Algal seston and periphyton distribution along a stream linking a chain of lakes on the Canadian Shield

The effect of natural impoundments on the longitudinal distribution of summer sestonic and periphytic algal biomass was assessed in a stream linking a chain of lakes in the Laurentian mountains of Quebec. Algal seston biomass was higher at lake outlets but declined rapidly downstream. Periphyton on gravel generally had larger biomasses at lake outlets, but on boulders, the dominant substratum, periphyton was unaffected by distance from the upstream lake. However, the only lake in the set larger than 100 ha, Lac Achigan, had a significant positive effect on the biomass of all the algal assemblages. Periphyton on boulders was significantly correlated with the distance from the headwater. The longitudinal trend in periphyton biomass was unimodal, with the peak around 4 km from the headwater. When the effect of lakes was statistically removed, algal seston distribution also followed a unimodal longitudinal trend parallel to that of periphyton. Simple geomorphic variables like distance downstream from the nearest lake and distance from headwater account for a significant portion of the variation of algal seston (52%) and periphyton (64%) in this stream.