Fish in high-flow zone of rivers may be the first victims of climate change

Maanantai 25.9.2023 klo 18.56 - Mikko Nikinmaa

Fish living in high-flow areas of rivers have chosen (or evolved in) that habitat, because they need a lot of oxygen for their high activity. Flowing water becomes well oxygenated, as it is continuously exposed to air. However, the amount of oxygen taken up by water decreases with increasing temperature, simultaneously as poikilothermic fish need more of it, as temperature increase speeds up their metabolism.

If temperature increases enough, fish heart cannot pump enough oxygenated blood to sustain metabolism. This we have clearly shown in our article Anttila et al, Comp Physiol Biochem A 275, 111340 (2023). Although the study was done on seabass, the principle holds also for river fish. The amount of oxygen initially reaching the gills, where it is taken up, further decreases the maximal temperature the fish can tolerate.

Zhi et al (Nature Climate Change 2023;  https://doi.org/10.1038/s41558-023-01793-3) have shown that the oxygen level in rivers throughout Europe and United States has decreased for the past forty years. While increased temperature was the main driver, the oxygen level decreased more than expected because of temperature increase. It is possible that the general eutrophication with increased oxygen consumption.

Regardless, associated with the physiological response to increased temperature of fish, the temperature-caused decrease in maximal amount of oxygen the water holds, can cause extinction of salmonids living in the fast-flowing head streams.

Physiological studies should be in the centre of climate change biology

Tiistai 12.4.2022 klo 15.49 - Mikko Nikinmaa

Climate change, and other environmental changes, affect the functions of organisms. The changes in populations and ecosystems follow these functional changes. If the functions of some organisms are not disturbed, the environmental change does not affect the ecosystem, if immigration and emigration can be accounted for.

These simple facts indicate that functional studies, i.e., physiology, should be in the centre of environmental biology. Indeed, a stone could have exactly the same molecules as an organism, but without functions it would still be a stone. However, physiological studies are marginalized in climate change research and environmental biology – there are less than 1/10^th of published physiological articles as compared to ecological articles within environmental biology. Furthermore, studies on animals account for less than 1/3^rd of the physiological studies.

In short, one carries out extensive ecological surveys and population genetic studies and observes that something has happened. This is the major problem with the research, it shows what has already occurred, but fails to evaluate why and how. With climate change research it is obvious that temperature increase plays a role, but only physiological studies can clarify, what the affected pathways are. Also, physiological investigations can answer in real time, if a disturbance is adequate to cause a perturbation in populations and ecosystems.

Climate change research as well as other environmental biology should be predictive. This requires that physiology becomes a central, not a marginal discipline. Studies require intensive, time-demanding work, which is often technically quite demanding. Because of this, the number of scientists working on physiological questions should be drastically increased. Only this makes it possible to turn environmental and climate change biology to predictive science, which is required to combat environmental problems.

Functions Determine Temperature Effects on Animals and Man

Torstai 30.9.2021 klo 19.05 - Mikko Nikinmaa

Climate change is one of the most studied topics of the present time. The web of science contains approximately 350 000 scientific articles studying the topic. Out of these, around 64 000 have studied the biology of climate change. However, only about 1500 of those appear to be concerned with animal or human physiology. This is very concerning, since any effects of temperature changes must be a result of physiological responses to the change. All the other findings on animals and man will be downstream consequences of the physiological adjustments. In view of the above, the recent compilation of reviews in the Journal of Experimental Biology is very welcome. Since it is open access, everyone can read it at https://journals.biologists.com/jeb/issue/224/Suppl_1.

As an example, polar bears have become iconic victims of climate change. There are several documentaries about skinny and diseased creatures about to succumb to increased temperatures as they find less food, when sea ice decreases because of an increase in temperature. However, if that were the only problem they are facing, their plight would probably be tolerable. But since their food availability is decreased, they need to spend an increased amount of energy to catch the prey. Because of the sea ice loss, the energy needed for locomotor activity is 3-4 times higher than expected. Another problem with endotherms is that an increase in temperature is less well tolerated than a decrease. Further, an increase in temperature is a serious problem in dry environments because water loss increases.

For ectothermic animals, temperature increase causes various problems. Dive durations of reptiles, turtles and amphibians decrease by about a third. This translates directly to decreased feeding efficiency. The thermal niches of fish become decoupled from the light-dark rhythms, which function as cues for reproduction etc. When thinking about thermal niches, it appears that both tropical and polar fish tolerate changes in temperature less well than temperate ones. Two overall problems are apparent in translating most temperature studies on fish to climate change scenarios. First, the temperature changes due to climate change are usually much slower than temperature changes imposed on fish in experiments. Second, individual variation as an important component of temperature responses of fish is hardly ever considered. Also, one knows very poorly, what the actual physiological mechanisms behind the measures of thermal tolerance are.

The major reasons why our understanding of physiological mechanisms behind the responses to climate change are poor is due to the fact that functional studies have not been considered important. However, any ecological or genetic response can only occur, if physiological changes take place earlier. Thus, predicting climate change effects on animal populations requires understanding how animals respond physiologically, not by observing changes that have already occurred (either ecologically or genetically).

Functional changes are at the heart of encironmental biology

Maanantai 2.3.2020 klo 15.21 - Mikko Nikinmaa

Environmental changes and pollution will only have an ecological effect, if they affect the function of some organisms in the ecosystem. Consequently, any environmental effect must be primarily functional, i.e. physiological. Toxicology is studying functional disturbances.

On the basis of the above three lines, environmental (and also evolutionary) biology must be based on functional studies and explanations. In view of this, it is very inappropriate that environmental physiology remains a minor discipline in environmental biology and toxicology, and evolutionary biology as compared to ecology and genetics. My ecologist and geneticist friends always disagree with this, and give the following arguments. The ecologist says that many of the effects are indirect, which thus shows that only ecological studies can explain the effects. However, the effect may be indirect to the species (or group) one is studying, but there must be a functional effect on some organism in the ecosystem. If there weren’t, there would be no change. The geneticist and evolutionary biologist says that only the genetic changes will be transmitted to future generations. Thus, if an environmental change has an effect in the genome, that will be the important effect. While it is true that only genetically coded effect will be transmitted over many generations, a genetic change will only manifest itself if it affects the functions of organism in such a way that fitness (i.e. the number of offspring reaching sexual maturity) is affected. If the genetic change does not affect any functions, it is neutral both from the environmental and evolutionary viewpoint.

In fact, function (i.e. physiology) is what makes a difference between a stone and organism. A stone could have exactly the same molecules as an animal, but without functions (physiology) it would still be a stone.