Getting to the bottom of lake clarity problem |

Getting to the bottom of lake clarity problem

Andy Bourelle

TAHOE CITY-New research indicates a declining amount of oxygen on the bottom of Lake Tahoe could become a new threat to the lake’s already declining clarity, the director of the University of California, Davis, Tahoe Research Group said Friday.

A layer of nutrients that could harm the clarity of the lake is locked in the ground at the bottom of Lake Tahoe, said Charles Goldman, who has studied Lake Tahoe for 41 years.

An “oxidized” layer of sediment is over the potentially harmful layer, holding it in.

However, if oxygen levels continue to decrease, the top layer could give way to nutrient-filled layer. Algal growth – the action believed to be the primary cause of the lake’s declining clarity – would flourish.

”If we ever lose the oxygen in the deep waters of Lake Tahoe, the battle will be lost,” Goldman said.

Fertilizers, sediment from erosion, air pollution and numerous other sources create what researchers call “nutrient loading” in Lake Tahoe. The combination of nitrogen and phosphorous from nutrients creates algal growth. In the early 1980s, Lake Tahoe went from being a nitrogen-dependent lake to a phosphorous-dependent one. What that means is there is significant amounts of nitrogen in Lake Tahoe. As phosphorous is loaded into the water, algal growth happens. But because there already is significant nitrogen, more of that element doesn’t immediately create more algae.

The oxygen on the lake’s bottom also is involved in the system. Over time, algae dies and descends to the dark area at the bottom of the lake. The bacteria breaks down the algae. However, to do that the bacteria uses oxygen, thus deteriorating the oxidized layer at the bottom, which locks in the potentially harmful nutrients.

So what’s the solution to stopping the oxygen depletion? It’s the same as stopping the algal growth: keep phosphorous from Lake Tahoe.

That can be accomplished by controlling stormwater runoff, stopping erosion, acquiring sensitive lands, restoring wetlands and completing similar projects in the Lake Tahoe Basin.

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