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Which aquatic species are equipped for a warmer future?
October 1, 2026 |
Researchers from the aquatic research institute Eawag study how changed whether conditions can affect aquatic life. Long periods of heat do not only increase water temperatures but also influence many other factors. This is why it is important to study how organisms respond to multiple environmental changes. In a study published in Global Change Biology, they found that warm-loving species perform well when resources are scarce, giving them a significant advantage in a warmer future. We spoke with Anita Narwani from the Department Aquatic Ecology to understand what happens in Swiss lakes during prolonged heat waves.
Anita, why are heat waves such a concern for lakes and rivers?
Heat waves don't just make the water warmer. They can change many of the physical and chemical conditions that aquatic life depends on. Warm water holds less oxygen, but many organisms need more oxygen when they are warm because their metabolism speeds up. This can be particularly stressful for animals that are adapted to cold, oxygen-rich water.
In lakes, there is another important effect. Previous research shows that warmer conditions can lead to longer and stronger periods of thermal stratification—when warm surface water sits on top of colder deep water and the two layers mix less. This means that lakes in Switzerland will tend to have longer periods in which the warm surface waters are well-lit but nutrient poor, while deeper waters will be cooler, darker and nutrient rich. The important point is that climate change can therefore change temperature, oxygen, light and nutrients at the same time. Aquatic organisms have to cope with this whole package of changes rather than temperature alone.
Levasseur et al. 2025 used experiments to investigate how combinations of changing lake conditions affect the growth of phytoplankton – the primary producers of nutrients forming the base of lake food webs. In their experiments they artificially generated many combinations of temperature, light and nutrient availability and documented which Swiss phytoplankton species and strains performed best under these novel sets of conditions.
Which species are likely to benefit, and which are likely to struggle?
One of the most interesting findings from Levasseur et al. 2025 is that some phytoplankton appear particularly well equipped for a warmer, more nutrient scarce future.
The researchers compared 19 populations of freshwater phytoplankton, representing 17 species isolated from Swiss lakes, under different temperatures, and (at the same time) different amounts of light, nitrogen and phosphorus. They found large differences between species, and importantly the differences among species changed with changing temperature. This meant that warming could completely change who is best competitor for scarce resources.
While exactly who wins in a competition varies with temperature, cyanobacteria stand out as potential winners in a warmer world. In the experiment, cyanobacteria tended to perform better than most other phytoplankton in both warmer waters with less light.
That combination could be especially useful in a warm, stratified lake. This finding is also consistent with observations that many cyanobacteria that profit from climate change tend to grow deeper in the lake, around the thermocline or metalimnion - the areas in the water, where temperature changes most rapidly with depth. There is less light there than at the surface, but they may have better access to nutrients coming from deeper water. The infamous blooms of Planktothrix rubenscens (often called the Burgundy-blood-alga) in many Swiss lakes, including Lake Zurich, are good examples of cyanobacteria taking advantage of this niche promoted by a warming climate.
So, their advantage isn't simply that "cyanobacteria like it hot." Some have a useful combination of traits: tolerance of high temperatures plus ability to grow with little light plus potential access to deeper nutrient supplies. Levasseur et al. 2025 conclude that their results support the idea that cyanobacteria could be favored by both warmer temperatures and limited light, although they also caution that their experiment contained only three cyanobacterial populations.
Can species adapt to climate change quickly enough, or is evolution too slow?
Evolution can sometimes happen surprisingly quickly, especially for microorganisms such as phytoplankton, which can go through many generations in a single year.
But evolution isn't the only way ecosystems respond to climate change. The species that are already there are different from one another. As environmental conditions change, some may become more successful while others become less successful.
Levasseur et al. found that some of the phytoplankton that were best adapted to warm temperatures also needed relatively few resources to survive and grow.
That means we could see major ecological changes without waiting for species to evolve. A lake community can change simply because the winners and losers change.
Over longer periods, evolution may also help some populations adapt – certainly experimental studies demonstrate that phytoplankton can evolve to withstand warmer temperatures. For slower-growing organisms such as fish, evolutionary change is likely to be much slower than for phytoplankton and may not rescue populations.
Why do warmer lakes often become nutrient-poor at the surface, and why does that matter?
Imagine a lake in summer as a layer cake. The sun heats the upper layer. That warm water is lighter and tends to stay on top, while colder, heavier water remains below. If the difference between the layers becomes strong, wind has difficulty mixing the lake from top to bottom.
The result can be that the surface water is very warm, has a lot of light, but increasingly few nutrients. The Metalimnion, which is the middle transition layer in a stratified body of water, is cooler, has less light and has more nutrients, similar as deeper water. The deep water is cold, has almost no light, but is rich in nutrients.
Previous studies show that warming temperatures can extend this stratified period and reduce the transport of nutrients from deep water back toward the surface.
Levasseur et al. 2025 then asked the next question: What does the combination of warmer temperatures and changing resource availability do to phytoplankton?
Their answer was that it depends strongly on the species. An especially interesting result was that the temperature at which phytoplankton were best at competing for scarce resources tended to be about 5°C cooler than the temperature at which they could grow fastest when resources were plentiful. So simply measuring the temperature at which an alga grows fastest doesn't tell us whether it will be successful in a real lake. Temperature, light and nutrients interact.
What does all this mean for the food web, fish, biodiversity and water quality?
Phytoplankton may be tiny, but they form the foundation of a lake's food web. If climate change influences which phytoplankton dominates, the consequences can therefore spread throughout the ecosystem.
Cyanobacteria are particularly important here. Many are relatively poor food sources for the small zooplankton that graze on phytoplankton. Levasseur et al. 2025 point out that this poor nutritional quality means that this shift toward more cyanobacteria can have consequences further up the food web. Some cyanobacteria also form blooms that contain toxins which can cause problems for drinking water quality, recreational activities and aquatic ecosystems.
At higher levels of the food web, climate change may particularly disadvantage cold-water fish. In Swiss lakes, whitefish (Coregonus spp.) and alpine minnows (Phoxinus spp.) are examples of fish well adapted to cold water. Their response will depend not only on temperature itself, but also on oxygen conditions and changes in the plankton and invertebrates they feed on. Populations of other genuinely warm-water fish species may expand as lakes warm.
The bigger message is therefore not simply "warmer lakes mean more cyanobacteria". It is that rising temperatures change several environmental conditions simultaneously, and species differ greatly in their ability to deal with those combinations. Levasseur et al.'s experiments suggest that some warm-adapted cyanobacteria already possess a particularly useful set of traits for the conditions expected in future lakes.
That can change the organisms at the very bottom of the food web. And these changes at the base of the food web can have cascading effects on zooplankton, fish, biodiversity and ultimately water quality.
Cover picture: Cyanobacteria on the surface of Greifensee. (Photo: Eawag, Stephanie Eyring)