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These longer stratification periods could have “far-reaching effects” on lake ecosystems, causing toxic algal blooms, fish death and increased methane emissions.

The study, published in Nature Communications, notes that the average seasonal stratification time of lakes in the northern hemisphere could be nearly two weeks longer by the end of the century, even under a low-emission scenario. It is found that stratification can take over a month longer for extremely high emissions.

If stratification periods continue to lengthen, “we can expect catastrophic changes in some marine ecosystems that could have irreversible effects on ecological communities,” the study’s lead author told Carbon Brief.

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

“Deadly Consequences”

When spring temperatures rise, many lakes begin the process of “stratification”. Warm air warms the surface of the lake and warms the top layer of water, which separates from the cooler layers of water below.

The layered layers do not mix easily, and the greater the temperature difference between the layers, the less mixed there is. Lakes generally layer between spring and autumn when hot weather keeps the temperature gradient between warm surface water and colder water lower.

Dr. Richard Woolway of the European Space Agency is the lead author of the paper which states that climate change is causing stratification to start earlier and end later. He tells Carbon Brief that the effects of stratification are “widespread and extensive” and that longer periods of stratification could have “irreversible effects” on ecosystems.

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

This can prevent the surface oxygen from sinking deeper into the lake and cause “deoxygenation” at depths where oxygen levels are lower and breathing becomes more difficult.

Lack of oxygen 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 told Carbon Brief that the decrease in oxygen levels at deeper depths includes fish in the warmer surface waters:

“Fish often migrate to deeper waters in summer to escape warmer conditions on the surface – for example during a heat wave at the lake. A decrease in oxygen at depth means that fish have no thermal refuge as they often cannot survive. when oxygen is present. ” Concentrations are too low. “

This can be very detrimental to lake life and even increase “fish deaths,” according to the study.

However, the effects of stratification are not limited to fish. The study notes that shifting to earlier stratification in the spring may also result in phytoplankton communities – a type of algae – growing earlier and no longer synchronized with the species that rely on them for food. This is known as the “trophic mismatch”.

Prof. Catherine O’Reilly, professor of geography, geology and the environment at Illinois State University, who was not involved in the study, added that longer shift periods could also “increase the likelihood of harmful algal blooms.”

The effects of climate change on lakes also extend beyond ecosystems. According to the study, low oxygen levels in lakes can improve the production of methane, which “is produced in and emitted from lakes at significant rates around the world”.

Woolway explains that higher warming can therefore lead to positive climate feedback in lakes, where rising temperatures mean greater emissions to warm the planet:

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

Beginning and separation

In the study, the authors determine historical changes in the stratification periods of lakes using long-term observation 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 the lake’s stratification period 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 following figure shows the average change in the days of sea stratification between 1900 and 2099 compared to the 1970-1999 average. The diagram shows historical measurements (black) as well as the scenarios RCP2.6 (blue) with low emission, RCP6.0 with medium emission (yellow) and RCP8.5 (red) with extremely high emission.

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

The illustration shows that the average layering time of the lake has already increased. However, the study adds that some lakes have a bigger impact than others.

For example, Blelham Tarn – the best-monitored lake in the English Lake District – now stratifies 24 days earlier and maintains stratification for an additional 18 days compared to its 1963-1972 averages, the study said. Woolway tells Carbon Brief that the lake is already showing signs of a lack of oxygen as a result.

Climate change increases the average length of stratification in lakes, as the results show by postponing the start of stratification earlier and “breaking up” the stratification later. The table below shows the projected changes in the onset, resolution and total length of the sea stratification under various emission scenarios compared to a baseline from 1970-1999.

The table shows that even under the low emissions scenario, the lake’s stratification time is expected 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 beginning of the stratification changed more significantly than the resolution of the stratification. The reasons for this are revealed by a closer look at the drivers of the stratification.

Warmer weather and weaker winds

The timing of the onset and breakdown of stratification in lakes is determined by two main factors – temperature and wind speed.

The influence of temperature on the lake’s stratification stems from the fact that warm water is less dense than cold water, Woolway tells Carbon Brief:

“By heating the water surface by increasing the air temperature, the water density decreases and different layers of warmth are also formed within a lake – cooler, denser water settles on the bottom of the lake, while warmer, lighter water forms a layer on top.”

This means that as temperatures rise due to climate change, the lakes begin to stratify earlier and stay stratified longer. At higher altitudes, the layers are likely to change more, Woolway told Carbon Brief, since “the lengthening of summer in regions with high latitudes is very obvious”.

The following figure shows the expected increase in the stratification duration of lakes in the northern hemisphere in the scenarios with low (left), medium (middle) and high (right) emission. Deeper colors indicate a greater increase in the stratification period.

Expected extension of the stratification duration in lakes in the northern hemisphere under the scenarios with low (left), medium (middle) and high (right) emissions. Photo credit: Woolway et al. (2021).

The figure shows that the expected effects of climate change on the duration of stratification are more pronounced in more northern latitudes.

The second factor is wind speed, explains Woolway:

“The wind speed also influences the time at which the stratification begins and collapses, with stronger winds mixing the water column and thus counteracting the stratification effect of an increase in air temperature.”

According to the study, wind speed is expected to decrease slightly as the planet warms up. The authors note that the expected changes in near-surface wind speed are “relatively small” compared to the likely rise in temperature, but add that this can still cause “significant” changes in stratification.

The study finds that air temperature is the single most important factor in determining when a lake begins to stratify. When looking at the stratification, however, it is found that the wind speed is a more important driver.

Meanwhile, Vachon says that wind speeds also affect methane emissions from lakes. He notes that the stratification prevents the methane produced at the bottom of the lake from rising, and that methane can rise to the surface after the stratification period has expired. However, according to Vachon, the rate at which the stratification ruptures will affect how much methane is released into the atmosphere:

“My work has shown that the amount of accumulated methane in groundwater that is eventually emitted depends on how quickly the stratification breaks up. For example, a slow and progressive break-up of the stratification will most likely allow and enable the water to become oxygenated Bacteria to oxidize methane to carbon dioxide. A quick break-up of the stratification – for example after storm events with high wind speeds – enables a more efficient release of the accumulated methane into the atmosphere. “

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

The authors note that these lakes have been stratified 3.5 days earlier every ten years since 1980, and their onset of stratification can vary by up to 48 days between some extreme years.

O’Reilly told Carbon Brief that “it is clear that these changes will lead lakes into uncharted territory,” adding that the paper “provides a framework for thinking about how much lakes will change in future climate scenarios.” .

Republished with permission from Carbon Brief.

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