Taxonomy and Distribution of Benthic Diatoms from Mono Lake, California, U.S.A.

Abstract

The benthic diatom flora of Mono Lake is described from samples taken from three localities at depths of approximately 1, 5, and 10 meters on rock and sediment substrates. Relative abundances of 30 taxa were recorded from duplicate counts of 500 valves from each of 18 samples. Dominant taxa included Navicula crucialis, Nitzschia frustulum, N. latens, N. reimerii sp. nov., N. monoensis, sp. nov. and Anomoeoneis sphaerophora var. minor var. nov.; taxonomy, morphology, and distribution of these taxa are considered. Navicula crucialis, previously recorded from saline lakes in North Africa and highly variable in outline, was most abundant in sediment samples. Anomoeoneis sphaerophora var. minor, Nitzschia reimeri, and N. monoensis were most abundant in deeper waters whereas both N. latens and N. frustulum occurred most frequently in collections from shallow waters. Smaller individuals of N. latens were broader than larger specimens, and some N. frustulum cells resembled N. austriaca. Nitzschia monoensis has numerous girdle bands and prominent external distal raphe ends. Navicula crucialis, A. sphaerophora var. minor, and Nitzschia monoensis were observed with internal valves. Mono Lake, located at the western edge of the Great Basin (Fig. 1), is a hypersaline (95 g/L in 1990) and alkaline (pH 10.0) (Herbst, unpublished data) remnant of Pleistocene Lake Russell. At a current surface elevation of 1,943 m, the lake has fluctuated considerably in the late Holocene (Stine, 1990). During the past 50 years, the lake has undergone a net decline of 14 m and a doubling of salinity, both attributable to diversion of tributary streams by the city of Los Angeles (National Academy of Sciences, 1987). The major ion content is dominated by sodium, with high anion concentrations of bicarbonate in addition to carbonate, chloride, and sulfate (Herbst, 1988). The benthic environment is composed of sand and organic-rich mud sediments over much of the basin. Hard substrates occur as localized calcareous tufa formations, pumice and alluvial granite cobble, and gravel along the west, north, and south shores (Herbst, 1990). The high salinity of Mono Lake restricts macrofaunal diversity to eight known benthic insects, all dipteran larvae (Herbst, 1988) and a single zooplankton species, Artemia monica Verrill. The dominant benthic insect is the alkali fly Ephydra hians Say (Ephydridae), a herbivore that grazes epilithic algae and epipelic microbial mats. Benthic algae from Mono Lake with which diatoms are known to occur include Ctenocladus circinnatus Borzi (Chlorophyceae) ' Portions of this study and support for D. B. H. were provided by a grant from the University of California Water Resources Center. We thank T. J. Bradley for informative discussions and C. W. Reimer for use of the Diatom Herbarium, Academy of Natural Sciences of Philadelphia. Publication costs, in part, are being met by a grant from the Darbaker Fund of the American Microscopical Society. TRANS. AM. MICROSC. Soc., 111(4): 338-355. 1992. ? Copyright, 1992, by the American Microscopical Society, Inc. This content downloaded from 207.46.13.64 on Sat, 03 Sep 2016 04:09:50 UTC All use subject to http://about.jstor.org/terms VOL. 111, NO. 4, OCTOBER 1992