Rose also identified a second problem: Deep water becomes less clear due to a host of factors, including erosion, algae growth, and fertilizer runoff from nearby agricultural fields and residential developments. Cloudier waters make plants less likely to survive, which means less photosynthesis and less oxygen below. And that, of course, is bad news for the lake creatures. “Just like humans, every complex life form on the planet depends on oxygen,” says Rose. “In water, it is in dissolved form.”
Each species has a unique critical oxygen threshold for survival. Deoxygenation particularly affects cold water fish such as trout, which needs 7 milligrams of oxygen per liter of water, and salmon, which needs 6 milligrams per liter. (Warm water species, like sea bass and carp, both need 5 milligrams per liter.)
“Even when you get down to low levels of oxygen concentration requirements, there are demonstrated impacts on the performance of individual organisms in the water,” says Peter Raymond, professor of ecosystem ecology at the University of Yale, who reviewed the article. “They don’t perform as well. They become stressed, as you can imagine.
Of particular concern is the combination of low oxygen and warmer water. For example, if temperatures and oxygen levels are not in the optimal range, it can skew the breeding schedule of fish, affecting the amount they breed. Warming waters can also overload or disable their immune systems, which can compromise the extent to which they can fight pathogens in a climate-altered environment.
Because fish are ectotherms, that is, they regulate their body temperature according to the outside temperature, their metabolism speeds up in warm waters, which increases the amount of oxygen they have. need to survive, says James Whitney, professor of biology at Pittsburg State University in Kansas. who was not affiliated with the study. “If it gets bad enough, they can suffocate, causing the fish to die,” says Whitney.
For example, during a drought in Kansas in 2018, Whitney recalls that the water in the streams was warmer and there was less due to the lack of rain. The fish swallowed oxygen from the surface water, but there was not enough for everyone, and some of them died.
Deoxygenation can become a vicious cycle. When lakes become anoxic, they accumulate sediment at the bottom, which then releases phosphorus, which can trigger algae growth on the surface. Lakes can develop harmful seaweed blooms, which consume any remaining oxygen. Some produce toxins that kill fish, mammals and birds; in extreme cases, they can also cause human illness and even death.
“It is not hypothetical that the organisms are affected. 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 has to happen globally: Reducing greenhouse gas emissions will prevent lake waters from warming up and losing their solubility. But local guarding matters too. “There is a direct impact on the climate here, but there is a lot to be done locally to maintain high oxygen concentrations,” agrees Rose.
Rose and several other study co-authors are helping GLEON (the global network of ecological lakes observatories), a group of scientists from around the world who focus on the conservation of freshwater resources. They share data to quickly detect ecosystem changes, as lakes are among the first to show measurable changes. Some of their recommendations include using data from one lake to learn about others and assessing risk 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 runoff. This can be coordinated by state agencies that manage water resources or individual lake associations. The Environmental Protection Agency also recommends that residents who live near water bodies use fertilizers according to label directions to prevent excess nitrogen and phosphorus from entering lakes, fertilizing inadvertently blooms algae.
“Proactive management is needed – or is going to be needed – in the future to even maintain the status quo,” says Rose. And by “the future” he does not mean decades. He means in the next two years. “It’s a permanent problem,” he says.
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