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Federal Research Center 
"Krasnoyarsk Science Center of the Siberian
Branch of the Russian Academy of Sciences"

 Федеральный исследовательский центр «Красноярский научный центр Сибирского отделения Российской академии наук»

Federal Research Center 
"Krasnoyarsk Science Center of the Siberian
Branch of the Russian Academy of Sciences"

Scientists have not found common patterns in the response of deep lake waters to climate change

26 November 2020 г.

Ученые не обнаружили общих закономерностей в реакции глубинных озерных вод на изменение климата
An international team of researchers have identified how the temperature of surface and deep waters in lakes has changed in summer over the last forty years. For the analysis, scientists used data of measurements made on 102 lakes on five continents. On average surface waters became warmer by a little more than 1.5 degrees Celsius under the influence of global climate change, while no general trend was revealed for deep waters. In some lakes, the water in the depth slightly warmed up, while in many others, on the contrary, it became colder. Scientists note that the temperature and the associated water quality in lakes is most likely to be influenced by a combination of factors which is unique for each water reservoir or region. The results of the large-scale study were published in the journal Scientific Reports.

There are almost 1.5 million lakes on earth. Despite the fact that almost a third of all fresh water is stored in several large lakes, numerous smaller lakes are also important sources of drinking water and fish, serving as other biological resources and places of recreation. From the ecological point of view, lakes play an important role in the carbon cycle and support life of numerous unique organisms. Climate change observed on the planet can have serious impacts on aquatic ecosystems. However, an accurate assessment of the effects of global warming on water temperature in lakes has not yet been made.

For many deep water bodies, stratification is typical in warm seasons, i.e. the separation of the strata into layers of different densities. This phenomenon is due to the physical properties of water, which has a maximum density at 4 ° C, while warmer and colder water should be lighter. In summer, in any large lake, a warm and light layer of water, so to say, floats on a cold and heavy one. In this case, the water temperature decreases with depth rather abruptly. The greater the difference in temperature between the surface and depth, the steeper this temperature jump is and the more difficult it is to mix the water in the lake. In the case of stable stratification in the lake near the bottom, there may be no oxygen, whereas the water temperature at different depths and the size of the surface and depth water layers determine the conditions for life of fish and other organisms. Thus, the water temperature at different depths, and  location of the temperature jump are important characteristics largely determining the quality of the water.

It is not easy to trace how the water temperature in lakes around the planet changes. In the case of surface waters, satellites can easily estimate the temperature. Scientists have already recorded that the surface of the lakes has become slightly warmer in recent years. To trace the processes in depth, direct measurements are needed. About fifteen years ago, lake researchers joined the international community to collect available long-term observational data. It turned out that there is still not enough data for global assessments of the specific response of water bodies to external influences. Only 102 lakes located in different climatic zones on five continents proved to be suitable for the purposes of long-term temperature analysis.

“This work is the result of collecting data for many years and collaboration of several dozens of scientists and institutions from 18 countries. We have found that deep lake water temperature responds to climate change in a very different way as compared to surface water. Moreover, it has turned out that characteristics such as the size of the lake or its location also do not greatly affect the variability of the temperature of deep waters. The observed global trends are most likely to be associated with local or regional climatic features or other external factors which need to be studied separately, ” says the first author of the article Rachel Pilla, post-graduate student at the Global Change Limnology Laboratory at the University of Miami (USA).

Having analyzed the results of direct measurements, scientists confirmed the previously recorded tendencies towards an increase in the surface water temperature and difference between the water density at depth and at the surface. This trend is observed on almost all continents and in all climatic zones, being associated with an increase in air temperature. In fact, the response of the temperature of deep waters to climate change is not obvious, judging at least by the available data set. In general, as the authors suggested, in lakes with a surface area of no more than one square kilometer the water in the depth has become slightly colder, while in larger ones, it has become a little warmer.

Such large-scale studies have become especially popular in recent years. This opportunity appeared due to the accumulation of data on the behavior of individual ecosystems. Previously, environmental scientists paid more attention to specific water bodies or to the life peculiarities of a single ecosystem. Now more and more attempts are being made to generalize the accumulated data and identify global patterns in the behavior of ecosystems. However, it is impossible to study all the lakes on the planet. Therefore, scientists hope that the behavior of characteristic water bodies will be typical for a larger class of objects. So, in the Asian part of Russia, Lake Baikal, the largest fresh water body on the planet, is considered as the reference system.

“One of the problems of such studies is associated with the existence of a significant number of long-term series of observations of the temperature regimes of lakes, with there being almost no parallel series of observations of water transparency, chlorophyll concentration, dynamics of the plankton community. On Lake Baikal, researchers of the Research Institute of Biology of Irkutsk State University have been conducting a comprehensive monitoring project since 1945, the so-called “point number 1”. In the framework of this project they monitor not only temperature and transparency of the water, but also the state of plankton (chlorophyll content, abundance and biomass of various phyto- and zooplankton). This is one of the oldest and most detailed series of lake data not only in Russia, but in the world as well. Owing to this, we can monitor and analyze large-scale processes of long-term changes associated with global climate processes. On Lake Baikal, we observe the processes of specialized endemic species being substituted for by cosmopolitan species, which is, in fact, a slow transformation of plankton. It is important to note here that it is the plankton community that is the foundation of any aquatic ecosystem, including Lake Baikal, ” says Maxim Timofeev, co-author of the study, Doctor of Biological Sciences, Director of the Research Institute of Biology of Irkutsk State University.

The researchers note that the difficulty in predicting temperature variability at depth makes the behavior of large water bodies less predictable under the climate change. On the other hand, for small lakes, the confirmed increase in surface temperature speaks for well-predicted impacts. These include a decrease in the concentration of oxygen at depth, likelihood of freezing in summer, and death of some species of fish or other organisms. In any case, for reliable global assessments of the behavior of lakes in the new climatic reality, a larger number of water bodies must be studied in detail.

“If to look at the map with the lakes used in the analysis, we will see the dominance of Western Europe and North America. This is not surprising - the study took into account only the results of continuous observations of the water temperature from the surface to the depth of the lake from 1970 to 2009. In Russia, there are only a few reservoirs meeting these requirements. At the same time, the main result of the article is that we cannot predict the response of a particular water body to climate change based on some general laws, and by the way, this is a rare case, when a prestigious journal publishes, so to say, a negative conclusion. Bad news for those who hope that an object can be examined and the results are extrapolated to others. If the quality of water in our lakes and reservoirs is important to us, we need to establish a system of continuous observations and research of large objects in different climatic and natural zones, "says one of the authors of the study, candidate of biological sciences, leading researcher of the Krasnoyarsk Science Center of SB RAS Egor Zadereev ...