Rose also identified a second problem: Deep water becomes less clear due to a multitude of factors including erosion, algae growth, and the flow of fertilizers from nearby agricultural fields and residential developments. Murkier waters make plants less prone to survive, which means less photosynthesis and less oxygen underneath. And that, of course, is bad news for lake creatures. “Like humans, every complex life form on the planet depends on oxygen,” says Rose. “In water, this is in dissolved form.”
Each species has a unique critical oxygen threshold for survival. Deoxygenation particularly affects cold water fish such as trout, which need 7 milligrams of oxygen per liter of water, and salmon, which need 6 milligrams per liter. (Hot water species, such as bass and carp, need both 5 milligrams per liter.)
“Even when we go down to low levels of oxygen concentration requirements, there are demonstrated impacts on the performance of individual organisms in water,” says Peter Raymond, professor of ecosystem ecology at Yale University. , who revised the document. “They are not behaving well. They become stressed, as you can imagine. ”
The combination of low oxygen and warmer water is particularly worrisome. For example, if temperatures and oxygen levels are not in the optimum range, it can distort the reproduction time of the fish, affecting the amount they reproduce. Warming water can also overheat or deactivate their immune systems, which can compromise the degree to which they can fight pathogens in a climate-altered environment.
Because fish are ectothermic, meaning they regulate body temperature based on the outside temperature, their metabolism accelerates in hot water, which increases the amount of oxygen they need to survive, says James Whitney, professor of biology at Pittsburg State University in Kansas, which was not affiliated with the study. “If it gets bad enough, they can suffocate, causing the death of fish,” says Whitney.
For example, during a 2018 drought in Kansas, Whitney recalled that the water in the streams was warmer, and there was less because of the lack of rain. The fish ingested oxygen from the surface waters, but it was not enough to turn them around, and some of them died.
Deoxygenation can become a vicious cycle. When lakes become anoxic, they build up sediment at the bottom, which then releases phosphorus, which can cause algae to grow to the surface. The lakes can develop harmful algale it blooms, which eat all that oxygen remains. Some produce toxins that kill fish, mammals and birds; in extreme cases, they can also cause human illness and even death.
“It’s not a hypothesis that organisms are going to be impacted. It’s going to happen,” says Raymond.
While there is no way to directly add oxygen to lakes, he points out, there are other ways to improve ecosystem health. The biggest change must happen at the global level: Reducing greenhouse gas emissions prevents lake water from heating up and losing solubility. But local care also counts. “There is a direct climate impact here, but there is a lot that can be done at the local level to maintain high oxygen concentrations,” Rose agrees.
Rose and several other study co-authors contribute to GLEON (Global Lake Ecological Observatory Network), a grassroots group of scientists from around the world who are focused on conserving freshwater resources. They share data to capture changes in the ecosystem first, since lakes are among the first to present measurable changes. Some of its recommendations include the use of data from a lake to learn about others, and risk assessment based on real-time measurements of local water temperature and dissolved oxygen levels. . Planting trees as buffer zones around lakes can prevent erosion, which can increase water clarity and reduce nutrient leakage. This can be coordinated by state agencies managing water resources or individual lake associations. The Environmental Protection Agency also recommends that residents living near water bodies use fertilizers according to the instructions on the label to prevent excess nitrogen and phosphorus from entering lakes. , inadvertently fertilizing the flowering algae.
“Proactive management is necessary — or will be necessary — in the future to even maintain the status quo,” Rose says. And for “the future,” it doesn’t mean decades. That means in the coming years. “That’s an ongoing problem,” he says.
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