Lake Tahoe geochemical study. 1. Lake chemistry and tritium mixing study

The study of vertical mixing in Lake Tahoe by temperature and oxygen measurements is limited to the top layer of the lake, since vertical gradients are extremely small at greater depth. Below 200 m the temperature gradient is about -1.6 X lo-” “C m-l, with little or no seasonal variation. A tritium profile was taken at a midlake station in 1973 together with samples for measurements on lake chemistry. Oxygen, nitrate, total inorganic carbon, and pI1 showed the effects of photosynthetic activity to a depth of about 80 m, well below the thermocline; average planktonic composition was calculated from these data. Silicate and carbonate alkalinity profiles were essentially uniform, showing that biological removal is negligible in comparison to the rate of vertical mixing. Essentially no variation in tritium concentration with depth was observed. Tritium measurements and input estimates in Crater Lake were used to calculate tritium concentration in Lake Tahoe between 1954 and 1973 and to cstimatc a ratio in tritium input of 32% precipitation to 68% vapor exchange. The tritium profile gives strong evidence for at least occasional complete mixing of Lake Tahoe. The last complete mixing may have occurred in March 1973 and at least one other such event took place between 1964 and 1968. Lake Tahoe is a large, clear lake covering an area of 499 km2 at an altitude of 1,897 m in the Sierra Nevada. Due to its great depth (max, 501 m; mean depth, 313 m) the temperature and chemistry are virtually unaffected by seasonal variations below the top layer of the lake. Furthermore, hypolimnetic vertical gradients are extremely small, making the problem of vertical mixing difficult to study. In August 1973, personnel from the Scripps Institution of Oceanography and from the Division of Environmental Studies of the University of California at Davis tested at Lake Tahoe the use of oceanographic methods for limnological studies. We here discuss geochemical measurements with reference to vertical mixing problems. We thank A. Birket, T. L. Broadwell, F. Dixon, D. Hasha, I-1. W. Paerl, R. C. Richards, and C. Toy for assistance in the ficld’ On leave from Swiss Federal Institutes of Technology (EAWAG), Diibendorf, Switzerland. work, and B. Horn Wilson, E. Toby Linck, and E. Slater for their work in processing samples. We are grateful to J. Edmond who made the silicate analyses. This study was supported by a grant from the National Science Foundation to the Isotope Laboratory, Scripps Institution of Oceanography. The tritium analyses were supported by NSF grant 29952 (principal investigator 1-I. E. Suess). The Schweizcr Gewgsserschu tzstiftung provided a grant to D. Imboden. Temperature and geochemical data A complete profile of water samples for geochemical analysis was taken at a midlake station (Table 1). Depth was measured by standard hydrographic techniques and temperature with reversing thermometcrs. Oxygen samples were fixed immediately and ?pH samples .were stored in full bottles to avoid exposure to the atmosphere; both measurements were made at the Lake Tahoe laboratory of UC, (Davis) within LIMNOLOGY AND OCEANOGRAPHY 1039 NOVEMBER 1977, V. 22( 6)