Hidden crises: underestimated impacts of climate change

Professor Hupfer, you are investigating how climate change affects oxygen levels in lakes. What observations have you made and what recommendations do you have?

Michael Hupfer: Long-term studies show that the warming of lakes is leading to a worrying decline in oxygen levels, especially in deeper layers. In a study for Germany, we showed that the surface temperature of 46 lakes analysed increased by 0.5°C per decade between 1990 and 2020. This increases temperature stratification in summer and hinders the exchange of oxygen between surface and deep water. Critical oxygen levels are already below 2 mg/L in more than half of the summer and autumn measurements, which is life-threatening for many organisms. 

Our model projections to 2099 show that under a pessimistic climate scenario (RCP 8.5), the summer stratification period could increase by up to 38 days. This would further reduce oxygen concentrations in deep water and threaten the habitats of many organisms. During the autumn months, large parts of deep zones could even become completely oxygen-free, with drastic consequences for fish, other organisms and the chemical processes in lake sediments. One possible solution is to reduce nutrient inputs such as nitrate and phosphate, which are often discharged from agricultural and urban sources. Our calculations show that reducing nutrient pollution could improve oxygen concentrations even under the pessimistic climate scenarios mentioned above. This approach could make a significant contribution to minimising the negative effects of climate change on the oxygen supply of lakes.

Many streams and rivers dry up periodically. Professor Gessner, you were part of an international consortium that investigated the impact of dry spells on the biodiversity of these waters. What did you find?

Mark Gessner: More than half of the world’s rivers periodically run dry. The resulting exposed sediments are a highly dynamic but poorly understood system. By analysing environmental DNA, we were able to comprehensively assess the biodiversity of microbes, invertebrates and plants in such sediments from 84 intermittent rivers in 19 countries. A key finding is that biotic interactions between bacteria, fungi, algae and protozoa influence the covariation of community composition more than environmental gradients. This outcome is surprising, because abiotic factors such as climatic conditions or nutrient availability were previously thought to dominate. Instead, we found that interactions between organisms are crucial for the stability and adaptability of the communities in dry river sediments. 

We also found evidence that resource scarcity and prolonged drought favour microorganisms that are particularly efficient at using scarce resources, and that these may play a key role in the functioning of these ecosystems. Although it is unclear how these communities will evolve in the long term under the influence of global changes, such as climate change and the increasing fragmentation of riverine landscapes, our results suggest that rivers and streams that are subject to periodic drying need to be explicitly included in river conservation strategies. At the same time, our data show that dry riverbeds are complex systems whose biodiversity is more strongly influenced by biotic processes than previously thought. These findings should be taken into account in future models aimed at predicting the impacts of climate change.

Climate change is known to alter the regional distribution of freshwater species in many ways. Dr Domisch, should the biological indices used for the ecological assessment of European rivers also be adapted as a result?

Sami Domisch: Benthic macroinvertebrates, for example, are used as biological indicators for the assessment of European watercourses. The composition of the species community of a water body type is compared with a reference state, i.e. the species community of a similar, natural water body type. If the distribution of species changes as a result of climate change, such a comparison may become difficult in the future, as the species communities in these natural, unpolluted water bodies may also change. 

There is a risk in focusing solely on the number of species, as 'more is not necessarily better': Within a species community, individual species can be unique in their ecological function, but also very similar to each other. A change in species distribution may be accompanied by a visible change (e.g. a species may no longer be observed in an area), but its ecological function may be taken over by another species, making the ecosystem equally resilient to environmental change. Of course, this is only possible to a certain extent. Conversely, a new species can only take on a few new ecological functions regionally. At present, the assessment indices correlate strongly with species occurrence, i.e. taxonomic diversity, but in order to be able to assess the ecological status of a water body in the future, it is important to also analyse functional resilience on the basis of species communities. 

Fish change their behaviour in warmer conditions. Dr Kalinkat, you have documented this in terms of hunting behaviour. What are the implications for the survivability of the species concerned and the stability of ecosystems?

Gregor Kalinkat: In a study we conducted together with the German Centre for Integrative Biodiversity Research (iDiv) and the Friedrich Schiller University Jena, we have shown that climate change is indeed altering the foraging behaviour of fish such as cod and flounder in the Baltic Sea: As temperatures rise, the metabolism of these fish increases, which means that they actually need more food. However, instead of hunting larger, more energy-rich prey, they eat more small and common prey such as small crustaceans, worms or brittle stars. This 'flexible foraging' may seem sensible in the short term to gain energy quickly, but it turns out to be inefficient in the long term. Our modelling suggests that it increases mortality and makes extinction more likely for species at the top of the food web. 

In particular, larger predatory fish species could starve because they cannot consume enough calories despite high food intake. This inefficient adaptation also has far-reaching consequences for the communities in the affected ecosystems. If predatory fish become extinct or severely depleted, the balance of the entire food web is upset. Smaller prey species may proliferate excessively, affecting habitat structure and resource availability. Our observations of Baltic Sea fish may also help to explain why their stocks have not recovered much recently, despite much reduced fishing quotas. However, similar behavioural adaptations could also occur in other groups of animals at the top of a food web, whether in aquatic or terrestrial habitats. This could make entire ecosystems more vulnerable to the effects of climate change.

Professor Wolinska, your research focuses on the relationships between parasites and their hosts. What does this actually have to do with climate change? 

Justyna Wolinska: In a meta-analysis of 60 studies, we showed that global warming significantly increases mortality from infections in cold-blooded animals such as crustaceans, fish and molluscs. As these animals are dependent on the ambient temperature, they are particularly sensitive to rising temperatures. In the case of bacterial infections, mortality increases with rising temperatures because the animals’ metabolism is accelerated and pathogens often grow faster in warmer conditions. In the case of fungal infections, the mortality rate of infected animals increases mainly at the thermal optimum of the fungus, i.e. it decreases when the temperature becomes too high. 

As a result, increased mortality of aquatic animals destabilises food webs and disrupts ecological processes such as the decomposition of organic matter. In addition, pathogens could spread as a result of warming, posing new risks to other species and potentially to humans. It is important that we continue to study the complex interactions between climate change, pathogens and hosts to better assess the long-term consequences for ecosystem function and stability.

Climate change requires a shift towards the increased use of renewable energies, including hydropower. Professor Jähnig, how will this affect the water bodies concerned?

Sonja Jähnig: In a review study, we have shown how diverse and numerous the negative impacts of hydropower on river biodiversity are. Using international examples, the study shows that this damage is caused by changes in river continuity, water flow, connectivity with floodplains and terrestrial habitats, and sediment and nutrient transport. Genetic exchange between populations is also affected by hydropower. In addition, dams act as physical barriers that impede species migration and disrupt their life cycles, while turbines significantly increase mortality rates – around one in five fish perish during passage.

 Hydropower plants also affect temperature and seasonal water flows, creating a mismatch between environmental conditions and a species’ reproductive cycle. It is therefore best not to build any new hydropower plants in biodiversity hotspots. We recommend the STREAM concept, which includes systematic planning, dismantling of redundant infrastructure, comprehensive socio-environmental impact assessment, participatory decision-making, and continuous monitoring and adaptive management. This is the only way to better understand and reduce the negative impacts of hydropower.

Professor Tetzlaff, you and your colleagues coined the term 'ecohydrological resilience' this year. What do you mean by this and how can this concept help us to better deal with the consequences of climate change?

Dörthe Tetzlaff: When climate extremes such as droughts or floods are discussed, the water already stored in our landscapes is often ignored. Under normal humid conditions, underground reservoirs are interconnected and extend both vertically and horizontally. Surface reservoirs, i.e. surface water, upper soil, but also vegetation, are sufficiently replenished by regular rainfall. In times of drought, surface reservoirs are depleted. Especially in dry periods, access to the so-called storage continuum, i.e. the amount of water stored underground in the soil and in the aquifers below, is crucial. This is because the volume and accessibility of this storage continuum determines whether water-related ecosystem services can be provided at landscape level at all times, from the wettest to the driest periods. 

In general, the greater the diversity of the landscape, the more connections to the storage continuum are potentially available. This increases hydrological resilience. The storage continuum ranges from areas with high storage capacity and ecohydrological resilience, such as wetlands, to areas with low storage capacity and low ecohydrological resilience, such as agricultural land and forest monocultures. It is therefore not only topography, soil types and geology that determine where water is stored in the landscape, but also the type of land use. 

In fact, the extent of a drought is largely determined by land cover, as different types of vegetation have different rates of evaporation and transpiration, which in turn depend on the availability of water in the subsoil. Droughts therefore do not affect whole regions or landscapes in the same way, but the effects of drought are specific and vary from one part of the landscape to another. 

Photos: David Ausserhofer/IGB