Thermal stress – how elevated water temperatures are changing aquatic ecosystems

This year, spring came exceptionally early, with high temperatures already setting off an explosion of vegetation in March. It has become pleasantly warm. Let him be the first to throw a stone who would prefer rains and frosts now. Let’s not kid ourselves, humans in our latitude will inherently flee to places that are dry and warm. It takes knowledge and awareness to admit that warmer does not mean better.

The progressive rise in average air and water temperatures is confirmed by hard data and numerous scientific studies. Poland has also seen a shift in thermal seasons toward longer summers and warmer winters [1], which can also be seen in rivers [2]. Climate change has meant that heat stress is beginning to affect all waters. Warming has both direct and indirect effects on aquatic organisms, and exacerbates existing problems such as eutrophication, pollution and the spread of disease. The pressure, which was often considered marginal, suddenly becomes a widespread problem.

Why rivers are getting warmer – heat stress and its many causes

The temperature of the waters is largely determined by solar radiation, so it varies depending on the time of day, season and latitude. It is correlated with air temperature, but the relationship is not linear. It also depends on the volume of water and local exposure due to channel geometry, shading and shoreline development. For example, the removal of riparian vegetation, including the cutting of natural riparian forests, reduces shading of the riverbed and contributes to warming waters. Also, the hydrology of the catchment affects the thermal regime. Warmer runoff from impermeable urban surfaces can significantly raise water temperatures, not to mention the effluent from power plants, industrial plants and treatment plants, which cause the so-called “water pollution”. thermal pollution.

Rivers have been transformed by humans for centuries and around the world, significantly modifying their thermal regimes. Disruption of hydrological connectivity through the construction of dams and weirs results in reduced flow velocity and water mixing dynamics, as well as increased exposure time, leading to faster heating. Also, widening and regulating river channels for transportation purposes increases the area and time of exposure to the sun. Low flows associated with water abstraction, regulation or droughts reduce the natural thermal buffering capacity, leading to faster heating of waters.

Scientists predict that river water temperatures will rise with climate change, and this is a trend that can already be observed over large areas of Europe, North America and Asia, although the warming will vary between areas. Thermal regimes of rivers vary considerably within and between catchments, even over short distances (less than 1 km). They change with diurnal, seasonal and interannual cycles, reflecting climatic conditions (e.g., seasonality will be lower in tropical regions than in temperate ones) and local conditions (e.g., thermal buffering by groundwater recharge). The results indicate that surface water temperatures will rise by 0.03°C per year in the UK, 0.001-0.08°C in the US, and 0.03-0.05°C in China.

Although the measured and projected changes in water temperature associated with climate change appear to be very small compared to the natural diurnal, seasonal and interannual temperature variability recorded in many temperate rivers, the chronic nature of these changes could have significant ecological impacts. A comprehensive review of the issue of the impact of thermal stress on animals inhabiting rivers, which has just been published in the journal WIREs Water [3], inspired me to look into the problem.

Thermal limits of aquatic animals

Changes in water temperature have a direct impact primarily on ectothermic organisms, such as fish and aquatic invertebrates. These organisms function within certain critical temperature ranges, beyond which they are unable to carry out physiological processes. Thermal ranges of certain physiological activities have also been determined for selected freshwater animals, such as. Foraging, growth or reproduction.

For example, for Atlanticsalmon (Salmo salar), the upper thermal limit for growth is 22.5°C, although they can feed at higher temperatures. Limiting temperatures can depend on life stage, body size and ecological interactions. Garten and Gentry [4] showed that larval dragonflies(Libellula auripennis and Macromia illinoiensis) with longer bodies have higher thermal maxima than smaller individuals of the species. It is also known that juvenile Atlantic salmon are more heat tolerant than adults due to a different ratio of body surface area to body weight [5].

Although studies typically focus on the upper thermal thresholds of organisms, elevated temperatures, even well below thermal maxima, have also been shown to affect the survival and physiology of aquatic animals, including diapause, phenology and reproduction.

Critical thermal maxima are of greater importance for organisms adapted to cooler environments, such as. living near springs, at high altitudes or higher latitudes. The same is true for organisms living in regions with very stable temperatures (e.g., tropical rivers with temperatures between 22 and 34°C), which have adapted to conditions close to their maxima. Even with small changes in temperature (<1°C), but lasting for longer periods (weeks or months), their thermal limits can be significantly exceeded.

Thermal acclimatization, or changes in physiology, behavior and phenology

Animals adapt to changes in environmental conditions through adaptation. The ability to acclimatize thermally is largely dependent on the characteristics of the habitat. Animals living in rivers with long-standing and stable thermal regimes (e.g., at high altitudes and high latitudes, in tropical zones or areas with significant groundwater recharge), may be less able to acclimatize to changing conditions than animals living in more dynamic environments.

While some organisms (including, importantly, some invasive species) may benefit from warming, others will lose from it. For example, survival rates of juvenile cyprinid fish in the Yorkshire Ouse River in the UK have increased over 15 years as a result of warming waters [6]. Numerous studies suggest that forkbeards (aquatic insects of the order Plecoptera) are particularly sensitive to increases in water temperature, and their numbers in Europe are declining with climate warming [7], while caddisflies (Trichoptera) are considered more tolerant animals, and temperature increases appear to be beneficial to them [8].

Changes in water thermals can also affect the phenology of organisms, or the timing of life events. Temperature affects all aspects of fish reproduction, including gamete development, fertilization and larval hatching. As the waters warm, fish spawning is likely to occur earlier and embryo development will be faster, according to the study. Migrations of Atlanticsalmon (Salmo salar) from rivers to the ocean in North America since the 1960s. In the 1970s. move by an average of 2.5 days per decade [9]. Fish eggs also develop faster in warmer water. The increase in temperature from 8°C to 12°C accelerated hatching from 63 to just 38 days after fertilization [10].

The phenology of aquatic insects is less well studied, but changes of only tenths of a degree Celsius can significantly affect the timing of events in their life cycle. Warmer water usually leads to earlier emergence of imago (adult form), as individuals reach maturity sooner. The consequences of altered phenology in aquatic ecosystems can be significant, as it leads to desynchronization of many processes, disruption of food base availability, predator-prey interactions and/or habitat availability. With seasonality occurring on a smaller scale in tropical regions, the consequences of thermal changes may be less noticeable than in temperate climates.

Fleeing the heat, or changes in migration range

As rivers warm, mobile animals are likely to move toward cooler areas. This phenomenon can affect single individuals (e.g., fish temporarily using local thermal refuges) or entire populations (e.g., migration toward mountain stream refuges). In the Rhône River, a 1.5°C rise in temperature over 20 years has led to the replacement of cold-water species such as dace by more tolerant chub and barbel [11]. Such escapes are beneficial to mobile species, but significantly disrupt interspecific competition at target sites.

Changes in geographic range depend on the ability of individuals and species to spread. While some organisms will be able to move to regions with more favorable conditions, those that already live at higher latitudes and have fewer opportunities to migrate toward the pole will have nowhere to go. Therefore, populations living near springs, in the upper reaches of rivers, and in mountainous regions are likely to be among the most vulnerable to warming. Not only will these organisms have nowhere to escape to, but they will also face additional competition from organisms migrating from lower altitudes/latitudes. We can expect that isolated or relict taxa, as well as those associated with specific habitats (e.g., the Arctic) are most likely to become extinct first.

Indirect effects of thermic changes on river ecosystems

In addition to the direct impact of warming on river ecosystems, rising water temperatures will have many indirect effects, such as exacerbating the impact of other pressures such as eutrophication, chemical pollution and disease.

As a result of rising temperatures, eutrophication, in particular, can increase significantly. In warmer rivers with reduced flow in summer, nutrient concentrations will be higher even with similar nutrient loads. This, in turn, will promote faster growth of algae, reduce photosynthesis of macrophytes and other primary producers, and lower dissolved oxygen levels (which are lower in warmer water), with further consequences for plants and animals. It may also turn out that previous remedial measures aimed at stopping trophy at the current level or lowering it, with increased water temperature, will be far from sufficient and greater reduction of nutrient loads will be necessary.

Temperature can affect the toxicity of pollutants, for example, ammonia, pesticides or polyaromatic hydrocarbons PAHs. There is also evidence that it alters the bioavailability, toxicity and bioaccumulation of toxic metals. Although studies have shown that higher temperatures generally enhance the harmful effects of pollutants, we have exceptions to this rule. For example, some pesticides degrade faster in warmer water. In general, chemical toxicity and increasing water temperature may interact, but this synergistic effect is likely to be variable and its impact at the population level difficult to predict.

Elevated water temperatures can also promote the spread of diseases, as they increase the likelihood of bacterial and viral infections becoming established and more widespread, as well as pathogens that have relatively high temperature thresholds. For example, a study by Marco-López and co-authors [12] predicts an increase in the frequency of salmonid diseases, including yersiniosis, boils and white spot, as a result of climate change. On the other hand, for pathogens that activate at lower temperatures, the risk of some diseases may decrease (e.g., hemorrhagic septicemia below 14°C). The consequences of climate change on the spread of animal diseases are not well understood, but are an area of great concern and in need of intensive research.

Can we make rivers more resilient to future warming?

It is not particularly revealing to say that the optimal resilience to climate change is a river free of anthropogenic disturbance, with good longitudinal, lateral and vertical connectivity, minimal changes in the flow regime and hydrology of the catchment area, with a preserved floodplain and natural vegetation of the riparian zone. It is also clear that around the world such rivers are now an extremely rare phenomenon.

The most vulnerable to the effects of climate change are watercourses lacking shade, with deforested banks and high exposure to sunlight. In an attempt to improve their resilience to rising temperatures, we should therefore place emphasis on restoring the vegetation of the coastal zone. Also, groundwater inflow largely regulates the thermal regime and limits the temperature amplitudes caused by solar radiation. It is also crucial to ensure links to floodplains and restore habitat heterogeneity.

The greater the ability of rivers to buffer thermal changes, the more resilient the biological communities inhabiting them will be to progressive climate change. In situations of increased warming, organisms can survive by taking advantage of thermal refugia (such as hyporeic upwelling, deep pools, groundwater tributaries or sections of the river system located higher where temperatures are more accessible). Protecting thermal refugia is therefore crucial to increasing the resilience of biological communities to climate change.

Rivers will continue to respond to changes in climate and land use in the catchment, hydromorphological transformations, pollution and other anthropogenic pressures. Although we cannot predict all the changes that will occur in aquatic ecosystems, managing rivers in a way that builds their resilience to a variety of pressures can, at least to some extent, mitigate the effects of these undesirable impacts. While it is true that the ecological effects of pollution and habitat loss are expected to outweigh the effects of climate change over the next century, focusing efforts on protecting and restoring refugia and building resilience in the river system will be key to at least partially adapting to climate-induced water temperature change.

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In the article I used, among others, works:

1. Czernecki B, Miętus M, 2017. The thermal seasons variability in Poland, 1951-2010. Theor Appl Climatol, 127, 481-493. https://doi.org/10.1007/s00704-015-1647-z
2. Marszelewski W., Jokiel P., Pius B., Tomalski P., 2022. River thermal seasons in the Central European Plain and their changes during climate warming. Journal of Hydrology, 610, 127945. https://doi.org/10.1016/j.jhydrol.2022.127945.
3. Johnson M.F., Albertson L.K., Algar A.C., et al, 2024 Rising water temperature in rivers: Ecological impacts and future resilience. WIREs Water, e1724. https://doi.org/10.1002/wat2.1724
4. Garten C.T., Gentry, J.B., 1976. Thermal tolerance of dragonfly nymphs. II. Comparison of nymphs from control and thermally altered environments. Physiological Zoology, 49, 206-213.
5. Breau C., Cunjak R.A., Peake S.J., 2011.Behaviour during elevated water temperatures: can physiology explain movement of juvenile Atlantic salmon to cool water? Journal of Animal Ecology, 80, 844-853. https://doi.org/10.1111/j.1365-2656.2011.01828.x
6. Nunn A.D., Cowx I.G., Frear, P.A., Harvey J.P., 2003. Is water temperature an adequate predictor of recruitment success in cyprinid fish populations in lowland rivers? FreshwaterBiology, 48, 579-588. https://doi.org/10.1046/j.1365-2427.2003.01033.x
7. Jourdan J., O’Hara R.B., Bottarin R.,Huttunen K.-L., et al., 2018. Effects of changing climate on European stream invertebrate communities: a long-term data analysis. Science of the Total Environment, 621, 588-599. https://doi.org/10.1016/j.scitotenv.2017.11.242
8. Hering D., Schmidt-Kloiber A., Murphy J., et al, 2009. Potential impact of climate change on aquatic insects: A sensitivity analysis for European caddisflies (Trichoptera) based on distribution patterns and ecological preferences. Aquatic Sciences, 71, 3-19. https://doi.org/10.1007/s00027-009-9159-5
9. Otero J., L’Abée-Lund J.H., Castro-Santos T., et al., 2013. Basin-scale phenology and effects of climate variability on global timing of initial seaward migration of Atlantic salmon(Salmo salar). Global Change Biology, 20, 61-75. https://doi.org/10.1111/gcb.12363
10. Crisp D.T. 1981. A desk study of the relationship between temperature and hatching time for the eggs of five species of salmonid fish. Freshwater Biology, 11, 361-368. https://doi.org/10.1111/j.1365-2427.1981.tb01267.x
11. Daufresne M., Roger M.C., Capra H., Lamouroux N., 2004. Long-term changes within the invertebrate and fish communities of the upper Rhône River: Effects of climatic factors. Global Change Biology, 10, 124-140. https://doi.org/10.1046/j.1529-8817.2003.00720.x
12. Marco-López M., Gale P., Oidtmann B.C., Peeler E.J. 2010. Assessing the impact of climate change on disease emergence in freshwater fish in the United Kingdom. Transboundary and Emerging Diseases, 57, 293-304. https://doi.org/10.1111/j.1865-1682.2010.01150.x