Oxygen-free zones can develop at the bottom of water bodies, especially at depth, as a result of warming. | © Solvin Zankl
As lakes warm, their oxygen content decreases. This is partly due to the oxygen solubility in water, which is lower at higher temperatures. In addition, warmer temperatures cause thermal stratification with a warmer surface layer on top of colder water. If this stratification persists for a long time in summer, oxygen can be depleted by organisms in the deep zone, but can not be replenished due to a lack of mixing. The resulting low oxygen concentrations can be problematic for all forms of higher life.
The research team used long-term data spanning over at least 30 years and hydrodynamic models to investigate how water temperature, lake stratification and oxygen concentrations in the deep layer of 46 German lakes change in response to climate warming.
Long-term data: Lake surface temperature has risen more strongly than air temperature by 0.5 degrees Celsius per decade over the past 30 years
Analysis of long-term data shows that the average annual surface temperature of all lakes has risen by 0.5 degrees Celsius per decade (°C/decade) between 1990 and 2020. This means that German lakes have warmed more strongly than the air over the same period, with an increase of 0.43 °C/decade. Water temperature in the deep water has remained almost constant.
Between 1990 and 2020, oxygen concentrations were below 2 milligrams per litre (mg/L) in 51 percent of summer measurements and even 62 percent of autumn measurements. This concentration is considered to be a critical threshold for the survival of many freshwater organisms. "The occurrence of low-oxygen conditions has increased in parallel with warmer temperatures, especially in autumn. This is due to the more stable thermal stratification," explained IGB researcher Robert Schwefel, lead author of the study.
The long-term data come from, among other places, Lake Arendsee in Saxony-Anhalt, where IGB operates its own monitoring station. | © Sylvia Jordan/IGB
Climate change: not increasing temperatures but longer thermal stratification is the main driver for deoxygenation
The researchers used hydrodynamic lake models to simulate future temperature and oxygen trends up to the year 2099 based on different emission scenarios of the Intergovernmental Panel on Climate Change (IPCC). Simulations were performed in twelve lakes for which high-resolution measurement data were available for model calibration.
Under the pessimistic emissions scenario RCP 8.5, which assumes a continuous increase in greenhouse gases until the end of the century, the surface temperature of the lakes would continue to rise by 0.3°C/decade until 2099. In the imtermediate scenario, RCP 4.5, the increase is only 0.18°C/decade. For the optimistic RCP 2.6 scenario, in which the increase in air temperature is limited to about 2°C above pre-industrial levels, only a minimal increase of 0.04°C/decade is predicted. Consistent with observations over the last thirty years, the temperature at depth would rise much less. "The increasing temperature difference between the surface and the deep layer increases the temperature stratification," explained IGB researcher Michael Hupfer, who led the project funded by the German Länder-Arbeitsgemeinschaft Wasser. The model calculations show that under the pessimistic emissions scenario, summer stratification would be extended by up to 38 days by the end of the century compared to the period from 2006 to 2016 (by 22 days for RCP 4.5 and 13 days for RCP 2.6).
The increasing stratification increases the risk of oxygen depletion in the deep layer: Using a simplified oxygen model, the team showed that oxygen concentrations in the deep layer would decrease by 0.7 to 1.9 mg/L in response to the extended stratification period (RCP 4.5 by 0.6 mg/L or RCP 2.6 by 0.2 mg/L). "This means that larger areas of the hypolimnion would remain hypoxic or even anoxic, especially in autumn. This would heavily impact fish habitats and the chemical conditions at the lake sediments," said IGB researcher Robert Schwefel.
Reduced nutrient loads could mitigate negative effects
To mitigate the effects of climate change, other stressors can be reduced, such as nutrient loads like nitrates and phosphates from urban and agricultural discharges. These act as fertilisers in water bodies and stimulate biological processes such as algae growth. In turn, oxygen consumption in the deep wates increase as more oxygen is depleted during subsequent decomposition processes.
In their study, the team also modelled the combined effect of climate change and reduced nutrient inputs on the oxygen balance. The good news: even in the most pessimistic emissions scenario, a reduction in nutrients by one trophic state level would lead to increased oxygen concentrations, which would mitigate the effects of warming. Even in the most pessimistic climate scenario, RCP 8.5, oxygen concentrations in the deep water increased when oxygen consumption in the deep water was assumed to be typical for conditions with reduced nutrient inputs. "Our results indicate that the negative effects of climate change on the oxygen balance of lakes can be effectively mitigated by increased efforts to limit nutrient inputs," said Michael Hupfer.
Background: emissions scenarios
The IPCC emissions scenarios are designed to model potential future climate developments based on different assumptions about societal, economic and technological changes. The RCPs (Representative Concentration Pathways) represent different scenarios that describe different levels of greenhouse gas emissions and their impact on global warming. The three main RCPs are:
- RCP 8.5: A scenario of continuously increasing emissions leading to a temperature increase of 4.3 to 5.4°C by 2100. This is the 'worst-case' scenario and would have severe consequences for the Earth's climate and ecosystems.
- RCP 4.5: A scenario in which greenhouse gas emissions peak around the middle of the 21st century and then gradually decline, limiting warming to 2.4-3.2°C by 2100. This is a moderate scenario.
- RCP 2.6: A best-case scenario involving strong and early mitigation action, potentially limiting temperature rise to 1.5-2°C, in line with the Paris Agreement targets. RCP 2.6 requires the deployment of carbon capture and storage (CCS) technologies and a rapid transition to renewable energy.
In summary, these scenarios outline a range of possible future developments, depending on how much and how fast humanity reduces its emissions. The path chosen will determine the climate impacts over the coming decades.
