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. Graphic: E. Jeppesen

By Lisa Borre
21 July 2014

(National Geographic) – 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. “Cyanobacteria 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.” [more]

Climate Change Already Having Profound Impacts on Lakes in Europe



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