For perspective on how climate change is affecting lakes, those of us here in the U.S. can just look across the pond, where scientists and the agencies involved in meeting the European Union’s Water Framework Directive have amassed an impressive body of research on the topic.
Not only are extreme weather events such as droughts and intense rainstorms becoming more common, climate warming is leading to increased algal growth and more frequent toxic algal blooms. It also affects the entire aquatic food web, including the number, size and distribution of freshwater fish species, according to the latest research.
New evidence from studies in Europe shows that a warming climate, in particular, is already having a profound impact on lakes, according to Dr. Erik Jeppesen at Aarhus University in Denmark. As I have noted in earlier posts, this is an important issue because other studies show that lake temperatures are on the rise throughout the world.
Two leading European freshwater research programs are REFRESH, studies of adaptive strategies to mitigate the impacts of climate change on freshwater ecosystems, and MARS, focused on the management of freshwater lakes, rivers and streams under multiple stressors, including climate change.
I learned about the extensive research by Jeppesen and his colleagues while attending the Joint Aquatic Sciences Meeting (JASM), a first-ever gathering of four freshwater science societies, in Portland, Oregon, in May this year. We were there as members of the Association for the Sciences of Limnology and Oceanography (ASLO). Jeppesen, who like me is also a member of the Global Lake Ecological Observatory Network (GLEON), gave a talk about the impacts of climate change on lakes and freshwater fish in Europe. Below is a summary of what they are finding in Europe and what they propose doing about it.
Climate warming is having a “eutrophication-like” effect on lakes
Among the impacts of climate change I’ve already written about, climate warming exacerbates lake eutrophication, a natural aging process whereby a lake becomes more enriched with nutrients and algal growth over time. This process, sometimes called “cultural” eutrophication because it is accelerated by nutrient pollution from humans (think Lake Erie), has become one of the greatest problems facing lakes throughout the world.
As water temperature increases, it has a similar effect on a lake as increasing nutrient loading, although the mechanisms are different, Jeppesen says. The natural mechanisms that control phytoplankton growth weaken in a warmer climate. The lake’s growing season is longer, the nutrients are more readily available, and predation on phytoplankton is lower. This leads to more algal growth.
Climate warming creates ideal conditions for algal blooms
Jeppesen’s research suggests that the more eutrophic a lake is, the more sensitive it is to warming water temperatures, especially in northern temperate lakes. Part of the reason is that eutrophic lakes tend to have large stores of nutrients in the sediments. With climate warming and less winter ice cover in recent decades, deep lakes remain stratified longer, with warmer water near the surface and cooler water at depth. Less mixing and a lack of oxygen in the deeper layers create ideal conditions for algae-loving nutrients, such as phosphorus, to be released from the sediments.
Higher temperatures in shallow lakes also leads to higher release of phosphorus in the summer, when algal blooms prevail due to higher metabolism in the lake bottom. Warmer water at the surface creates ideal conditions for algal blooms, including toxic ones. “Cyanobateria like it hot,” said Jeppesen (citing Professor H. Pearl in the U.S.), “which is part of the reason why we’re seeing more toxic algae blooms.”
Climate warming is affecting the food web and fish populations
Another well-documented effect of climate warming is on fish populations, which play a key role in the trophic dynamics of lakes. Researchers are finding changes in the types of fish and the size and age structure of the fish population in recent decades, Jeppesen says. There has been a shift towards a dominance of fish species that can tolerate or adapt to a wide range of temperatures. Some of these species are what Jeppesen calls “bad guys” because they feed directly on zooplankton, such as water flea, that help control phytoplankton growth.
In Europe, the food web is being disrupted as lakes warm. The trend favors species such as bream and carp, which prey on zooplankton instead of some salmonids, pike and perch, which are considered “good guys” because they can prey on smaller fish. Bream and carp also disturb the sediment, leading to increased nutrient release.
Warmer lakes favor fish populations with smaller and fast-reproducing individuals, which is similar to the effect eutrophication has on lakes. A study published in the Journal of Limnology, in 2014, stated, “The response of fish to warming has been surprisingly strong, making them ideal sentinels for detecting and documenting climate-induced modifications in freshwater ecosystems.”
Climate warming and nutrient enrichment have a synergistic effect
The effects of nutrient enrichment and climate warming are a “double-whammy” for lakes. Evidence suggests that nutrient loading and warming water temperature have a synergistic effect, leading to further degradation of water quality than would be expected with one or the other mechanism. This can be understood by looking at lakes that have low nutrient inputs across a wide range of climatic zones or where management interventions have successfully reduced excessive nutrient loading.
One example of the latter is Lake Maggiore, in northern Italy, where the phosphorus loading declined from 1970 through the mid 1990s, and then leveled off through 2010. Surface water temperature has been on the rise during this same period. In the mid 1990s, the population of coldwater fish species such as trout and whitefish declined dramatically in Lake Maggiore, as determined by fisherman catches, while warmwater species such as shad increased. This shift occurred despite a reduction in nutrient loading that would have otherwise favored the larger, coldwater fish species living in more nutrient poor waters.
Further reducing nutrient loading is a win-win for lakes
If water quality managers are frustrated by the slow response of lakes to nutrient loading reductions, they have good reason to be. Warming water temperatures are making the challenging task of achieving water quality goals a whole lot harder. While it is difficult to separate out the climate signal from other stressors on lakes, especially eutrophication, the latest research is closing in on doing just that. Based on the data from European lakes, experts are calling for even more reductions in nutrient loadings to offset the impacts of rising water temperatures.
Addressing the problem of eutrophication requires additional reductions in nutrient pollution than would be necessary in the absence of human-induced climate change. The take away message from Europe is that nutrient reduction efforts need to be stepped up, not just to meet today’s water quality goals, but also to increase the lake’s resilience to warming trends that are already underway.
“It’s a win-win,” said Jeppesen. “The benefits are clear because of the synergistic effects between temperature and nutrient loading. Reducing the amount of nutrients available also increases the resilience of a lake to climate change.” In an earlier post, I noted that this is an approach being embraced on Lake Tahoe, and more recently by the USEPA, which is developing climate change resiliency criteria that some Great Lakes restoration initiative projects will have to meet to receive federal funding.
Jeppesen delivered his message with a sufficient dose of Danish-style humor, at one point showing a slide of a seal sunning itself on a rock with a cartoon-like thought bubble, “It takes time. Let’s wait and see…” He said, “Even though we humans may change our minds from day to day about whether climate change is real, depending on how hot or cold the weather is outside, the lakes and their biological communities are feeling it. They know global warming is real. The fish know it’s getting warmer.” Presumably the seals do, too.
For climate adaptation planning purposes, it’s not just hypothetical what might happen to lakes. There is enough evidence on the impacts of climate change in recent decades and from comparing lakes across different climatic zones to take action now. Jeppesen said, “It’s time to act!”
Lisa Borre is a lake conservationist, freelance writer and avid sailor. With her husband, she co-founded LakeNet, a world lakes network, and co-wrote a sailing guide called “The Black Sea” based on their voyage around the sea in 2010. A native of the Great Lakes region, she served as coordinator of the Lake Champlain Basin Program in the 1990s. She is now an active member of the Global Lake Ecological Observatory Network.