Project at a Glance

Title: Characterization of year-round sensitivity of California's montane lakes to acidic deposition

Principal Investigator / Author(s): Sickman, J O

Contractor: Marine Science Institute and Department of Biological Sciences, UC Santa Barbara

Contract Number: A5-203-32

Research Program Area: Ecosystem & Multimedia Effects

Topic Areas: Acid Deposition, Ecosystem Impacts


Surface-water chemistry in four watersheds in the Sierra Nevada, California was studied year round during the period October 1986 through June 1988. The watersheds were located on both sides of the range and at altitudes greater than 2,800 m. The basins were predominantly granitic, and the surface waters had acid neutralizing capacity (ANC) generally less than 100 eq L-1. Two lakes were situated on the eastern escarpment of the Sierra Nevada: Ruby Lake (3724'50"N, 118'46' 15"W) and Crystal Lake (3735'36"N, 11901'05"W). The western Sierra lakes were located in Sequoia National Park: Topaz Lake (3637'30"N, 11838'11"W) and Pear Lake (3636'02"N, 11840'00"W). The sampling program was designed to coincide with different seasonal conditions in the watersheds (e.g. snowmelt, summer and winter stratification, lake mixing and maximum snowpack accumulation). All four lakes were found to be dimictic. They were ice-covered an average of five to six months of the year. The shallow lakes, Topaz (maximum depth, 5 m) and Crystal (maximum depth, 14 m), had weak thermal stratification during ice-free periods and stronger stratification under-ice. The deeper lakes, Ruby (maximum depth, 35 m) and Pear (maximum depth, 27 m), had strong temperature stratification during both winter and summer. These periods of stratification were associated with anoxia and large accumulations of ANC in the hypolimnion of Pear Lake. During February 1988 ANC reached 600 eq L-1 in the bottom water of Pear Lake. More than 50% of the ANC at this time was contributed by ammonium; permanent ANC was on the order of 250 eq L-1. Ruby Lake had no hypolimnetic accumulation of ANC during winter anoxic periods. ANC was highest at a depth of 1 m and this was probably increased by inflowing stream water. Topaz and Crystal Lakes had slightly depressed dissolved oxygen concentrations in their hypolimnia but very little accumulation of ANC under ice. The major solutes in the subsurface waters of the lakes reached their minima during the latter part of snowmelt and gradually increased through the remainder of the year. Lowest pH (5.7 - 6.0) was observed during snowmelt in these waters. Near-bottom concentrations of solutes generally had less seasonal variation and were higher than subsurface values. Lowest pH (5.8 to 6.0) was observed during periods of anoxia in the near-bottom waters. Mean snow depth and snow chemistry were similar among watersheds and between years. Volume-weighted mean pH ranged from 5.3 to 5.5, and the dominant cations were hydrogen and ammonium. The order of anions based on concentration was acetate = nitrate > sulfate > formate = chloride. Atmospheric, wet deposition of ammonium, basic cations and acidic anions can be greater during the period from May through October than during the period of snow accumulation. The majority of hydrogen loading occurred as snow. Mass balance calculations indicate that the watersheds in Sequoia National Park (Topaz and Pear) produce less than half as much ANC (per hectare) as the Ruby and Crystal watersheds. Because of the coarse temporal spacing of the samples during snowmelt, ANC yield from the watersheds was probably overestimated. In addition, estimates of atmospheric loading to some watersheds were underestimated because of coarse spatial distribution of the snow surveys. After making conservative corrections of ANC yield and atmospheric loading, the ratio of ANC yield to atmospheric loading of hydrogen ion ranged from about one at Pear Lake to about five at Crystal Lake.

For questions regarding this research project, including available data and progress status, contact: Research Division staff at (916) 445-0753

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