Climate Change Already Having Profound Impacts on Lakes in Europe

Photo: Lake Maggiore, Italy. Credit: mbdortmund, Wikimedia Commons.
Lake Maggiore in Italy is an example of a lake already feeling the effects of climate change. In the 1990s, the population of coldwater fish species such as trout and whitefish declined dramatically. Photo Credit: mbdortmund, Wikimedia Commons.

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.

Photo: E. Jeppesen giving a talk. Credit: E. Jeppesen.
Erik Jeppesen talks about the effects of climate change on lakes and freshwater fish. Warming water temperatures favor fish species he refers to as “bad guys” because they feed on species that help control phytoplankton growth.They also disturb lake sediments, releasing nutrients that fertilize algal blooms. Photo courtesy of E. Jeppesen.

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.”

Image: Lake degraded by nutrient loading and warming. Source: E. Jeppesen.
A diagram showing changes in the food web of a Danish lake degraded by nutrient loading and warming. Note the smaller fish sizes and increased amounts of algae in the degraded and warm lake examples. Image credit: E. Jeppesen.

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.

Photo: Erik Jeppesen in the Azores. Credit: E. Jeppesen.
Danish scientist Erik Jeppesen holds a carp, which he calls one of the “bad guys” being found in warming lakes in Europe. Photo courtesy of E. Jeppesen.

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.


  1. Michael A. Mundy
    Coral Gables, Florida
    November 12, 2015, 9:27 am

    Thanks for the erudite article on the causal factors underlying the unhealthy state of Lake Maggiore and others in general globally. Unfortunately, as the naysayers commenting hereon highlight, better environmental and health promotion efforts are urgently needed to halt and reverse this increasingly dire ecological development. It is a primary means to create widespread acceptance of emerging related facts, science and solutions as interconnected new brain physiological and psychological research highlights in breaking down the “us versus them” tribal responses and thus affording greater collective focus on aligning human behavior for sustainable change. The US being no exception despite changing attitudes seemingly associated with a correlation between the frequency of many recent national, natural disasters, climate change and human activity and behavior. Particularly, where special interests through their surrogates – lobbyists, politicians and others – with impunity are adamant in maintaining the status quo to their benefits at grave costs to the environment, public good and welfare. Write on…you are on a honorable, noble path and thank you.

  2. Mike Riley
    July 22, 2014, 7:47 am

    Has there been any study of lakes inside the US? I’m with Logan Martin Lake Protection Association we do chemical and biological monitoring of Logan Martin lake in which we send our results to Alabama Water Watch. In our case we are showing improvements in dissolved oxygen (probably from decrease pollution) and we record water temperature. I’m sure the data is out there from other state organizations that can provide that data.

    • Lisa Borre
      July 22, 2014, 2:11 pm

      Thanks for you question, Mike. The most recent study of lakes in the U.S. was the National Lakes Assessment: A Collaborative Survey of the Nation’s Lakes (2009) by USEPA. The report is based on a random survey of U.S. lakes conducted in 2007 and is not nearly as definitive on the topic of climate change as the research coming out of Europe. The NLA does talk about how climate change is affecting lakes here in the U.S. (see pages 79-80 in the full report), but the study is not looking at changes through time. An updated survey of lakes was conducted in 2012 but results aren’t available yet.

      The monitoring work that your lake association is doing on Logan Martin Lake is critically important for understanding how lakes are changing. Not only is this helpful for use on your lake, but it can be useful for others in statewide, national and even global studies. It would make sense that you are seeing improvements in dissolved oxygen with a decrease in pollution. Thanks for your efforts and keep up the good work!

  3. Pro Libertate
    July 22, 2014, 6:49 am

    Can’t find this post – must have been lost. I don’t think you would censor facts that contradict your hypothesis, that would be unscientific:

    Living on lake Geneva, the biggest lake in Europe, I can say with confidence that this is utter nonsense!

    HOW could those lakes change due to “warming”, WHEN THERE IS NO WARMING???

    According to the IPCC models, the Alpine region, since 2005, is supposed to experience HOT summers and MILD winters with almost no snow, except at high altitudes.
    Instead, since 2007, we have FREEZING COLD WINTERS, cold springs, cool summers and tons of snow every winter, even right next to lake Geneva, which usually doesn’t get much snow.

    Right now, as I type this, daytime temperatures for most of July were at 13-15C instead of the normal seasonal temperatures of 27 to 35C.

    The month of may was officially way below average.

    We have had excessive cold all spring, had to keep on the heating for a long time.

    This is the 3rd year in a row like this. Last year, I was so imprudent as to not wear a winter jacket in june – I almost froze to death!

    In 2013, supposedly a “hot” year, we had summer temperatures for 3 weeks – the rest of the time, it was like now – FREEZING COLD!

    And I’m not making this up. Here is a PEER REVIEWED STUDY that confirms that in the entire Alpine region, temperatures have been FALLING since 2000 and the amount of snow and ice has INCREASED:;jsessionid=25F918D398269BA54BAFC0FE8BBCCF94.d03t03

    So where in the world do you get your data from?

    How can anyone claim that “the lakes change due to warming” when a study of the exact temperatures shows that THERE WAS NO WARMING?

    • Lisa Borre
      July 23, 2014, 5:48 pm

      Pro Libertate, Thanks for your comments/questions. It gives me the opportunity to mention another example of a European lake (Geneva) affected by climate change. First, I would like to point out that the warming trends I refer to in the post are long-term trends, not the short period since 2007 that you mention.

      I will not argue with you about the cold winters you’ve experienced of late, having lived in Europe myself from 2007-2010 and visited Lake Geneva several times during that period. But it is important for all of us to understand that even during a warming trend, there is considerable variability in temperature. You can have relatively cooler periods preceded or followed by warmer ones. To call the long-term warming trend “utter nonsense” is a misunderstanding of the amount of variability that can occur from year-to-year and even decade-to-decade.

      To clarify, I refer you to the water temperature data provided in the review paper by Jeppesen et al. (2012) published in Hydrobiologia: “Impacts of climate warming on the long-term dynamics of key fish species in 24 European lakes.” They found the mean water temperature of Lake Geneva increased by 0.17 degrees Centigrade per decade since 1986, and lake stratification occurs one month earlier than it did 30 years ago. This has impacted the fishery, with conditions favoring warmer water species and a dramatic reduction in Arctic char, a cold water species, as measured by fish catches.

      Even with the “freezing cold winters” you describe since 2007, mean lake temperature, while relatively cooler than its peak in the early 2000s, was still above average for the period dating back to 1960 (See Figure 11 on page 13 of Jeppesen et al. 2012).

      What is important to remember about lakes is that they are integrators of changes in their surrounding environment, and as a result, they are very reliable indicators of changes in climate.

      You cite a study of climate trends for the Alpine region which refers to the period since 2000, but I am surprised you didn’t mention the author’s main conclusion based on an analysis of air temperature and snow data: “This is consistent with the recent ‘plateauing’ (i.e. slight relative decrease) of mean winter temperature in Switzerland and illustrates how important decadal variability is in understanding the trends in key snow indicators.” They, too, emphasize the importance of climate variability in understanding their data. A long-term view and looking at changes in similar ecosystems (in this case lakes) throughout an entire region (Europe) puts climate trends in proper perspective.

  4. Stefan Metzeler
    July 22, 2014, 6:45 am

    As published today, the lack of fish in the lakes is due to excessive water purification, which doesn’t leave enough nutrients. It has nothing to do with “warming”, given that there has been no detectable warming in the alpine region:

    • Lisa Borre
      July 23, 2014, 5:53 pm

      Stefan, thanks for the comment. It is true that fish biomass goes down with nutrient loading reductions, and this is seen in many lakes worldwide, Erik Jeppesen says. There are indications of this on Lake Maggiore, where fishing yields have declined since the 1980s, but it doesn’t explain the shift to more warm water species in the lake. The changes in the fish communities are likely triggered by multiple stressors, including warming water temperature, changes in the trophic state of the lake and the introduction of invasive species, according to a review paper by Jeppesen et al (2012).

      To your point about no detectable warming in the alpine region, see my reply to “Pro Libertate,” which includes further explanation. While it has been relatively cooler in recent years, there is an overall warming trend going back to 1960. I refer you to the above paper by Jeppesen and one of my previous posts on the topic: Alpine Lakes in Austria Reflect Climate Change.

  5. vm
    July 21, 2014, 8:00 pm

    \\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.\\

    important point here. they disproved the idea that more fertilizer runoff was to blame instead of climate change

    • Lisa Borre
      July 23, 2014, 6:23 pm

      Thanks for the comment, vm. The situation on Lake Maggiore is one of multiple stressors at work (see reply to Stefan’s comment). The nutrient reductions I was referring to are those that were achieved as part of lake restoration efforts in the 1970s, when the lake began to make a recovery in water quality. However, other climate trends are now at work to further complicate lake restoration efforts. According to a study of 24 lakes in Europe by Jeppesen et al. (2012), more intense rain events, coupled with periods of drought, are leading to increased pollutant loads from the Maggiore catchment. Reducing fertilizer runoff from farms is an important focus of lake restoration efforts.

  6. Ray Del Colle
    July 21, 2014, 7:57 pm

    “Scientists have known about global warming for decades. It’s real. Let’s move on to what we can do about it.”

    • Lisa Borre
      July 23, 2014, 6:25 pm

      Thanks for your comment, Ray.

  7. Betsy
    Lake. Michigan
    July 21, 2014, 2:08 pm

    Again you have distilled a lot of science into easy to comprehend information for a lay person like me.
    Thank You,

    • Lisa Borre
      July 23, 2014, 6:23 pm

      Thanks for reading, Betsy.

  8. miller
    United States
    July 21, 2014, 12:42 pm

    Global temperatures are at peak since the advent of satellites to monitor the surface temperatures of oceans in the 1960s. Not a very big window of time for making meaningful conclusions.

    • Lisa Borre
      July 21, 2014, 3:19 pm

      Thanks for the comment, Miller. It gives me the opportunity to remind readers that lakes are considered good indicators of climate change for a variety of reasons, including some mentioned in this post. We are fortunate to have in-lake measurements of water temperature that pre-date satellite technologies, such as on Lake Tanganyika, a record that goes back 100+ years. Lake sediments are another source of useful long-term climate-related information.

  9. John Hartz
    July 21, 2014, 11:43 am

    Lisa Borre:

    Kudos on a well-written and well-researched article.

    I have posted a link to it on the Faebook page of Skeptical Science.

    • Lisa Borre
      July 21, 2014, 1:45 pm

      Thanks, John, for reading and sharing.