Effect of microhabitat features, seston quality, and periphyton on abundance of overwintering black fly larvae in southern Quebec

Maximum biomass of overwintering larvae of simuliids before emergence was measured in 34 riffles from 16 lake outlets in southern Quebec in April 1985. Biomass was correlated with measurements of current velocity, water depth, periphyton abundance, distance from the lake, and indices of seston quantity and quality. The quantity of seston tended to increase with distance from the lake mainly because of an increase in inorganic material. Seston quality, as measured by chlorophyll concentration, protein concentration, and chlorophyll and protein content per unit of mass of seston, tended to decrease with increasing distance from the lake. Chlorophyll concentration in the water and in the seston and protein content of the seston were the best correlates of black fly biomass in these sites. Multiple regression models were used to describe the microdistribution of the three species of simuliids present (Prosimulium mixtum/fuscum, Stegopterna mutata, and Simulium vittatum). Biomass of all three species varied with distance from the lake, current velocity, water depth, and chlorophyll concentration. In addition, both Prosimulium and Stegopterna biomasses were negatively correlated with standing stocks of periphyton. A large-scale model predicting mean biomass in riffles was also constructed with distance from the lake, mean current velocity and depth, and a topographic estimate of lake trophy. This model was successfully validated on an independent data set collected in 1984 from 32 riffles in seven lake outlets.