Elevated water levels and flooding around the Great Lakes have generated recent debate about the role of global warming, but researchers have known for years that lake temperatures are rising. To illustrate how lakes are changing, scientists have created a lake thermal classification system and noted that a significant proportion of lakes could be reclassified as warmer types as global temperatures rise.
Changing Thermal Classifications
Temperature plays a key role in lake ecology and affects phenomena such as species distribution and organism growth rate. But although there are lake classification systems for characteristics such as trophic state, which measures biological activity, there has been no temperature-based system.
Writing in Nature Communications, researchers have proposed a classification of lakes into nine thermal regions based on surface water temperatures. Corresponding roughly with latitude, the nine regions are, from north to south, northern frigid, northern cool, northern temperate, northern warm, northern hot, tropical hot, southern hot, southern warm, and southern temperate.
“The thermal classification allows any lake in the world to be allocated to a thermal type,” said Stephen Maberly, a limnologist and ecophysiologist at the UK Centre for Ecology & Hydrology who was the lead author of the study. “This will allow the responses of lakes with similar thermal characteristics to be compared or lakes with different characteristics to be contrasted. For example, the growing interest in effects of warming could be described for lakes from different thermal regions. It will also help work at one particular site to be contextualized at a global scale.”
Maberly and collaborators from U.K. universities in Dundee, Glasgow, Reading, and Stirling, as well as the Dundalk Institute of Technology in Ireland, used satellite data from over 700 lakes around the world collected twice a month for more than 16 years. They performed a statistical analysis of the data and established a classification system, which they applied to all areas of the world with a lake model.
Researchers applied different future climate scenarios to project how the distribution of the thermal regions could change. They found that 12%, 27%, and 66% of lakes would be reclassified to a lower-latitude thermal region under scenarios of low, medium, and high greenhouse gas concentrations, respectively. Under the last scenario, the number of northern frigid lakes will drop by 79%, whereas northern temperate and northern warm lakes will increase by 166% and 228%, respectively.
Warmer surfaces can generate a host of direct and indirect effects on lakes. These effects range from impacts on species (including fish such as salmon, trout, and arctic char that require cool water) to changes in ice cover, which may affect how people use lakes.
“Indirect effects will be linked to changes in the critical physical structure of lakes where typically, in summer warm surface water is less dense and floats on top of cooler, more dense water at depth—so-called stratification,” said Maberly. “This creates two very different environments with consequences for algal growth, deoxygenation at depth, and cycling of nutrients within the lake. Warming will increase the strength and duration of stratification, promoting development of cyanobacterial (blue-green algal) blooms, especially in nutrient-rich lakes and promoting deoxygenation at depth.”
Teasing Out the Effects of Warming
Although lakes can act as sentinels of change, they are the result of complex forces at play that make determining the effects of climate change very difficult, said John D. Lenters, an honorary fellow at the University of Wisconsin–Madison’s Center for Limnology who was not involved in the study. He called the work a robust and intensive analysis of lake surface water temperature (LSWT) data and model output but said it fails to make the case for the merits of its new approach over climate or air temperature classifications.
“There wasn’t anything that was all that surprising in the results, e.g., LSWT varies according to (mostly) latitude and elevation,” said Lenters. “And so why not just classify LSWT according to these and other more easy-to-relate-to geographic and/or climatic variables? I would think that would be of more interest to readers.”
Meanwhile, Maberly and his collaborators plan to expand their lake temperature modeling work to analyze other global aspects of future temperature in lakes. They also want to widen their analysis from temperature to other variables that can be detected with satellites.
“Ultimately, by bringing different global data sets together for lakes and their catchments we hope to define different lake biomes and the variables that control them,” said Maberly.
—Tim Hornyak (@robotopia), Science Writer