Sep. 30, 2018
ScienceEnvironment

Lakes Under Climate Stress

Rising Temperatures and Extreme Weather Events put Pressure on Freshwaters

  • Fig. 1: The cycle of a typical dimictic lake: mixing of the water column takes place twice a year.Fig. 1: The cycle of a typical dimictic lake: mixing of the water column takes place twice a year.
  • Fig. 1: The cycle of a typical dimictic lake: mixing of the water column takes place twice a year.
  • Fig. 2: Comparison of the internally and externally caused fertilisation of lakes in the context of global warming: nutrient accumulation in water encourages the growth of algae, particularly the development of Cyanophyceae. (Photos: stockadobe.com – Maurice Tricatelle, Wolfilser, Design: Unicom GmbH)
  • Fig. 3: At Berlin’s Lake Müggelsee, wind speeds above 30 m/s were recorded during the storm Xavier on 5 October 2017 (blue line). This led to an abrupt, drastic increase in the lake’s turbidity (red line). Data was recorded using an automatic measuring station, operated on the Müggelsee by the Leibniz-Institute of Freshwater Ecology and Inland Fisheries (IGB). Such continuous measurements enable researchers to capture the response behaviour of ecosystems in real time.
Global climate change affects all ecosystems, including lakes – and is therefore the focus of long-term research. At the Lake Müggelsee, a team of scientists is exploring how climate change affects our freshwaters, and what we can expect in the future.
One of the most visible effects of global warming are undoubtedly changes in phenology – the timing of seasonal events in the course of the year. When temperatures rise, lakes freeze over later and more rarely, and the ice starts melting earlier. Due to the increase in light and heat, algae start to develop earlier in spring: on average, almost one month earlier in lakes of the northern temperate zone. The many indirect consequences of climate change, such as altered oxygen and nutrient dynamics, are even more serious, but invisible to the naked eye. Such changes are a threat to the long-term balance of lakes [1].
Alarming Increase in Temperature in Recent Decades
Increases in air temperature result in an increase in water temperatures, as is the case with Lake Müggelsee, located in southeast Berlin. The eutrophic shallow lake has been the subject of long-term research since 1976. The extensive data series reveal that, since measurements began, the temperature of the lake’s surface water in summer has increased by 0.54°C per decade, i.e. an increase of more than 2°C over the past 40 years. Many other lakes in Germany and the rest of the world are experiencing similar warming trends. The global average warming rate for lake summer surface water is 0.34°C per decade [2]. Consequently, summer surface water temperatures have increased more considerably and rapidly in some cases than comparable air temperatures.

Annual Cycle Out of Synch
The thermal cycle of lakes reacts particularly sensitively to warming. This cycle governs the seasonal mixing of the water column. Oxygen and nutrients are distributed evenly throughout the lake. Since temperature differences between surface water and hypolimnic water prevent intermixing in summer and winter, most lakes in our part of the world undergo circulation during the months of spring and autumn (dimictic lakes, Fig.

1). Exceptions include lakes that are too shallow to be able to develop continuous stratification in summer. In such lakes, the water is regularly mixed throughout summer (polymictic lakes). Other lakes, on the other hand, do not freeze over in winter because they are located in a warmer climate or are too deep. Consequently, such lakes also mix during winter and only develop stratification in summer (monomictic lakes).
However, an increase in hot weather periods in summer causes also shallow, polymictic lakes such as the Müggelsee, to stratify more frequently, sometimes even for several weeks during summer. In seasonally stagnant lakes (monomictic and polymictic lakes), milder winters and higher temperatures cause stratification to begin earlier in spring, and possibly also to end later in autumn. Very deep lakes such as Lake Constance no longer cool down sufficiently in warm winters and, as a result, no longer mix down to the bottom every year. In other words, climate change is altering the stratification types of lakes, usually to the detriment of mixing.

Oxygen Depletion, Nutrient Contamination and Algal Blooms
The changed thermal structure and reduced ice development lead to indirect effects such as different light, oxygen and nutrient dynamics, which is a cause for concern among researchers. It is predicted that the oxygen concentration in lakes will decline. This phenomenon can best be observed in deep water layers in late summer, since higher water temperatures result in higher oxygen consumption. Due to more prolonged stratification, zones may be created where oxygen is no longer repleted in good time by the onset of circulation.
The lack of oxygen causes a further problem: chemical processes trigger the release of nutrients previously bound in the sediment, such as phosphorus. This climate-induced intensification of the internal fertilisation of lakes counteracts the major efforts taken in recent decades to reduce external nutrient inputs and the associated eutrophication of lakes. In addition, increased precipitation, such as due to intense rainfall events, result in increasing nutrient loads from the catchment area, especially of dissolved organic carbon (DOC).
High water temperatures, prolonged stratification and high nutrient concentrations encourage the growth of algae, particularly the development of Cyanophyceae, which are optimally adapted to such conditions (Fig. 2). Under climate change, all three variables change to the advantage of these microorganisms. In this way, Cyanophyceae may form dense carpets, especially at the water surface, severely limiting the recreational value of a lake.

Blanket Statements Virtually Impossible
Lakes react differently to climate change, depending on the type of lake and their catchment area. It is therefore virtually impossible to make blanket statements. However, it appears to be the case that lakes are becoming warmer and more oxygen-deficient in the wake of climate change, and that the thermal structure of lakes are undergoing long-term change.
In addition, some lake ecosystems are likely to change abruptly once critical boundaries – referred to as tipping points – are exceeded. Short-term changes are possible following extreme events. When the low-pressure system Xavier struck on 5 October 2017 with wind speeds above 30 m/s, for example, the turbidity of the Müggelsee changed abruptly (Fig. 3). The entire water body of the lake was mixed in the space of a few hours, resulting in the resuspension of lake sediment throughout the water column.
Stressors such as higher nutrient loads and poorer water quality, an increase in extreme events (storm events, heat waves and flooding) and greater water consumption and scarcity distort the effects of the long-term warming trend, making it difficult to predict future developments. In an effort to summarize previous findings in this area, a dossier on “Lakes under Climate Change“ was recently published, which gives a universally comprehensible explanation of the underlying natural processes, and highlights possible future scenarios.

Water Management Faces Major Challenges
The consequences of climate change could have a serious impact on the protection and use of lakes. Water managers are confronted with new uncertainties and challenges. However, climate impact research and long-term monitoring programs could contribute to the development of robust adaptation strategies. Numerous researchers agree that, if lakes are to be preserved not only as habitat, but also for the different functions they offer humans (e.g. as drinking water reservoirs, for inland fisheries, for recreation and tourism), there needs to be holistic, flexible and long-term water management that takes into account the dynamics of entire catchment areas.

Dossier
In the Dossier “Seen im Klimawandel: Diagnosen und Prognosen aus der Langzeitforschung (Lakes under climate change: diagnosis and prognosis from long-term research)“, experts summarize the findings generated from climate impact research. The Dossier can be downloaded free of charge.

 

Contact
Prof. Rita Adrian

Leibniz-Institute for Freshwater Ecology and Inland Fisheries
Berlin, Germany
adrian@igb-berlin.de

References

[1] Adrian R et al. 2016. Environmental Impacts—Lake Ecosystems. In: Quante M, Colijn F. North Sea Region Climate Change Assessment, Regional Climate Studies. p. 315-340. DOI:10.1007/978-3-319-39745-0_10.

[2] O’Reilly CM et al. 2015. Rapid and highly variable warming of lake surface waters around the globe. Geophysical Research Letters. 42(24):10773-10781. DOI:10.1002/2015GL066235.

[3] Dossier (English spoken) “Seen im Klimawandel: Diagnosen und Prognosen aus der Langzeitforschung (Lakes under climate change: diagnosis and prognosis from long-term research)“; Leibniz-Institute of Freshwater Ecology and Inland Fisheries (IGB); DOI:10.4126/FRL01-006407562



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