How invasive species trigger algal blooms and linger toxins

We said countless times that algae blooms are a growing concern for many freshwater ecosystems around the world, and explored why and how they affect humans and nature. We even looked at how humans play key roles in this issue directly, by polluting the water systems, and indirectly, due to climate change.

However, there is a whole other dimension left to talk about: These blooms are often caused by invasive species that outcompete native organisms for resources. They are plants or animals that are not native to an area and can cause harm to the environment, economy, or human health. They can outcompete native species for resources, disrupt food webs, and alter habitats. They can spread naturally, such as on the feet or feathers of migratory birds, or be introduced by humans accidentally.

To be able to better predict the future challenges of water management for the sake of humans and the rest of the biosphere, we need to understand this worrying phenomenon and its effects. In this blog, this is exactly what we will do, looking at the case study of Lake Mendota.

Lake Mendota

Lake Mendota is a freshwater lake located in Madison, Wisconsin, USA. It is the largest of the four lakes in the Yahara River chain and covers an area of about 39 square kilometers. The lake is home to a diverse range of species, including fish, birds, amphibians, and other aquatic organisms. Some of these species, such as the common loon and the lake sturgeon, are considered threatened or endangered, making the preservation of Lake Mendota’s ecosystem critical for their survival.

Scientists are drawn to Lake Mendota for its unique properties and ecological significance. As one of the most studied lakes in the world, with detailed records going back over 40 years, Lake Mendota provides an ideal location for researchers to explore topics such as water quality, nutrient cycling, and aquatic ecology. Additionally, the lake’s long history of scientific investigation and monitoring provides a valuable baseline for understanding changes in the ecosystem over time

Using these baselines, a recent study carried out by University of Wisconsin-Madison scientists has shed light on the effects of two invasive species, the spiny water flea and the zebra mussel, on the microbial community of Lake Mendota in Madison, Wisconsin.

Invasive species

Invasive species are plants or animals that are not native to an area and can cause harm to the environment, economy, or human health. They can outcompete native species for resources, disrupt food webs, and alter habitats. Ecosystems take extraordinarily long to reach delicate balances, reaching a “goldilocks zone” where nothing goes to waste, and no one can “win”.

But invasive species have no natural regulatory mechanisms in their new environments, leading to catastrophic consequences. Invasive species can be introduced naturally, such as bits of plant matter hanging onto migratory birds, accidentally, such as through ballast water from ships, or intentionally, such as when people release pets or plants into the wild.

Scientists are thus interested in studying invasive species and their effects on ecosystems because they can have major impacts on the environment and human health. By understanding how these species spread and interact with native species, scientists can develop strategies to prevent and control their spread. The study found that the invasions of spiny water fleas, first observed in 2009, and zebra mussels, first observed in 2015, caused major changes in the microbial community of Lake Mendota.

Spiny water fleas and zebra mussels

Before we move on, let’s introduce the culprits. If you have any knowledge of North American aquatic ecosystems, you likely already shiver thinking about these two infamous pests.

Spiny water fleas are small crustaceans native to Eurasia that arrived in the Great Lakes region in the 1980s. They have a long spine, which they use for defense and for catching prey. They feed on small zooplankton, which are a vital part of the food chain in freshwater ecosystems. As very few animals in North America either prey on them or know how to defend themselves from them, these fleas have free reign to terrorize freshwater habitats, making other hunters go hungry.

Similarly, zebra mussels are small freshwater mussels native to the Black Sea region of Europe. They were first discovered in the Great Lakes in the late 1980s and have since spread to other freshwater systems throughout the continent. They filter feed on phytoplankton and are very efficient at it, which can have significant impacts on the food chain and the water quality of affected ecosystems. Moreover, they have a hard shell and can attach themselves to hard surfaces like rocks, boats, and pipes, which can cause significant damage to infrastructure and impact the recreational use of affected water bodies.


The study points out three main consequences of invasive species on the microbial community. Firstly, they looked at cyanobacteria themselves and found a worrying trend: the blooms occur earlier each year. This disrupts the natural cycle of the lake and can even lead to harmful algal blooms, preventing the clear phase that usually happens during spring. During this time, the water should be clear, and sunlight should be able to penetrate through to the bottom of the lake, allowing plants and various algae to photosynthesize and produce oxygen at all depths. This clear water also allows for the growth of native species such as algae and diatoms, which provide a food source for many small aquatic animals.

Additionally, the clear water stimulated the growth of submerged aquatic vegetation, which provides an important habitat for fish and other aquatic animals to hunt and rest in. By disrupting this phase, the entire food web suffers, leading to a loss of biodiversity and a potential ecosystem collapse. Moreover, the diversity of the cyanobacteria increased significantly: not just a few new species, but entirely different genera, families, and even orders.

Secondly, the concentration of toxins produced by cyanobacteria increased during the summer months. This increase was observed despite the fact that the total number of cyanobacteria remained relatively constant. The reason for this could be that different strains of cyanobacteria produce different types of toxins, and the invasive species were found to be promoting the growth of those strains that produce more potent toxins. Other environmental factors, such as changes in nutrient availability or variations in temperature, could also be driving an increase in toxicity. This is particularly concerning because cyanobacteria toxins can be harmful to both humans and animals that come into contact with the water, potentially causing skin irritation, respiratory problems, and even liver damage

Lastly, the populations of other bacteria in the lake have been changed due to the invasive species. This is a surprising observation as it was not thought possible before. Heterotrophic bacteria were considered to be significantly less affected by changes in the environment than other communities. These kinds of changes in the microbial community can have far-reaching consequences on the entire ecosystem of the lake, including the food web, nutrient cycling, and water quality. If this process is not understood soon, it has the potential to cause irreversible damage if not addressed.


These findings have significant implications for both water management and biodiversity. The study highlights the interconnectedness of microbes with the broader food web and their susceptibility to long-term environmental change. As a result, it is crucial to monitor and prevent algal blooms caused by invasive species in order to protect freshwater ecosystems and the organisms that rely on them.

The full effects of increased toxicity are not well understood, but they will need to be closely monitored, or else countless species, including humans, will suffer. These toxins will continue to accumulate within animals like shellfish and crustaceans and spread throughout the food web.

The study’s lead author, Robin Rohwer, emphasized the importance of long-term data collection and monitoring to help understand the impacts of invasive species on freshwater ecosystems. Such data can help scientists identify unexpected connections between microbes and the broader food web and can aid in the development of effective strategies for preventing and managing algal blooms.

The findings of the University of Wisconsin-Madison study demonstrate the far-reaching impact of invasive species on freshwater ecosystems and the importance of monitoring and preventing algal blooms. By protecting freshwater ecosystems, we can also protect human health, the economy, and the natural world as a whole.