Rivers Under Stress

All of us feel under pressure sometimes and are affected by stress in one way or another. Just like humans, the natural environment can be under pressure from stress, such as high temperatures, pollution, and habitat destruction (Figure 1). Often, the environment is affected by multiple stressors—more than one kind of stress at the same time. Delicate ecosystems like rivers are very sensitive to multiple stressors, and the plants and animals living in them can be severely affected. Rivers are essential for us, too, because we depend on them for drinking water and food production. Yet, they contain <1% of the surface fresh water on our planet [1], making it even more important to protect what little river water is available.

How can we protect our rivers if we do not know how multiple stressors affect them? Fortunately, scientists can create experimental miniature streams to safely test the effects of multiple stressors on a river, without actually harming the river or its inhabitants.

When 1 + 1 = 3

When multiple stressors put pressure on rivers, the effects on the aquatic organisms can sometimes be different from what we would expect [2]. Imagine it is a particularly warm and dry summer, and a river is affected by both a low water level, due to poor rainfall, and a high water temperature. There are many animals living in this river, and some are more sensitive to these stressors than others. Mayfly larvae are sensitive to both stressors. A low water level alone would kill one out of five mayfly larvae, and high temperature alone would also kill one out of five larvae.

So how many mayfly larvae die when the two stressors are present at the same time?

We would expect that 1 + 1 = 2, meaning that we could just add up the number of mayfly larvae dying from each stressor. This would mean that two mayfly larvae out of five are lost in the multiple-stressor scenario. However, we actually see that three out of five larvae die, which gives us a 1 + 1 = 3 situation (Figure 2). How can this be?

To answer this question, we must first understand how low water levels and high temperature put pressure on the river. Rivers are rich in oxygen because, when the water is flowing, it mixes with the air. Oxygen is essential for mayfly larvae and other aquatic animals. But if the speed of the water movement is reduced, less water and air are mixed together, which can decrease the amount of oxygen in the water. This can happen when the water level in the river is low because the water flows more slowly. An increase in water temperature can also reduce the amount of oxygen in the river, because oxygen does not dissolve well in warm water. But this still does not explain why we lose more mayfly larvae than expected when both stressors are present. So, what are we missing?

We must remember that rivers are complex ecosystems in which many kinds of organisms interact with each other. Stressors can change the way organisms interact. In our example, several kinds of algae live in the river, along with the mayfly larvae. Some algae prefer warm, slow water, and they multiply quickly under these conditions—using large quantities of oxygen as they do so. These algae reduce the already limited amount of oxygen in the water, which kills even more mayfly larvae than low water levels or high temperature alone.

Keep in mind that multiple stressors can interact in various ways. For example, we might also have a 1 + 1 = 1 situation, in which the harmful effect of multiple stressors on the river is less than expected. We could even have a situation in which 1 + 1 = 0, in which nothing changes because the two stressors cancel each other. But the most dangerous situation for a river is the 1 + 1 = 3 scenario.

How Can We Study Multiple Stressors?

To better understand the effects of multiple stressors, we can directly observe rivers by looking at changes in water quality and the organisms living there. For example, we could see what happens to a river after a hot summer with little rain. But every river is different, so it is impossible to know if the changes we see are caused only by low water levels and high temperatures, or whether they are caused by other stressors we did not consider. This could result in false conclusions and actions taken to protect the river that might not be effective.

An alternative to observing rivers themselves is to use aquariums or other types of containers in laboratory experiments, in which we can have everything under our control. The advantage of these experiments is that we can safely test various stressor combinations without harming the river. We can also have many aquariums with the same conditions, which are called replicates. If we add a stressor and observe the same change in all the aquarium replicates, we can be confident that the stressor we used is causing the change that we see.

The main disadvantage of using laboratory aquarium experiments is that they are over simplified compared to a real river. This is because fewer types of organisms are present in the experiment, and conditions like light and temperature do not change the same way they do in nature. To study multiple stressors, we need a realistic experiment in a natural setting, which means outside. We also need replicates so that we can be certain that our measurements are correct. At the same time, we do not want to harm the river itself. So, what can we do?

Exstream: Studying Miniature Streams

We can use water from a river to create artificial, miniature streams in an outdoor experiment (Figure 3). By pumping water from the river into small enclosures, we can simulate the natural conditions of the river. To create the right habitats for various river organisms, we add sand, gravel, and stones taken from the river, to make them feel comfortable. We also add fallen leaves that can be used as food by organisms such as the mayfly larvae in our example. By pumping water from the river, organisms that live in the river are carried into these experimental miniature streams. Like in a real river, they can decide to stay, or they can leave if the conditions are not right. For example, if mayfly larvae feel stressed, they can move out of the miniature stream through a hole in the enclosure, by way of the water current.

We call this system of miniature streams ExStream [3], and it can be manipulated like aquariums in a laboratory. For example, we can add warm water to simulate high water temperature, or we can control the speed of the water by changing the size of the inlet opening of the miniature streams.

If we manipulate only some of the miniature streams and let the others remain in natural conditions, we can compare them. The differences between manipulated and natural miniature streams tell us about the effects of the stressor. As we have many miniature stream replicates, we can see if the effect of the stressor is the same in all stressed miniature streams. By using stressors alone or in various combinations, we can identify the individual and combined effects of stressors on aquatic organisms like algae, insects, crustaceans, parasites, bacteria, and fungi. This helps us to understand what we can do to protect our rivers.

Conclusions

In nature, things are much more complicated than in our mayfly example. We only looked at two stressors and two organisms, mayfly larvae and algae. But many more organisms live in rivers, and more than two stressors can be present at the same time. The many river organisms can each be affected by different stressors, which can have multiple effects on the ways the organisms interact with each other. This complexity is what makes the consequences of multiple stressors so unpredictable, and why it is important to try to understand their effect on rivers. From the above example, we can learn that it is essential for slow-flowing river sections to avoid high water temperatures, for example by planting trees on the river’s banks—this offers shade to cool down the water.

Miniature stream experiments allow us to identify and understand stressful situations when 1 + 1 = 3. This is crucial given how delicate and complex rivers are and how important they are for us humans and for nature. Only by understanding how stressors interact can we effectively protect our rivers now and in the future.

Glossary

Habitat Destruction: ↑ Damaging or reduction of the living space of an organism or groups of organisms, for example by building a dam in a river.

Stressors: ↑ Any environmental change that might affect (mostly negatively) organisms or entire ecosystems. Examples include fertilizers and pesticides that enter the water, or increased water temperatures and lower water levels in rivers.

Ecosystems: ↑ All the organisms in a defined environment (a lake or forest) that live and interact with each other.

Aquatic Organisms: ↑ All living creatures, for example, plants and animals, living in environments made up of water, like rivers, lakes, and the ocean.

Larvae: ↑ Young insects are called larvae. After hatching from the egg, aquatic insects look very different from their adult flying forms. During this stage, the insects feed and grow.

Replicates: ↑ Copies of identical conditions within a scientific experiment. Only if the results are consistent among the replicates, we can be certain that they are not produced by chance.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

The idea for this contribution was developed within the framework of the Integrated Research Training Group (IRTG) of the Collaborative Research Centre RESIST (CRC 1439). In particular, we thank the IRTG management team, Vanessa Wirzberger, Verena Brauer, Bernd Sures, and Michael Eisinger for their support. The study was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) - CRC 1439/1 - project number: 426547801.