Periphyton community structure and dynamics in a subarctic lake

Periphyton species composition was analysed at 20 stations around an island in a large (1239 km2) oligotrophic lake in subarctic Quebec (Lac à l'Eau Claire; latitude 56°10′N, longitude 74°30′W) to describe the mature communities colonizing the upper littoral region and to evaluate periphyton abundance and distribution relative to the physical environment. Four major communities could be clearly distinguished in the field by their macroscopic features, specifically colour (black, brown, and green) and growth form (filamentous or encrusted), as well as by their standing stock (cover and areal Chl a concentration) and photosynthetic characteristics. (1) Black crust—This community was dominated by the cyanobacterium Gloeocapsa, with highest percent cover in shallow waters (≤ 0.25 m) protected from wave action by offshore boulder barriers. Photosynthesis under full sunlight was low per unit biomass (0.7 μg C∙(μg Chl a)−1∙h−1). (2) Brown film—This community was dominated by Calothrix, with Gloeocapsa 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 chlorophyte Gloeocystis, with Oscillatoria as subdominant, and colonized shallow depths (≤ 0.25 m) in the shaded underlayer of rocks. It had slow, light-limited photosynthetic rates (0.1 μg C∙(μg Chl a)−1∙h−1). (4) Green filaments—This community was dominated by Ulothrix zonata, with associated chlorophytes and diatoms, and was a rapidly growing assemblage characterized by the highest diversity, species richness, and productivity per unit biomass (3.5 μg C∙(μg Chl a)−1∙h−1). It occurred on gravel beds at depths < 0.5 m and was restricted to the well-illuminated south-facing shores of the island. Communities 1, 2, and 3 had similar maximum standing stocks throughout the period of sampling (mean of 1.3 μg Chl a∙cm−2), whereas the Ulothrix community rose from 1.9 μg Chl a∙cm−2 in late July to 5.5 μg Chl a∙cm−2 by mid-August. The overall rich biodiversity of the Lac à l'Eau Claire periphyton (> 200 taxa recorded) may reflect the diversity of microenvironments and intermediate disturbance in the upper littoral zone. Key words: cyanobacteria, chlorophytes, diatoms, epilithon, periphyton, photosynthesis.