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These longer periods of stratification could have “far-reaching implications” for lake ecosystems, the paper says, and can drive toxic algal blooms, fish die-offs and increased methane emissions.

The study, published in Nature Communications, finds that the average seasonal lake stratification period in the northern hemisphere could last almost two weeks longer by the end of the century, even under a low emission scenario. It finds that stratification could last over a month longer if emissions are extremely high.

If stratification periods continue to lengthen, “we can expect catastrophic changes to some lake ecosystems, which may have irreversible impacts on ecological communities,” the lead author of the study tells Carbon Brief.

The study also finds that larger lakes will see more notable changes. For example, the North American Great Lakes, which house “irreplaceable biodiversity” and represent some of the world’s largest freshwater ecosystems, are already experiencing “rapid changes” in their stratification periods, according to the study.

‘Fatal Consequences’

As temperatures rise in the spring, many lakes begin the process of “stratification.” Warm air heats the surface of the lake, heating the top layer of water, which separates out from the cooler layers of water beneath.

The stratified 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 autumn, when hot weather maintains the temperature gradient between warm surface water and colder water deeper down.

Dr Richard Woolway from the European Space Agency is the lead author of the paper, which finds that climate change is driving stratification to begin earlier and end later. He tells Carbon Brief that the impacts of stratification are “widespread and extensive,” and that longer periods of stratification could have “irreversible impacts” on ecosystems.

For example, Dr Dominic Vachon – a postdoctoral fellow from the Climate Impacts Research Centre at Umea University, who was not involved in the study – explains that stratification can create a “physical barrier” that makes it harder for dissolved gases and particles to move between the layers of water.

This can prevent the oxygen from the surface of the water from sinking deeper into the lake and can lead to “deoxygenation” in the depths of the 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 Centre for Environmental Research, who was not involved in the study.

Lead author Woolway tells Carbon Brief that the decrease in oxygen levels at deeper depths traps fish in the warmer surface waters:

“Fish often migrate to deeper waters during the summer to escape warmer conditions at the surface – for example during a lake heatwave. A decrease in oxygen at depth will mean that fish will have no thermal refuge, as they often can’t survive when oxygen concentrations are too low.”

This can be very harmful for lake life and can even increase “fish die-off events” the study notes.

However, the impacts of stratification are not limited to fish. The study notes that a shift to earlier stratification in spring can also encourage communities of phytoplankton – a type of algae – to grow sooner, and can put them out of sync with the species that rely on them for food. This is called a “trophic mismatch.”

Prof 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 harmful algae blooms.”

The impact of climate change on lakes also extends beyond ecosystems. Low oxygen levels in lakes can enhance the production of methane, which is “produced in 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 lakes, where rising temperatures mean larger planet-warming emissions:

“Low oxygen levels at depth also promotes methane production in lake sediments, which can then be released to the surface either via bubbles or by diffusion, resulting in a positive feedback to climate change.”

Onset and Breakup

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 also run simulations of future changes in lake stratification period under three different emission scenarios, to determine how the process could 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 plot shows historical measurements (black), and the low emission RCP2.6 (blue), mid emissions RCP6.0 (yellow) and extremely high emissions RCP8.5 (red) scenarios.

Change in lake stratification duration compared to the 1970-1999 average, for historical measurements (black), the low emission RCP2.6 (blue) moderate emissions RCP6.0 (yellow) and extremely high emissions RCP8.5 (red). Credit: Woolway et al (2021).

The plot shows that the average lake stratification period has already lengthened. However, the study adds that some lakes are seeing more significant impacts than others.

For example, Blelham Tarn – the most well-monitored lake in the English Lake District – is now stratifying 24 days earlier and maintaining its stratification for an extra 18 days compared to its 1963-1972 averages, the study finds. Woolway tells Carbon Brief that as a result, the lake is already showing signs of oxygen depletion.

Climate change is increasing average stratification duration in lakes, the findings show, by moving the onset of stratification earlier and pushing the stratification “breakup” later. The table below shows projected changes in the onset, breakup and overall length of lake stratification under different emission scenarios, compared to a 1970-1999 baseline.

The table shows that even under the low emission scenario, the lake stratification period is expected to be 13 days longer by the end of the century. However, in the extremely high emissions scenario, it could be 33 days longer.

The table also shows that stratification onset has changed more significantly than stratification breakup. The reasons why are revealed by looking at the drivers of stratification more closely.

Warmer Weather and Weaker Winds

The timing of stratification onset and breakup in lakes is driven by 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 cool water, Woolway tells Carbon Brief:

“Warming of the water’s surface by increasing air temperature causes the density of water to decrease and likewise results in distinct thermal layers within a lake to form – cooler, denser water settles to the bottom of the lake, while warmer, lighter water forms a layer on top.”

This means that, as climate change causes temperatures to rise, lakes will begin to stratify earlier and remain stratified for longer. Lakes in higher altitudes are also likely to see greater changes in stratification, Woolway tells Carbon Brief, because “the prolonging of summer is very apparent in high latitude regions.”

The figure below shows the expected increase in stratification duration from lakes in the northern hemisphere under the low (left), mid (center), and high (right) emission scenarios. Deeper colors indicate a larger increase in stratification period.

Expected increase in stratification duration in lakes in the northern hemisphere under the low (left), mid (centre) and high (right) emissions scenarios. Credit: Woolway et al (2021).

The figure shows that the expected impact of climate change on stratification duration becomes more pronounced at more northerly high latitudes.

The second factor is wind speed, Woolway explains:

“Wind speed also affects the timing of stratification onset and breakdown, with stronger winds acting to mix the water column, thus acting against the stratifying effect of increasing air temperature.”

According to the study, wind speed is expected to decrease slightly as the planet warms. The authors note that the expected changes in near-surface wind speed are “relatively minor” compared to the likely temperature increase, but they add that it may still cause “substantial” changes in stratification.

The study finds that air temperature is the most important factor behind when a lake will begin to stratify. However, when looking at stratification breakup, it finds that wind speed is a more important driver.

Meanwhile, Vachon says that wind speeds also have implications for methane emissions from lakes. He notes that stratification prevents the methane produced on the bottom of the lake from rising and that, when the stratification period ends, methane is allowed to rise to the surface. However, according to Vachon, the speed of stratification breakup will affect how much methane is released into the atmosphere:

“My work has suggested that the amount of accumulated methane in bottom waters that will be finally emitted is related to how quickly the stratification break-up occurs. For example, a slow and progressive stratification break-up will most likely allow water oxygenation and allow the bacteria to oxidise methane into carbon dioxide. However, a stratification break-up that occurs rapidly – for example after storm events with high wind speed – will allow the accumulated methane to be emitted to the atmosphere more efficiently.”

Finally, the study finds that large lakes take longer to stratify in spring and typically remain stratified for longer in the autumn – due to their higher volume of water. For example, the authors highlight the North American Great Lakes, which house “irreplaceable biodiversity” and represent some of the world’s largest freshwater ecosystems.

These lakes have been stratifying 3.5 days earlier every decade since 1980, the authors find, and their stratification onset can vary by up to 48 days between some extreme years.

O’Reilly tells Carbon Brief that “it’s clear that these changes will be moving lakes into uncharted territory” and adds that the paper “provides a framework for thinking about how much lakes will change under future climate scenarios.”

Reposted with permission from Carbon Brief.