The Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB) demonstrated that the increase in summer storms, driven by climate change, is creating an unprecedented risk for deep and clear water lakes: the massive proliferation of toxic cyanobacteria.
The findings, published in Limnology and Oceanography Letters, challenge traditional environmental management strategies and pose new public health challenges.
The experiment in Lake Stechlin
A team led by Hans-Peter Grossart, Stella Berger, and Jens Nejstgaard designed an experiment at the LakeLab of Lake Stechlin (Germany) to simulate the effect of a severe storm in deep and clear waters.
- 24 experimental enclosures of nine meters in diameter and 20 meters deep were used.
- In some enclosures, a deep mixing of the water column was induced, mimicking the effect of a summer storm.
- Other enclosures remained as unaltered controls.
The methodology allowed for the separation of the effects of mixing from other external factors and the replication of the experiment under realistic conditions.
Results: cyanobacteria proliferation
The deep mixing mobilized nutrients and phytoplankton from the lower layers to the illuminated surface, causing an abrupt increase in algal biomass.
- In the first phase, mobile cryptophytes (Cryptomonas) dominated the community.
- Subsequently, filamentous cyanobacteria (Dolichospermum) proliferated, reducing water transparency.
- Finally, colonial diatoms (Asterionella formosa) showed significant growth.
Dr. Berger explained that in deep and clear lakes, algae can grow in lower layers thanks to sunlight penetration. When a storm drags them to the surface, they find ideal conditions to reproduce quickly.
Ecological and climatic impacts
The study describes a sequence of processes following the simulated storm:
- Alteration of thermal stratification.
- Uplift of nutrients such as phosphorus, nitrogen, silicon, and inorganic carbon.
- Stimulation of primary production and changes in phytoplankton composition.
The accumulation of biomass in deep waters can:
- Aggravate deoxygenation.
- Favor eutrophication.
- Alter carbon and nutrient flows to the bottom.
Models derived from the experiment estimate that a single storm could increase the annual production of cyanobacteria by 20%, and successive storms would have an even greater impact.
Health risks and lake management
Toxic cyanobacteria pose a danger to humans and animals. Professor Grossart emphasized that the warming of lakes affects not only shallow, nutrient-rich ecosystems but also deep and crystalline lakes.
These results challenge the traditional management strategy focused on reducing external nutrients. Blooms can originate from internal mechanisms, such as mixing caused by storms, even in lakes with low nutrient levels and without recent human-origin inputs.
Emerging threats and need for adaptation
The IGB concludes that deep and clear lakes, previously considered stable, face emerging threats that require:
- Proactive management strategies.
- Monitoring adapted to extreme weather phenomena.
- A deeper understanding of internal processes activated by climate change.
The increase in algal biomass could favor carbon sequestration, acting as negative feedback on global warming. However, it can also intensify oxygen loss and perpetuate eutrophication cycles, compromising water quality and the conservation of high-value ecosystems.
