Clear appearances are deceptive, the water quality of the Stechlinsee has deteriorated considerably in the past 20 years. I Photo: Michael Feierabend
Many lakes are suffering from a “man-made” problem: they contain excessive nutrients, especially phosphorus, which enters aquatic ecosystems from wastewater or agricultural sources. Even more worryingly, various observations from around the world have recently shown that even some remote lakes which are largely unaffected by humans exhibit enhanced nutrient concentrations, resulting in stimulated productivity. This is also the case with Lake Stechlin, which generations know and appreciate as a crystal clear lake. However, we have observed dramatic changes in recent years.
Long-term measurements taken by IGB show that phosphate concentrations in the lake are now four times higher in spring than they were a decade ago. We were also able to detect highly accelerated oxygen depletion in the water of Lake Stechlin. Oxygen depletion is expanding from the bottom of the lake to ever shallower areas from year to year. The average depth of visibility is now less than six metres; 20 years ago it was nine metres.
Lake Stechlin – caught in a vicious circle?
All the signs are that Lake Stechlin is caught in a vicious circle in which several factors exacerbate each other. This leads to a rapid deterioration in water status. Higher phosphate levels in the lake result in increased algal growth. Some of the elevated algal biomass sinks into the depths of the lake and is deposited in the sediment, where carbon and phosphorus accumulate. When algal biomass decomposes, dissolved oxygen is consumed in the water, and may be fully depleted in a short space of time, particularly towards the bottom of the lake. The loss of oxygen alters the chemical conditions at the boundary layer between sediment and water. This causes the release of phosphate previously bound in the sediment, resulting initially in an input of substantial quantities of the nutrient into water layers close to the sediment. When mixing occurs throughout the lake next spring, the nutrient is distributed evenly in the water, ready for consumption by algae during the next growth period. And the vicious circle continues.
IGB’s long-term data series demonstrates that this vicious circle does indeed play a role in Lake Stechlin. It is irrelevant how the development was unleashed. A dramatic loss of oxygen can be observed at the end of the lake stratification period, before full mixing of the water body begins in autumn, particularly since 2016. Oxygen depletion is expanding from the bottom of the lake to ever shallower areas from year to year, not only at the deepest point at the north bay (69.5 metres), but also at the shallower bays to the west and south. This goes hand in hand with redissolution of phosphate from the sediment. As a consequence, phosphate concentrations in Lake Stechlin are now four times higher in spring after full mixing than they were a decade ago, a level that classifies it as a nutrient-rich lake.
A similar picture emerges regarding the total quantity of phosphorus in the water column. Simple calculations show that the water volume of Lake Stechlin contained a total of around 2,000 kilogrammes of phosphorus between 2000 and 2010, when the lake was still classed as nutrient-poor; by spring 2020, this figure had tripled. As a result, there has also been a strong increase in algal biomass over the past decade. This has a negative impact on the clarity of the water, reflected also by diminished visibility. Whereas the annual mean depth of visibility was around nine metres two decades ago, it now averages at less than six metres. In periods of pronounced algal blooms, visibility of even less than three metres was measured at times.
And the reasons: power plant or climate change?
Although the processes that result in a nutrient build-up in Lake Stechlin are precisely documented, it is difficult to determine the definitive cause. A massive intervention at the time was a water circuit that had been installed especially for the purpose of cooling Rheinsberg nuclear power plant. This process involved around 300 million litres of water being taken daily from an adjacent lake, Nehmitzsee, and being discharged, ten degrees warmer, into Lake Stechlin. The nutrient inputs that entered the system were much higher than is currently the case. Even so, they had little impact on Lake Stechlin.
This could be explained by the fact that, when the power plant was in operation, dense stocks of large mussels and underwater plants in the water passage linking the two lakes acted as a permanent nutrient trap. The shutdown of the cooling circuit in 1989 also interrupted the water flow in the channel connecting the two lakes. As a result, the mussel and plant populations were no longer effective at removing nutrients.
Another possible cause is related to climate change. This assumption is supported by the phenomenon that several of the world’s highly remote lakes, which are largely unaffected by humans, exhibit similar tendencies to Lake Stechlin. A key factor is the altered stratification and mixing behaviour of the lake: rising water temperatures under climate change result in the prolongation of summer stratification, when the warm surface water layer is separated from the cold deep water layer. This prolongs not only the growth period in the sunlit surface water, but also the period of time during which the oxygen reservoir is depleted in the deep water and phosphate is increasingly redissolved from the sediment. In the case of Lake Stechlin, this period is now around 30 days longer than it was 30 to 40 years ago.
What can be done?
We agree that, all things considered, it is irrelevant how the development was unleashed. Undertaking further scientific studies is not a mandatory precondition for reaching informed decisions on what action to take. Instead, the key question is whether the current developments can – and should – be tolerated. Or whether deliberations and efforts are required to slow down, stop, or even reverse the negative trend. Then the immediate follow-up question has to be: what action is worth considering and what action should be ruled out? This ultimately requires a social and political response.
