Work on water clarity on a time schedule |

Work on water clarity on a time schedule

An early version of a water clarity model for Lake Tahoe should be ready within 18 months, which, coupled with information in a just-released draft assessment of Tahoe’s environment, will help decision-makers understand better what is causing the lake’s declining clarity – and how to stop it.

In the meantime, however, restoration projects must continue until officials have a better idea of how to prioritize them, said researcher John Reuter.

“We don’t have an unlimited amount of time. Implementation of these restoration projects can’t be held back. That’s the nature of adaptive management,” said Reuter, of the University of California, Davis, Tahoe Research Group. “It’s not like all the restoration will be done in one night.”

Reuter, who participated in creating a Working Draft of the Lake Tahoe Watershed Assessment, said three primary questions must be answered in order to try to stop Tahoe’s declining clarity. Where does the material – nitrogen, phosphorous and sediments – come from that is causing the clarity decline? How much of a reduction in those nutrients is going to be necessary? And what is the best way to reduce those nutrients?

At least a partial answer to the first question is available in the draft Watershed Assessment, an 800-page-plus document recently made public. In the assessment, U.C. Davis has identified a “nutrient budget.” That budget tells researchers, generally, where the nitrogen and phosphorous comes from, the combination of which leads to algal growth and therefore contributes to the decline in clarity.

The budget identifies what percentages of the nutrient loading are caused by the atmosphere, stream runoff, runoff from other sources and groundwater leaking into the lake. Reuter said researchers hope to further refine that budget to be more specific. Officials, for example, could specify what kind of impact wood-burning stoves have or how much road salts are a factor.

When the water clarity model is completed, it will be used as a predictive tool to help answer the second question.

Nitrogen and phosphorous contribute to the algal growth primarily responsible for Tahoe’s declining clarity. The lake right now is considered a phosphorous-dependent system. That means as phosphorous enters the lake, algae grows. When nitrogen enters, algal growth doesn’t necessarily accelerate because there isn’t enough phosphorous in the lake.

The water clarity model should be able to tell researchers and decision-makers how much of the total phosphorous loading needs to be kept out of the lake to stop the declining clarity.

Additionally, as it’s refined, the model may be able to help answer the third question. Officials may be able to predict what certain environmental restoration projects will do to reduce the phosphorous loading. That will help decision makers such as the Tahoe Regional Planning Agency prioritize what needs to be done.

“There’s no single project, not 10 projects, where if they are done that will solve the problem. There are many of them,” Reuter said. “We really have to take the watershed approach to it.”

Visibility into the depths of Lake Tahoe has declined more than a foot a year for the past 30 years. Because of the numerous potential causes – including development, automobile exhaust, erosion and many other sources – creating a comprehensive model for the whole Tahoe watershed is a long, time-consuming process, Reuter said. Work started on it in early 1998.

Annual nutrient loading in Lake Tahoe.

– 418 metric tons of nitrogen.

– 10 percent from runoff

– 20 percent from streams

– 15 percent from groundwater

– more than 50 percent from atmospheric deposition

– 45 metric tons of phosphorous

– 24 percent from runoff

– 29 percent from streams

– 9 percent from groundwater

– 27 percent from atmospheric deposition

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