Limnological conditions in Mono Lake: contrasting monomixis and meromixis in the 1990s

Abstract

Mono Lake is a large, saline lake, located in the North American Great Basin and is subject to large variations in freshwater inflow as climatic conditions and diversion schemes have changed; consequently, major variations in chemical stratification occur. A transition from monomixis to meromixis occurred from 1994 to 1995. Lake-wide surveys of temperature, conductivity, dissolved oxygen, ammonium and chlorophyll profiles, Secchi depth and light attenuation, and Artemia monica abundances conducted throughout 1993, 1994, 1995, 1996 and 1997 document the contrasts between monomixis and meromixis. During the monomictic conditions in 1993 and 1994, the lake thermally stratified in March and mixed to the bottom by December, the hypolimnion became anoxic in late March and the water column was oxygenated to the bottom by December. During meromictic conditions in 1995, 1996 and 1997, the absence of holomixis during winter resulted in persistent anoxic conditions beneath the chemocline, an accumulation of ammonium in the monimolimnion and depletion in the mixolimnion, and low mixolimnetic chlorophyll concentrations in the spring and autumn. A comparison of the density differences between 2 and 28 m due to thermal versus chemical stratification indicated thermal stratification predominated in 1993 and 1994, while in 1995, 1996 and 1997 chemical stratification dominated the density differences. Ammonium, the limiting nutrient in Mono Lake, was lower in the upper mixed layer throughout 1996 and 1997 compared to the monomictic years, 1993 and 1994. During 1996 and 1997, the annual maxima in Secchi depths were among the deepest observed during the past 19 years, and reflected the lower phytoplankton abundance caused by decreased availability of nitrogen as a result of strong chemical stratification and the absence of a period of holomixis. In both 1996 and 1997, maturation of the spring generation of Artemia was slowed, peak abundance of the first generation of adult Artemia was a month later and percent ovigery, fecundity, and body size were reduced as compared to 1993 and 1994.