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Forecasting thermal mortality in aquatic ectotherms

Confirming what was previously proposed, aquatic invertebrates display a notably limited tolerance to increased temperatures in river water. However, their adjustment to such warmer environments needs a gradual process. These findings stem from the recently published article in Global Change Biology by a team of ecologists, including Ignacio Peralta-Maraver from MNat.

The study introduces a new methodology to measure how animals can perform under warming stress, making it easier to upscale laboratory work with real field conditions. Typically, warming tolerance of aquatic organisms is evaluated by subjecting them to a controlled environment in the laboratory. Under these conditions, the water temperature is progressively increased over a short time window of few hours, observing the critical temperature at which the organisms collapse. For example, amphipod species began to die at around 34 Celsius degrees under laboratory conditions. Thus, considering that natural rivers typically don't get warmer than 27 Celsius degrees, it was previously thought that these animals had a safety buffer of 7 degrees before death occurred. However, the short timeframe used in the laboratory do not provide a complete picture - much like a person can handle an hour in a sauna but shouldn't stay in there the entire day.

The amphipod especies Gammarus roeseli. Photo by Michal Manas.

The study, which uses amphipod species as model freshwater organisms, delves into how these life forms cope with intense and variable episodes of warming over extended periods, rather than merely brief hours. The research shifts away from the conventional focus on a singular critical temperature, instead exploring the likelihood of survival across more substantial and realistic timescales.

In simpler terms, the previously assumed safety margin proposed for the studied species doesn't hold up when examined over the span of weeks rather than mere hours. Also, whether the amphipods had acclimated to warmer or colder water made a substantial difference, as well as if they live in conditions of limiting oxygen or not. Just like other ectothermic animals, amphipods need more oxygen in warmer water. Therefore, they demonstrate superior heat tolerance if they have previously acclimated to conditions demanding higher oxygen consumption, or to environments characterized by lower oxygen availability.

Extreme heat poses significant strain on amphipods, with temperature alone not telling the whole story. The duration of exposure plays a vital role, as longer periods of high temperatures increase the stress experienced by these organisms. Merely relying on a specific temperature threshold is insufficient to measure their heat tolerance. Additional factors such as time and water oxygen levels must be considered for a comprehensive understanding of their ability to cope with extreme heat.

The work of Peralta-Maraver and his colleagues provide a better understanding of the critical temperature thresholds for animals in their natural environment and suggest a potential 60 to 100 percent increase in warming stress-related deaths due to climate change in the next 100 years. This innovative approach allows for prompt actions, such as improving water oxygen levels, to protect at-risk species. While global warming poses global challenges, water quality can be enhanced through regional interventions.

Verberk, W. C. E. P., Hoefnagel, K. N., Peralta-Maraver, I., Floury, M., & Rezende, E. L. (2023). Long-term forecast of thermal mortality with climate warming in riverine amphipods. Global Change Biology, 00, 1– 11.


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