The widely accepted notion that freshwater lakes warm up as temperatures rise may be false. Surprisingly, not all lakes conform to this general trend, according to a new study led by the Nanjing Institute of Geography and Limnology of the Chinese Academy of Sciences.
For the investigation, an international team of researchers sampled 345 lakes across 12 countries.
The goal was to uncover the underlying factors determining how temperate lakes react to atmospheric warming. The primary focus was on lakes in the United States and Europe.
“Atmospheric warming heats lakes, but the causes of variation among basins are poorly understood,” wrote the study authors.
One of the most intriguing discoveries was that the surface and deep waters of these lakes did not have a uniform response to atmospheric warming. In fact, surface water temperatures rose more rapidly compared to their deeper counterparts.
However, multi-decadal trends in the relationship between air temperature and water temperature often diverged or even showed an opposite trend for individual lakes, noted the researchers.
The team identified a combination of factors such as local climate, land cover, geomorphology, and water transparency that played pivotal roles in explaining these divergent temperature changes between water and air.
Lakes that were clear, cold, and deep, particularly those located at higher elevations and amidst natural land use, showed the most pronounced reactions to atmospheric warming.
In an interesting twist, lakes that suffered from nutrient pollution, often signified by green algae cover, displayed diminished sensitivity to global warming compared to clear, less polluted lakes.
“Since the nutrients in green surface waters can act as a shade, [they keep] the deep waters cool,” said study first author Dr. ZHOU Jian.
Another noteworthy finding was that as the atmosphere’s temperature rises, lakes become progressively less responsive to these changes.
“As lakes warm up, they start to lose water by evaporation and this acts to cool them off. Just like we sweat to cool off, lakes ‘sweat’ by evaporating water to get rid of excess heat,” explained study co-author Professor SHI Kun.
The broader implications of these findings are pivotal for environmental policies. The study emphasizes the necessity of recognizing the diverse ways in which lakes respond to climate warming, especially when crafting adaptation and mitigation strategies.
“Lake management practices need to consider where lakes are, what they are surrounded by, and how sensitive they are to unique and interactive effects of climate change and human activities,” said study co-author Professor QIN Boqiang.
Lakes are vital barometers of the broader impacts of climate change. Their health and stability are intricately linked with the health of surrounding ecosystems and communities.
Lakes are heavily influenced by global warming, leading to a variety of environmental impacts.
As global temperatures increase, lake surface temperatures also rise. This warming can lead to changes in water stratification, with warm water staying at the top and cooler water at the bottom, affecting aquatic life.
In colder regions, many lakes form ice covers in winter. With global warming, the duration of this ice cover is reduced.
A lack of ice can cause changes in the seasonal cycles of lakes. Reduced ice cover can also lead to increased evaporation, affecting lake water levels.
Warmer water temperatures change the species composition of lakes. Some species might thrive, while others might struggle or even disappear. Invasive species that prefer warmer waters may also become more prevalent.
Increased temperatures, combined with nutrient pollution (from agricultural runoff, for instance), can promote the growth of harmful algal blooms.
These blooms produce toxins that are harmful to aquatic life and humans and can lead to dead zones in lakes where oxygen levels are too low for most organisms to survive.
Temperature changes can disrupt the balance of different organisms in a lake. For example, if one species reproduces earlier because of the temperature, it can affect the species that rely on it for food.
Apart from the effects of reduced ice cover leading to evaporation, global warming can alter rainfall patterns. Some lakes might experience reduced water levels due to decreased rainfall, while others could be at risk of flooding.
Lakes act as carbon sinks, absorbing and storing carbon. However, increasing temperatures and changes in vegetation around lakes alter the amount of carbon they can store and the rate at which they release carbon back into the atmosphere.
Increasing temperatures can reduce the oxygen levels in lakes, affecting water quality. Warmer waters are also more conducive to the growth of pathogens, potentially impacting both aquatic life and human health.
The research is published in the journal Nature Communications.
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