PERIPHYTON COMMUNITY STRUCTURE AND DYNAMICS IN A SUB-ARCTIC LAKE

Periphyton species composition was analysed at 20 stations around an i sland in a large (1239 km?) oligotrophic lake in subarctic Quebec (Lac a l'Eau Claire; latitude 56 degrees 10'N, longitude 74 degrees 30'W) to describe the mature communities colonizing the upper littoral regio n and to evaluate periphyton abundance and distribution relative to th e physical environment. Four major communities could be clearly distin guished in the field by their macroscopic features, specifically colou r (black, brown, and green) and growth form (filamentous or encrusted) , as well as by their standing stock (cover and areal Chi a concentrat ion) and photosynthetic characteristics. (1) Black crust-This communit y was dominated by the cyanobacterium Gloeocapsa, with highest percent cover in shallow waters (less than or equal to 0.25 m protected from wave action by offshore boulder barriers. Photosynthesis under full su nlight was low per unit biomass (0.7 mu g C . (mu g Chl a)(-1) . h(-1) ). (2) Brown film-This community was dominated by Calothrix, with Gloe ocapsa and Phormidium as subdominants. Maximum abundance was at 0.5 m, with photosynthetic rates that were similar to the black community. ( 3) Green crust-This community was dominated by the mucilaginous chloro phyte Gloeocystis with Oscillatoria as subdominant, and colonized shal low depths (less than or equal to 0.25 m) in the shaded underlayer of rocks. It had slow, light-limited photosynthetic rates (0.1 mu g C . ( mu g Chl a)(-1). h(-1)). (4) Green filaments-This community was domina ted by Ulothrix zonata, with associated chlorophytes and diatoms, and was a rapidly growing assemblage characterized by the highest diversit y, species richness, and productivity per unit biomass (3.5 mu g C . ( mu g Chl a)(-1) h(-1)). It occurred on gravel beds at depths less than or equal to 0.5 m and was restricted to the well-illuminated south-fa cing shores of the island. Communities 1, 2, and 3 had similar maximum standing stocks throughout the period of sampling (mean of 1.3 mu g C hl a . cm(-1)), whereas the Ulothrix community rose from 1.9 mu g Chl a . cm(-2) in late July to 5.5 mu g Chl a . cm(-2) by mid-August. The overall rich biodiversity of the Lac a l'Eau Claire periphyton (>200 t axa recorded may reflect the diversity of microenvironments and interm ediate disturbance in the upper littoral zone.