By Ayesha Tandon
New research shows that lake “stratification periods” – a seasonal separation of water into layers – will last longer in a warmer climate.
These longer stratification periods could have “far-reaching implications” for lake ecosystems, according to the article, and can lead to toxic algal blooms, fish kills and increased methane emissions.
The study, published in Nature Communications, finds that the average period of seasonal lake stratification in the Northern Hemisphere could last nearly two weeks longer by the end of the century, even under a low emissions scenario. He finds that stratification could last more than a month longer if emissions are extremely high.
If stratification periods continue to lengthen, “we can expect catastrophic changes in some lake ecosystems, which may have irreversible impacts on ecological communities,” the study’s lead author told Carbon Brief. .
The study also reveals that the large lakes will see more noticeable changes. For example, the North American Great Lakes, home to “irreplaceable biodiversity” and representing some of the largest freshwater ecosystems in the world, are already experiencing “rapid shifts” in their stratification periods, according to the study.
As temperatures rise in the spring, many lakes begin the process of “stratification.” The warm air heats the surface of the lake, heating the upper water layer, which separates from the cooler water layers below.
Laminated layers do not mix easily and the greater the temperature difference between the layers, the less mixing there is. Lakes generally stratify between spring and fall, when warm weather maintains the temperature gradient between warm surface waters and cooler waters at depth.
Dr Richard Woolway from the European Space Agency is the lead author of the paper, which finds that climate change is pushing stratification to start earlier and end later. He tells Carbon Brief that the impacts of stratification are “widespread and widespread” and that longer periods of stratification could have “irreversible impacts” on ecosystems.
For example, Dr Dominic Vachon – a postdoctoral fellow at the Center for Climate Impacts Research at Umea University, who was not involved in the study – explains that stratification can create a “physical barrier” that makes it more difficult for dissolved gases and particles to move between layers of water.
This can prevent surface water oxygen from sinking deeper into the lake and can lead to “deoxygenation” in deeper water, where oxygen levels are lower and respiration becomes more difficult.
Oxygen depletion can have “fatal consequences for living organisms”, according to Dr. Bertram Boehrer, a researcher at the Helmholtz Center for Environmental Research, who was not involved in the study.
Lead author Woolway tells Carbon Brief that decreasing oxygen levels at deeper depths trap fish in warmer surface waters:
“Fish often migrate to deeper waters during the summer to escape warmer conditions at the surface – for example during a heat wave on a lake. A decrease in oxygen at depth will mean that fish will not have no thermal refuge, as they often cannot survive when oxygen concentrations are too low.
This can be very harmful to lake life and may even increase “fish kill events,” the study notes.
However, the impacts of stratification are not limited to fish. The study notes that a shift to earlier spring stratification may also encourage communities of phytoplankton – a type of algae – to grow earlier and may get them out of sync with species that depend on them for food. This is called a “trophic mismatch”.
Professor Catherine O’Reilly, a professor of geography, geology and the environment at Illinois State University, who was not involved in the study, adds that longer stratified periods could also “increase the likelihood of proliferation harmful algae”.
The impact of climate change on lakes also extends beyond ecosystems. Low oxygen levels in lakes can increase the production of methane, which is “produced and emitted from lakes at globally significant rates,” according to the study.
Woolway explains that higher levels of warming could therefore create a positive climate feedback in the lakes, where rising temperatures mean greater global warming emissions:
“Low oxygen levels at depth also promote the production of methane in lake sediments, which can then be released to the surface via bubbles or diffusion, resulting in a positive feedback on climate change.”
Beginning and breaking
In the study, the authors determine historical changes in lake stratification periods using long-term observational data from some of the “best-monitored lakes in the world” and daily simulations from a collection of lake models.
They are also running simulations of future changes in the stratification period of the lake under three different emission scenarios, to determine how the process might change in the future. The study focuses on lakes in the northern hemisphere.
The figure below shows the average change in lake stratification days between 1900 and 2099, compared to the 1970-1999 average. The graph shows the historical measurements (black) and the RCP2.6 low emissions (blue), RCP6.0 medium emissions (yellow) and RCP8.5 extremely high emissions (red) scenarios.
The graph shows that the average stratification period of the lake has already lengthened. However, the study adds that some lakes experience greater impacts than others.
For example, Blelham Tarn – the best-monitored lake in the English Lake District – is now stratifying 24 days earlier and maintaining its stratification for an additional 18 days compared to its 1963-1972 averages, according to the study. Woolway tells Carbon Brief that as a result, the lake is already showing signs of oxygen depletion.
Climate change increases the average duration of stratification in lakes, the results show, by moving the onset of stratification earlier and pushing the “break” of stratification later. The table below shows the projected changes in onset, breakup, and overall length of lake stratification under different emission scenarios, relative to a baseline of 1970-1999.
The table shows that even under the low emission scenario, the stratification period of the lake is projected to be 13 days longer by the end of the century. However, in the Extremely High Emissions Scenario, it could take 33 days longer.
The table also shows that the onset of bedding changed more significantly than the break in bedding. The reasons for this are revealed by taking a closer look at the drivers of stratification.
Warmer weather and weaker winds
The timing of the onset and breakdown of stratification in lakes depends on two main factors: temperature and wind speed.
The impact of temperature on lake stratification is based on the fact that warm water is less dense than cold water, Woolway explains to Carbon Brief:
“Warming the surface of the water by increasing the air temperature causes the density of the water to decrease and also causes the formation of distinct thermal layers in a lake – colder and denser water settles to the bottom of the lake, while the warmer, lighter water forms a layer at the top.”
This means that as climate change causes temperatures to rise, the lakes will begin to stratify earlier and remain stratified for longer. Lakes at higher elevations are also likely to experience greater stratification changes, Woolway told Carbon Brief, because “the extension of summer is very apparent in high latitude regions.”
The figure below shows the projected increase in stratification time for Northern Hemisphere lakes under the low (left), medium (center), and high (right) emission scenarios. Deeper colors indicate a greater increase in the stratification period.
The figure shows that the expected impact of climate change on the duration of stratification becomes more pronounced at higher latitudes further north.
The second factor is wind speed, explains Woolway:
“Wind speed also affects the timing of the onset and breakdown of stratification, with stronger winds acting to mix the water column, thus acting against the stratification effect of the temperature increase of the air.”
According to the study, the wind speed is expected to decrease slightly as the planet warms. The authors note that the expected changes in wind speed near the surface are “relatively minor” compared to the likely increase in temperature, but they add that this may still lead to “substantial” changes in stratification.
The study reveals that air temperature is the most important factor when a lake begins to stratify. However, when looking at stratification failure, wind speed is found to be a more important factor.
Meanwhile, Vachon says wind speed also has implications for methane emissions from lakes. He notes that stratification prevents the methane produced at the bottom of the lake from rising, and that when the period of stratification ends, the methane can rise to the surface. However, according to Vachon, the rate at which stratification breaks down will affect the amount of methane released into the atmosphere:
“My work has suggested that the amount of methane accumulated in bottom waters that will eventually be emitted is related to how quickly stratification breakdown occurs. For example, a slow and gradual breakdown of stratification will most likely allow oxygenation of the water and allow bacteria to oxidize methane to carbon dioxide. However, stratification failure that occurs quickly – for example after storms with high wind speeds – will allow accumulated methane to be emitted into the atmosphere more efficiently.
Finally, the study finds that large lakes take longer to stratify in the spring and generally remain stratified longer in the fall, due to their higher water volume. For example, the authors focus on the North American Great Lakes, which are home to “irreplaceable biodiversity” and represent some of the largest freshwater ecosystems in the world.
These lakes have stratified 3.5 days earlier every decade since 1980, according to the authors, and their onset of stratification can vary by up to 48 days between some extreme years.
O’Reilly tells Carbon Brief that “it is clear that these changes will move lakes into uncharted territory” and adds that the paper “provides a framework for thinking about the magnitude of changes lakes will experience under future climate scenarios.” “.
Republished with permission from Carbon Brief.
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