ABIOTIC AND SEASONAL FACTORS INFLUENCING TRACE-METAL LEVELS (CD, CU, NI, PB, AND ZN) IN THE FRESH-WATER AMPHIPOD GAMMARUS-FASCIATUS IN 2 FLUVIAL LAKES OF THE ST-LAWRENCE-RIVER

The potential use of the freshwater amphipod Gammarus fasciatus as a b iomonitor of metal pollution has been assessed in two fluvial lakes of the St. Lawrence River. Metal (Cd, Cu, Fe, Mn, Ni, Pb, and Zn) concen trations were measured twice at 22 sites in Lakes St. Pierre and St. L ouis, both in oxic sediments and in amphipods. Concentrations of free- metal ions at the sediment-water interface were estimated from lake wa ter pH and sediment/water sorption equilibria. Multiple linear regress ion models forecasting trace metal (Cd, Cu, Ni, Pb, and Zn) levels in amphipods were developed, using abiotic variables (sediment-bound meta ls, sediment organic carbon, free-metal ions, hydrogen ions, dissolved calcium; and dissolved organic carbon) and biotic (size class) variab les. These models explained 73-81% of the observed variation, except i n the case of Zn (68%) and Cu (61%), indicating that G. fasciatus is a promising biomonitor for Cd, Ni, and Pb in littoral environments. Sea sonal patterns of amphipod contamination were identified for Fe, Ni, a nd Pb, summer concentrations being higher than those measured during t he fall. Size-dependent variation in amphipod metal levels was found, smaller organisms being more contaminated than larger ones.