Rose also identified another problem: Deep waters become less clear due to a multitude of factors, including erosion, algae growth, and manure runoff from nearby agricultural fields and housing estates. Turbid waters make plants less likely to survive, which means less photosynthesis and less oxygen below. And that, of course, is bad news for lake creatures. “Just like humans, every complex form of life on the planet depends on oxygen,” says Rose. “In water it’s 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. (Types of warm water, such as bass and carp, need 5 milligrams per liter.)
“Even when you go down to a low level of oxygen concentration need, it will show an impact on the performance of individual organisms in the water,” says Peter Raymond, a professor of ecosystem ecology at Yale University, who reviewed the paper. “They are not doing well. They become stressed, as you might imagine. ”
The combination of little oxygen and warmer water is of particular concern. For example, if temperatures and oxygen levels are not in the optimal range, this can distort the reproduction time of the fish, which affects the amount they reproduce. Heated waters can also overfill or deactivate their immune system, which can jeopardize the degree to which they can fight pathogens in a climate-altered environment.
Because fish are ectotherms, meaning they regulate body temperature based on outside temperature, their metabolism speeds up in warm waters, increasing the amount of oxygen they need to survive, says James Whitney, a professor of biology at Pittsburgh State University in Kansas, who was not associated with the study. “If it gets bad enough, they can suffocate and cause fish to be killed,” Whitney says.
For example, during the Kansas drought of 2018, Whitney recalls that the water in the streams was warmer, and there was less of it due to lack of rain. The fish extracted oxygen from the surface waters, but it was not enough for the tour, and some even died.
Deoxygenation can become a vicious circle. When the lakes become anoxic, they form a sediment at the bottom which then releases phosphorus which can trigger the growth of algae on the surface. Lakes can develop harmful algae blooms, who eat all that is left of oxygen. Some produce toxins that kill fish, mammals and birds; in extreme cases they can cause human illness and even death.
“It is not hypothesized that the organisms will be affected. That will 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 globally: Reducing greenhouse gas emissions will stop the warming and loss of solubility of lake waters. But local care is also important. “There is a direct climate impact here, but a lot can be done locally to maintain high oxygen concentrations,” Rose agrees.
Rose and several other co-authors in the research contribute GLEON (Global Network of Ecological Observatories on the Lake), a core group of scientists from around the world who are focused on conserving freshwater resources. They share data to capture ecosystem changes early, as lakes are among the first to show measurable shifts. Some of their recommendations include using data from one lake to learn about others and assess risk based on measuring local water temperature and dissolved oxygen levels in real time. Planting trees as a buffer zone around the lake 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 living near water bodies use fertilizer according to the instructions on the label to prevent excess nitrogen and phosphorus from entering the lakes, inadvertently fertilizing algae blooms.
“Proactive governance is needed – or will be needed – in the future to even maintain the status quo,” Rose says. And by “future” he does not mean decades. Thoughts for the next few years. “This is a constant question,” he says.
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