LISA BORRE is a lake conservationist and writer who contributes the National Geographic‘s “Water Currents” blog. By permission we quote from work of hers that appeared in that blog: Climate Change Already Having Profound Impacts on Lakes in Europe and Warming Lakes: Barometers of Climate Change?
Global assessment shows 95% of lakes are warming
In 2010, National Geographic News reported on the results of the first comprehensive global study of lake temperature trends. The study — conducted by researchers at NASA’s Jet Propulsion Laboratory (JPL) in California using satellite data — found that in the last 25 years, the world’s largest lakes have been steadily warming, some by as much as 4°F (2.2°C). In some cases, the trend is twice as fast as the air temperature trend over the same period.
Climate warming is having a “eutrophication-like” effect on lakes
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. 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.
Studies by Dr. Erik Jeppesen at Aarhus University in Denmark note that 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.