Last Updated on Thursday, 28 June 2012 21:17
Thursday, 28 June 2012 00:00
Status and Trend of Tahoe Water Quality
“One of our highest priorities has always been and continues to be water quality,” says Joanne Marchetta, Executive Director of the TRPA. While some news is good—the winter clarity interim target of 78.7 feet was met in the five-year period of the report—other water quality news is not good.
photo: Boys on pier at Valhalla by Michelle Sweeney
June 29, 2012
By Michelle Sweeney
This article builds on the previous publication, Threshold Evaluation Report, An Introduction, published June 22 at TahoeProject.org.
The Threshold Evaluation Report (Threshold Report) is an important tool for informing decisions about the Tahoe Basin environment. Produced every 5 years, the Threshold Report is the “check” element of the Tahoe Regional Planning Agency’s (TRPA) management cycle. Water quality is the subject of Chapter 4 of the 2011 Threshold Report. The information is organized into categories that correspond to the following geographic designations: pelagic (deep water), littoral (shore zone), tributaries, surface runoff, groundwater and other Basin lakes. “One of our highest priorities has always been and continues to be water quality,” says Joanne Marchetta, Executive Director of the TRPA. While some news is good—the winter clarity interim target of 78.7 feet was met in the five-year period of the report—other water quality news is not good.
The lake’s shoreline is a significant area of concern. Scientists utilize data on indicators such as plant and animal species status and water clarity. “All of those indicators taken together are suggesting possible relationships and problems in our nearshore,” says Marchetta. (For more information about some of these indicators visit the aquatic invasive species section at TahoeProject.org.) “Nearshore scientific work, which is ongoing and incomplete, is needed to understand these relationships in the nearshore,” says Marchetta. “In this Regional Plan we are doing all that we can do, all that we know how to do, based upon current scientific information for the nearshore.” There is insufficient data on the nearshore to determine status or trend of the nearshore environment—information to this effect is absent from the Threshold Evaluation water quality chapter. There is however enough information about pelagic Lake Tahoe and its tributaries to facilitate the determination of status and trend for these geographic designations.
The overall status of the pelagic waters is somewhat worse than targets and the trend is moderate decline. Data shows an increase in phytoplankton primary productivity and a decrease in annual average secchi depth, both of which indicate a declining condition. In the case of phytoplankton primary productivity the status is considerably worse than target and the annual average secchi depth is only somewhat worse than target. It is only the winter average secchi depth that is trending in a positive direction in the pelagic category. Winter average secchi depth is at or somewhat better than target, an improved condition.
Meanwhile, the overall status of tributary water quality is considerably worse than target and the trend is little or no change. The tributary data shows moderate improvement with regard to suspended sediment and total phosphorus concentration and little or no improvement with regard to nitrogen concentration. Phosphorus and nitrogen concentration in tributaries of Lake Tahoe are considerably worse than the targets set for each while suspended sediment concentration is only somewhat worse.
In short, water quality has been, and needs to continue to be a focus of environmental improvement in order for the environmental goals for Lake Tahoe to be met. Chapter 3, Section 8 of the draft Regional Plan Update document discusses hydrology and water quality, describing policies for managing the watershed that are aimed at improving water quality.
Tahoe Regional Planning Agency Regional Plan | Draft 2011 Threshold Evaluation Report