Since zebra mussels can attach themselves to hard surfaces, boat owners are always at risk of potentially helping zebra mussels spread across waters if they're not careful. To prevent this, boat owners have to be cautious and make sure that they're thoroughly cleaning their boats, especially if they've recently traveled in infested water.

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Zebra mussels also pose threats to people who like to visit beaches. The shells of zebra mussels can wash up on the shore of beaches and cut people's feet because of how small and sharp they are.

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I think it's safe to say that if my native beach was infested with zebra mussels, I would not be happy.

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I'm glad you brought that up because I recently spoke with a friend who is native to Chicago and has plenty of experience with zebra mussels.

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Hi, my name is Louise. My name is Claire, and we're both from Chicago on beautiful Lake Michigan. Um we both have like pretty similar experiences with zebra mussels. I think that my first exposure was going camping as a kid. We went to this smaller lake in Wisconsin, and they had signs everywhere about making sure you clean your boats and watch the underside of docks and all these things because that's where the mussels were and all your yeah.

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And was on a pretty small lake in an area where there's a lot of little lakes like that. And I remember the US Sandau noticed it that we weren't allowed to go in the lake unless we brought water tools. And it was because of the deeper muscles. And there was all different technology. I remember that really scared me as a child. And I thought the deeper muscles looked really scary, and I was really scared of getting my feet titled, and so I refused to go in the lake culture of like teeny years after the market because I didn't want the zebra mussels to get me. And then I think that as I grew up, I kind of was more curious about what that was about, and I did a little bit of Google searching, a little research, and I think that's kind of when I learned more about like what zebra mussels were and what an invasive species was, and the environmental impact of invasive species like zebra mussels and how they take away the available phytoplankton and things like that, and how it undermines the entire ecosystem, which just kind of further made me scared of zebra mussels.

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Zebra mussels were likely introduced by boat through something known as ballast water. However, this is not at a recreational boating scale, but at a global shipping scale. Ballast water is utilized by deep draft commercial cargo vessels for steadying the ship while loading and transiting cargo. Historically, it was taken in or dumped out at major ports when ships needed to adjust the weight when loading and unloading. The total ballast water capacity for ships entering U.S. ports averages at 3.2 million gallons, although it can range up to 74 million.

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This provides plenty of opportunities for uptake of aquatic species from across the world and then their subsequent release at whatever the cargo's destination might be. Due to the shallowness of Lake St. Clair, where zebra mussels were first discovered, in order to lighten up, vessels must dump their ballast water to get from Lake Erie to Lake Huron, Michigan, and Superior.

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This is part of the reason why. The Great Lakes are the most invaded freshwater system in the world, with one new invasive aquatic species discovered every six to seven months in the 1980s. Because of this, the United States and Canada have made great strides in regulation aimed at minimizing the introduction of new invasive species. Ballast water exchange was first implemented in the late 1980s, essentially encouraging ships to flush out the ballast of near shore water and its associated organisms, replacing it with salty open ocean water in organisms that are less likely to be able to survive in the Great Lakes.

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Flushing of ballast water is successful because it disrupts osmoregulation, a mechanism by which organisms maintain internal cellular stability by regulating or adapting the salt content of their body fluids to change in the salt content of their external environment.

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Osmotic stress affects necessary metabolic functions and kills freshwater organisms. The sudden salinity increase causes organisms to experience significant osmotic stress, which results in high mortality.

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Something interesting that I recently found is that zebra mussels inconvenience hydroelectric companies because of how they settle and build up in their infrastructure causing damage. Zebra mussels are known to attach and build up inside water pipes, which can restrict water flow and cost these companies a lot of money to get their pipes cleaned and have chemical treatments done.

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It was estimated that between the years of 1993 and 1999, zebra mussels cost the power industry $3.1 billion. Which really puts into perspective how significant of a problem these are for hydroelectric companies. This not only affects the companies, but can also inconvenience regular people like us because of how it can get in the way of us being able to access clean drinking water if the issue is not handled properly.

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Zebra mussels aren't considered a form of biological pollutant due to their permanency as populations and because of their reproductive and competitive capabilities that make them extremely difficult to mitigate once they're introduced.

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Part of the issue of managing their spread is that it's difficult to treat such a widespread pest when other species in the system would also be at risk by the effective treatments. Either that or the treatments require too much money or intensive work. Let's go over some examples of what we've heard.

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Copper-based treatments are a popular option for molluscides.

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Aquatic organisms are sensitive to lower levels of copper and high levels are toxic.

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When in the water, it binds to their gills and can result in cardiovascular collapse in zebra mussels. When ingested, it gets absorbed into the gut, transported to the liver, and then incorporated into proteins that can potentially damage its DNA.

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In a scientific study at Pelican Lake, Minnesota, researchers aimed to determine a proper copper concentration for a treatment that would target zebra mussels without harming non-target species. They pumped in infested lake water with zebra mussel villagers, adult zebra mussels, and non-target species in various tanks. They treated each tank with one of four copper treatments and also monitored water chemistry, which means pH, temperature, alkalinity, dissolved oxygen, specific conductance, and total ammonia nitrogen.

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They found that the greater difference a site experiences in water chemistries meant a greater difference in copper toxicity. They found that site specificity is important for determining copper treatments in lake management. Treatments need to be created with careful understanding of the site in order to minimize off target effects on other species, although that makes the process much more difficult and intensive.

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This specificity and also necessary considerations of other organisms is part of the issue when it comes to controlling zebra mussels. The costs are too high.

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One example that got close to eradication is from Lake Winnipeg. The story starts in October of 2013 when one singular zebra mussel was found attached to a boat hull. This was the first time anyone had seen any evidence of there being zebra mussels in this body of water, so everyone immediately sprung into action.

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Because of the time of year that this happened, everyone was especially eager to get rid of any traces of zebra mussels because the water was going to freeze in a couple of weeks, and if any zebra mussels were left in the water, they would be hard to find later and would be ready to reproduce in the spring.

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They continued to monitor the lake and try really hard to stop any more zebra mussels from appearing. But unfortunately, in the summer, nine zebra mussels were found on a vessel in the lake. Now the focus is shifting from trying to eliminate zebra mussels to trying to stop them from spreading any further. And many watercraft inspection programs have been implemented to prevent zebra mussels from spreading by attaching to boats.

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I think this story is important because even though eradication wasn't successful, it really shows how difficult it is to eradicate them. Stories like this contribute to people's negative perceptions of zebra mussels and is also one of the reasons why some people think their introduction into large bodies of water is inevitable. Lake Waco is popular for recreation and also importantly provides drinking water to the area. It's at high risk of invasion because its water chemistry makes it highly conducive to muscle establishment. Because of this, since 2009, the city has been extremely proactive about management. They've installed signage, alerting lake goers about the risk of transporting zebra mussels on boats. They also have volunteer boat ramp inspectors who in 2014 detected a small colony of zebra mussels on a private boat ramp. The Texas Parks and Wildlife Department immediately removed the boat ramp, sighted the owner, and within three weeks had implemented a benthic barrier covering the area directly surrounding the boat ramp where they were first found.

SPEAKER_02 26:04

I know benthic refers to anything associated with the bottom of the lake, but what exactly is a benthic barrier?

SPEAKER_05 26:11

In this case, the benthic barrier was a 3,900 square meter gas impermeable plastic sheet. It took two days to lay plastic sheets across the entire section of the lake floor, and it was only removed after nearly five months. After they removed it, they found two live muscles under the mats, but they removed them, and ultimately the strategy was effective enough to hinder reproduction of the surviving population, and they have still shown no evidence of zebra mussels. The method was also successful because the impermeability of the barrier reduced dissolved oxygen content to suboptimal concentrations below the barrier, which also physically just stopped the bellagers from being able to disperse.

SPEAKER_02 26:53

Considering the proliferation of zebra mussels, I can imagine that this case was effective in great part due to the early action that was taken by the Parks Department.

SPEAKER_05 27:02

Exactly. However, with this method, obviously other organisms were not going to be able to survive either. Aquatic plants were suppressed and died, and harmful chemicals such as hydrogen sulfide and ammonia, which were produced under the low oxygen conditions under the barrier, may have been released afterwards. Shockingly, this reminds me of a Russian military policy that is devastating in similar ways. On multiple occasions, the Russian military has destroyed its own land instead of facing defeat. This has happened during World War I, World War II, and also most recently during their invasion of Ukraine. During World War II, the Russian army was surprised by a sudden German invasion in 1941 and were forced to evacuate. However, instead of just retreating, they burned down crops, villages, they destroyed bridges, factories, and water infrastructure all on their way out.

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The Russian army was able to save itself from defeat, but at the expense of its own people's lives. What a desperate way to respond. Imagine what the wreckage must have looked like for the people that lived there. How long did those effects last? This strategy effectively eliminated zebra mussels at the expense of everything else living in that area. Interestingly, this method mirrors the military strategy referred to as Scorched Earth, where an army will destroy its own resources to prevent the enemy from using them.

SPEAKER_05 28:33

Well, this begs the question: if zebra mussels can only be stopped through militant destruction of a native environment and its ecosystem, is it worth it? To what lengths are city officials, municipal water infrastructure employees, or conservationists willing to stop these creatures?

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Or are they inevitable?