Ms. Adrian, what environmental changes are lakes already exposed to?
Rita Adrian: Lakes are influenced to a great extent by global warming and the input of substances from their respective catchment area. Both effects act in parallel, increasing the risk of the eutrophication of lakes – the excessive enrichment with nutrients disturbing the lake's ecological balance.
Increases in air temperature result in an increase in water temperatures. Since 1985, the global average of the temperature increase of the surface water of lakes in summer has increased by 0.32°C per decade. The warming trend of Berlin’s shallow Müggelsee since 1976 has in fact been 0.54°C per decade. Consequently, the upper limit of increases in air temperature adopted in the Paris Agreement in 2015 has already been exceeded in lakes.
What effect does the temperature rise have on lakes, how do they react to it?
Their thermal structure changes. We have observed an increase in the duration of the thermal stratification of lakes in summer. In productive lakes such as Berlin’s Müggelsee, the extension of the duration of thermal stratification leads to an increase in oxygen-free conditions in hypolimnic water, and subsequently to the release of nutrients such as phosphorus that were previously bound in sediment. This climate-induced lake-internal fertilisation promotes the eutrophication of lakes and consequently the development of Cyanophyceae blooms. Cyanophyceae are perfectly adapted to high temperatures, stable thermal stratification and high nutrient concentrations – under climate change, all three variables change to the advantage of these microorganisms. Other prominent changes in ecosystems in the context of global climate change are changes in phenology – the timing of distinct seasonal events. We are seeing drastic changes in this area: the ice starts breaking up earlier, which improves the light conditions in lakes. As a result, algae start to develop earlier in spring: on average, almost one month earlier in lakes of the northern temperate zone.
And what is your prediction for our lakes in the future?
It appears to be the case that lakes are becoming warmer and more oxygen-deficient in the wake of climate change, and that thermal structures will change in the long term. In addition, lake ecosystems are likely to undergo abrupt change once critical boundaries – referred to as tipping points – are exceeded.
Is it possible to predict such abrupt changes?
No, unfortunately not. We have investigated known occurrences of abrupt changes in European and American lakes for early warning signals. These are generic statistical changes in a time series that theoretically predict rapid changes. The mechanisms underlying the abrupt changes in the tested lakes are very well known, thanks to empirical time series spanning 30 to 40 years. We have tested four such signals and ascertained that they were not early warning signals that are able to predict abrupt changes with certainty.
Which topics will your group address in the future?
We will continue to conduct our climate impact research. Biodiversity, the metabolism of lakes, the role of temporal scales, and the effects of extreme events will be our key areas. To be more specific, we are exploring the following questions: How is biodiversity created and how can it be preserved? What time scale do we need to look at in order to understand the dynamics of lakes? Will we fail to identify central mechanisms if we only take measurements at monthly or weekly intervals? Global networks such as GLEON and NetLake, in which data of a high temporal resolution is measured in lakes around the world, now offer us a global perspective of the situation. Intense data exchange and close cooperation take place within such networks. Data management and the development of global data portals will increase in importance in our bid to improve the visibility and accessibility of data. It also remains important for our group to ensure that our findings are incorporated into policy. For example, our work has been incorporated into IPCC (Intergovernmental Panel on Climate Change) reports. IGB has a voice in global climate research, which we want to continue raising.
The interview was conducted by Wiebke Peters.