Scientists have warned that human activities are causing a phenomenon known as "browning" in fresh water, which is threatening aquatic animals such as rainbow trout.
What's happening?
Researchers from the Research Institute for Farm Animal Biology and the University of South Bohemia have found that fish are exhibiting higher stress responses because of increasing levels of organic compounds in waterways, according to a news release.
For instance, greater amounts of humic substances and fulvic acids — both of which are formed from the decomposition of plant and animal matter — are appearing in fresh water from agriculture, urbanization, deforestation, and rising global temperatures.
As these compounds accumulate, they not only turn waterways an unsightly brown color but also make it harder for fish to survive because they reduce oxygen levels.
In a four-week experiment on young rainbow trout, researchers observed that at a concentration of 50 milligrams of carbon per liter, the fish showed signs of distress at the molecular level.
The 34 genes impacted are crucial to a healthy immune response, detoxification, and reducing oxidative stress, according to Alexander Rebl of the Fish Genetics Working Group at FBN.
"We need to take the browning of water bodies seriously as an environmental problem. Even if we don't see any external damage yet, our results clearly show that fish already react to low levels of pollution," Thora Lieke, a researcher at the University of South Bohemia, said. "Now we need to look more closely at the long-term effects on fish populations and the whole aquatic ecosystem."
The findings were published in the Journal of Hazardous Materials.
Why is browning in waterways concerning?
The team explained that the browning of fresh water could have long-term impacts on the health of fish.
"Constant stress costs the animals energy," the release stated, adding that chronic stress could make them more vulnerable to disease. This could have knock-on effects on the entire food web, as reduced visibility in the water can hinder the amount of sunlight that reaches the depths, making it harder for plants to grow. Many fish feed on plants, so with less food available, aquatic animals may not have enough to eat.
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Never 👎 Furthermore, browning disrupts nutrient cycling and increases the amount of planet-warming carbon dioxide and methane in the atmosphere.
Browning may also increase waterborne illnesses, contaminated seafood, and poor drinking water for humans who swim, eat fish, and get drinking water directly from freshwater sources.
What's being done to promote healthier freshwater bodies?
The team noted that future studies are needed to better understand the long-term consequences of browning on aquatic animals and form a plan to protect waterways. Gaining insight into how the changing climate and human activities affect fresh water is also important, as humans depend on clean water for food, including rainbow trout.
Farmers and researchers are addressing nutrient runoff from cattle farming and fertilizer use on crops by planting diverse groundcover plants to absorb rainwater better and developing a copper-based hydrogel to capture excess nitrates used in fertilizers. Both methods can reduce the amount of nutrients that reach waterways, giving fish and other aquatic life better chances to survive.
If you'd like to help our fish friends, consider installing a rain garden or planting native flowers or shrubs in your yard to soak up rainwater and prevent runoff from entering freshwater bodies.
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