Inhabitants, tourists and conservationists were heavily concerned when Lake Stechlin, the "gem" located 80 km north of Berlin that is widely known for its great clarity, suddenly became turbid after a violent summer storm in July 2011. What was the reason? With the help of data collected through their long-term monitoring program, limnologists from the Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB) tried to reconstruct the events. "Our results suggest that the storm caused both nutrients and phytoplankton from the deeper water layers to reach the surface, where the algae could rapidly grow under the high-light conditions," says Dr. Peter Kasprzak, the first author of the article in "Ecosystems". "In addition, water transparency was critically affected by large quantities of suspended calcite particles, which had formed due to the high photosynthetic activity of the phytoplankton."
Whether the succession of events suggested by the analysis of the long-term data was actually correct was assessed in a field experiment at IGB’s LakeLab – a research platform floating on Lake Stechlin that consists of 24 enclosures, each 9 meters in diameter and twenty meters deep. In the experiment, the scientists simulated a strong summer storm in four of the enclosures, while four additional enclosures remained unchanged and served as controls. With the results of this second study published in "Global Change Biology", IGB could now verify the hypothesis.
Due to temperature-related differences in water density, deep, nutrient-poor lakes such as Lake Stechlin are characterized by stable water stratification during summer. The lakes develop a warmer epilimnion (upper layer) reaching down to 8 to 11 meters depth that is separated from the colder meta- and hypolimnion (deeper layers). This stratification has major influences on the physical, chemical and biological processes in lakes. Stratification prevents the exchange of nutrients between the deeper water layers and the epilimnion. The available nutrients in the epilimnion are quickly consumed by the phytoplankton, so that the algal population size becomes limited by nutrient availability and the lake remains relatively clear. As a result, light can penetrate into the more nutrient rich metalimnion. In addition to the algae in the epilimnion, a distinctive phytoplankton population occurs in these deeper layers, consisting mainly of cyanobacteria that are able to grow with less light.
Due to extreme summer storms, which are predicted to become more common during the course of climate change, the stratification erodes by mixing the water layers. This phenomenon was mimicked by the IGB scientists in the LakeLab enclosures with the help of water pumps. Subsequently, the response of phytoplankton was followed over six weeks. "In fact, algal development in the epilimnion was substantially boosted after the mixing," says Dr. Darren Giling, the first author of the experimental study published in "Global Change Biology". The reason is that nutrients and phytoplankton have been moved from the deeper water layers to the surface, where the algae profited from the nutrients and the ample light supply, leading to a massive algal bloom. "We were particularly surprised to see that this high phytoplankton activity and the increased turbidity of the surface water persisted for more than four weeks, even though the simulated storm lasted only four hours," explains Giling.
With the LakeLab experiment, IGB was able to essentially reproduce the events observed after the natural heavy storm event of 2011. Furthermore, the results suggest that increases in the frequency or severity of storms can conspicuously alter the metabolism of such clear, deep lakes. "The prospects for pristine lakes are worrying," summarize Darren Giling and Peter Kasprzak unanimously. Therefore the international community must take even more rigorous measures against climate change at the political level. At the same time, further efforts are needed to limit the input of nutrients to lakes like Stechlin.
Further information at the LakeLab.
Kasprzak P., Shatwell T., Gessner M.O., Gonsiorczyk T., Kirillin G., Selmeczy G., Padisák J., Engelhardt C. (2017) Extreme weather event triggers cascade towards extreme turbidity in a clear-water lake. Ecosystems. DOI: 10.1007/s10021-017-0121-4
Giling D.P., Nejstgaard J.C., Berger S.A., Grossart H.-P., Kirillin G., Penske A., Lentz M., Casper P., Sareyka J., Gessner M.O. (2017) Thermocline deepening boosts ecosystem metabolism: evidence from a large-scale lake enclosure experiment simulating a summer storm Global Change Biology 23: 1448–1462. Doi:10.1111/gcb.13512