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Changes in nutrient dynamics caused by rising water temperatures and altered stratification patterns due to climate change are promoting the growth of harmful algal blooms. This is the outcome of a new long-term study led by the University of Bayreuth and conducted in the Franconian Lake District. The researchers report their findings in the journal Water Resources Research.
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Why it matters
Reservoirs currently hold around 10% of global freshwater storage capacity and fulfil a wide range of functions, from hydropower generation and recreation to providing drinking water. High water quality is essential, especially for direct human use. However, harmful algal blooms are increasing worldwide in reservoirs, leading to a deterioration of water quality and posing risks to human health due to algal toxins, particularly those produced by blue-green algae. It is already known that climate change is raising water temperatures and thus altering nutrient dynamics in lakes and reservoirs. Until now, however, the combined long-term effects of changing nutrient inputs and the impact of climate change on nutrient processing within lakes have remained insufficiently understood. Only with insights such as those provided by the Bayreuth long-term study can the long-term effects of climate change on water quality in reservoirs be described precisely, and suitable restoration measures can be developed.
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Rising water temperatures due to climate change accelerate many biological processes: e.g. algae grow earlier and more intensively, microorganisms break down organic matter more rapidly, and nitrogen is converted and thus consumed more quickly. The typical summer stratification of lakes – warm surface water and cold deep water – also begins earlier with increasing temperatures and lasts longer. This results in very limited exchange between surface and deep water, causing phosphorus to accumulate in deeper layers while oxygen no longer reaches the lower parts of the lake.
In their study, the research team led by Prof. Dr. Stefan Peiffer at the Hydrology research group, Bayreuth Centre of Ecology and Environmental Research (BAYCEER), analysed an extensive dataset covering 19 years (2000–2019). They examined water temperature, stratification duration, nutrient composition and oxygen content in four Franconian reservoirs: the shallow Altmühlsee and the three deep, interconnected Brombach reservoirs.
“The data show a significant increase in water temperature of up to one degree Celsius per decade as well as an increase in stratification duration of up to eighteen days per decade. The duration of oxygen depletion also rose by up to thirty-five days per decade,” says Peiffer. In all lakes, nitrogen concentrations decreased, whereas phosphorus concentrations increased. This altered nitrogen‑phosphorus ratio raises the risk of harmful blue-green algal blooms: algae require both nutrients to grow, so decreasing nitrogen levels in the water would normally limit algal growth. However, blue-green algae can fix atmospheric nitrogen and use it for growth. When nitrogen becomes scarce in the water, they gain a competitive advantage and proliferate more strongly than other species.
“For the first time, we have been able to show using real long-term data that increased water temperatures due to climate change lead to a higher risk of harmful blue-green algal blooms as a result of changes in stratification dynamics and enhanced internal phosphorus load from lake sediments. Our findings therefore highlight the urgent need to consider the impacts of climate change on the health and restoration of lakes worldwide, especially given that climate change is expected to intensify over the coming decades,” says Peiffer.
The study was conducted in close cooperation with the Ansbach Water Management Authority, which provided the long-term monitoring data. It was funded through the EU Horizon 2020 Programme with a Marie Skłodowska-Curie Grant (813438).
Prof. Dr. Stefan Peiffer
Chair of Hydrology
BAYCEER
University of Bayreuth
E-mail: stefan.peiffer@uni-bayreuth.de
K. S. As, M. A. Münch, G. Trommer, A. Pudelko, T. Behrends, S. Peiffer. Global Warming Enhances Nitrogen-Limitation in a Temperate Reservoir System Under Continued External Load. Advancing Earth and Space Sciences (2026)
DOI: https://doi.org/10.1029/2025WR040978
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Biologie, Meer / Klima, Umwelt / Ökologie
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