PERIPHYTON IN THE CLEAR AND COLORED WATER MASSES OF THE ST.-LAWRENCE RIVER (QUEBEC, CANADA) - A 20-YEAR OVERVIEW

Periphyton was collected on navigational buoys in the Montreal island sector of the Sr. Lawrence River during 1994-95 to compare biomass and species composition in the transparent ''green'' waters originating f rom the Great Lakes with those found in the colored, more turbid ''bro wn'' waters; coming from the Ottawa River. Periphyton biomass (chlorop hylla, ash free dry mass, density and biovolume) was depressed in brow n waters and differences in species composition were observed at the c lass (i.e., diatoms more important in brown waters) and species level( i.e., Melosira varians more abundant in brown waters). Comparison of 1 994 and 1995 with 1982 and 1973 data supported the observed difference s in biomass between brown and green waters, yet showed no major tempo ral shifts in periphyton species composition despite reductions in pho sphorus loadings 20 the Great Lakes and the St. Lawrence River during that 20-yr period. Biomass (density and biovolume) of Cladophora and r elative abundance of chlorophytes appeared much higher in 1982 than re corded in 1994 and 1995. Cladophora influences diatom community compos ition by providing a substratum for epiphytes and thus represents a ke y species controlling the structure of periphyton assemblages. Future monitoring efforts in the St. Lawrence River should focus on this spec ies and include a standardization of sampling and enumeration methods.