PERIPHYTON AS AN INDICATOR OF WATER-QUALITY IN THE ST-LAWRENCE-RIVER (QUEBEC, CANADA)

The performance of various algal indices to document improvements in w ater quality across a low nutrient concentration gradient was assessed during 2 years in the St Lawrence River (Quebec, Canada). Water-quali ty variables and periphyton samples were collected on navigational buo ys near Montreal during the spring, summer and fall of 1994 and 1995. Exposure to urban wastewater varied widely within the sector surroundi ng the island of Montreal, with some areas upstream receiving no direc t effluents and areas further downstream receiving treated and untreat ed wastewater. Faecal coliform concentrations provided a good tracer o f effluents and were significantly correlated to nutrient concentratio ns (r= 0.33-0.72, p < 0.001) and water transparency (r = 0.70, p < 0.0 01). Despite a strong gradient in faecal coliform concentration (< 2 t o > 20 000 UFC/100 mi), algal biomass and diversity did not reflect di fferences between sites with varying levels of urban wastewater. Taxon omic composition of periphyton communities, particularly the presence of the cyanophyte Plectonema notatum Schmidle, was related (r = 0.48, p = 0.004) to exposure to urban effluents. Variables describing season al changes (temperature, Julian day, river discharge, conductivity, NO 2-NO3) explained a large fraction of total variance (38-52% of total v ariance) and thus exerted the predominant influence on algal biomass a nd species composition in the St Lawrence River. Variables describing the presence of effluents explained 1-22% of the variance in compositi onal data. Subtle changes in periphyton species composition were the o nly response to different levels of exposure to urban wastewater in th e Montreal area, which represented relatively small differences in com parison to natural seasonal variability. (C) 1998 Elsevier Science Ltd . All rights reserved.