ULTRASTRUCTURE AND SEASONAL GROWTH-PATTERNS OF MICROBIAL MATS IN A TEMPERATE CLIMATE SALINE ALKALINE LAKE - GOODENOUGH-LAKE, BRITISH-COLUMBIA, CANADA

Goodenough Lake is a small (ca. 1 by 0.6 km) saline-alkaline lake on t he Caribou Plateau in British Columbia, Canada, which has a mean depth of <1 m, allowing the growth of substantial prokaryote-dominated bent hic microbial mats over the entire lake bottom. Microbial mats were in vestigated on both a detailed ultrastructural level and at a macroscop ic level to determine, on a lakewide scale, the seasonally influenced changes that occurred over the course of a spring to fall growth seaso n. The acceleration of phototrophic activity with warming of the lake water in spring could be followed by increases in dissolved oxygen lev els, which quickly reached supersaturation early in the summer. The gr azing activity of brine fly larvae (Ephydra hians) was found to have a significant influence on mat growth and development patterns. On an u ltrastructural level, several types of filamentous cyanobacteria were the main phototrophic organisms present, even in anaerobic parts of th e mats, where they were accompanied by purple bacteria. The dominant c yanobacterium present was a large oscillatoriacean form which, in cert ain parts of the lake, formed unusual columnar structures that had a v ery low diversity of microbial species in comparison to mats from othe r regions of the lake. The microbial community also differed in areas of the lake where less saline groundwater springs emerged from the lak e bed.