California Environmental Protection Agency

Air Resources Board

 No. 96-5

September 1996

 RESEARCH NOTES

Brief Reports to the Scientific and Technical Community

 Research Division, John R. Holmes, Ph.D., Chief

P.O. Box 2815, Sacramento CA 98512

Background:

Methods:

Results:

Significance and Application:

Related Projects:

Acidic deposition has been reported on both slopes of the Sierra Nevada mountains and in the Lake Tahoe basin. Granitic soils such as those found in the Sierra Nevada have very little acid buffering capability; thus an increase in the acidity of rain and snow in that area could increase the acidity of Sierran lakes and streams. Acidification of lakes and streams can seriously affect the health of aquatic biota. By comparing data from naturally acidic Sierran lakes with data from nearby nonacidic lakes, this study assessed the long-term effects of hyperacidity on aquatic biota in the Sierra Nevada.

Chemical conditions and the presence or absence of fish and amphibian populations were determined for 104 lakes in the Bench Lake/Mt. Pinchot area of Kings Canyon National Park in the Sierra Nevada in the summer of 1992. Thirty-three of the lakes were chosen for detailed analysis of their chemical and biological characteristics. Eight of these lakes are considered acidic (pH

In the 104-lake survey, pH ranged from 5.0 to 9.3. The pH of 10 of these lakes was below 6.0. Typically, Sierran waters are dominated by calcium, sodium, and bicarbonate ions. In this study, sulfate was found to be the dominant anion in 19 of the 33 lakes in the smaller survey; eight of these were acidic. The source of acidity and sulfate was found to be sulfuric acid produced by the oxidation of pyrite found in metamorphic and granitic rocks in the area. Below pH 6, increases in acidity caused by sulfuric acid resulted in increased concentrations of aluminum, sum of base cations (calcium, magnesium, potassium, and sodium), and nitrate. Increased mineral solubilities explain the increased aluminum and base cation concentrations, but the reason for the nitrate increase is not clear.  Trout were found in 7 of the 33 lakes. Their distribution appeared to be explained primarily by stocking history. Tadpoles of the yellow-legged frog (Rana muscosa) were restricted almost exclusively to lakes lacking fish, and exclusively to non-acidic lakes. Adults were more tolerant of the presence of fish and of acidic conditions. The sensitivity of tadpoles to low pH appeared to be greater in the field than in previous laboratory experiments. The distribution of trout appeared to have significant effects on the distributions and abundances of amphibian and invertebrate taxa, with large, mobile, and conspicuous taxa rare or absent in trout lakes, but relatively common in lakes lacking trout. The distributions of most macroinvertebrate taxa showed no relationship to lake acidity; however, one caddis-fly larva was rare or absent in acidic lakes. Several species of large-bodied microcrustacean zooplankton also were rare or absent in acidic lakes but common in non-acidic lakes.

This naturally acidic area may provide a good model for acidification effects on aquatic ecosystems. The results of this study suggest that increased acidification of high Sierra lakes would result in elimination of larval amphibians, large microcrustaceans, and a few macroinvertebrates from lakes and a decline in microcrustacean species richness. However, the most profound human impacts to date on high Sierra aquatic communities appear to be related to the presence of introduced trout species in the waters.

The Air Resources Board has also funded these related projects (ARB contract numbers are given in parentheses): Aquatic Biota in the Sierra Nevada: Current Status and Potential Effects of Acid Deposition on Populations (A932-138) and Aquatic Amphibians in the Sierra Nevada: Current Status and Potential Effects of Acidic Deposition on Populations (A932-139).