Metal accumulation in American wild celery (Vallisneria americana Michx.) in the St. Lawrence River: effects of water depth and exposure to current

Abstract

The sensitivity of Vallisneria americana Michx. as an indicator species of metal concentrations was assessed through the examination of its spatial (between- and within-site) and temporal (short- and long-term) variability. Eight macrophyte beds located in the St. Lawrence and Ottawa rivers were selected to contrast metal concentrations found in plants exposed to different types of waters, upstream and downstream of their confluence, in the Greater Montreal urban area. Comparisons among sites revealed higher metal concentrations, both in water and in plant tissues, at sites exposed to Ottawa River "brown" waters than at sites exposed to St. Lawrence River "green" waters. Within each site, samples represented a broad range in terms of water depths, exposure to current, incident light intensity, and total plant biomass. At all sites, metal concentrations in plant tissues were lowest in the shallow water found in sheltered, dense beds of submerged aquatic vegetation and increased in deep, open-water areas beyond the limits of dense vegetation. This persistent gradient may result from local differences in plant growth rates, exposure to currents, and (or) metal bioavailability. The use of a sampling strategy designed to control for within-site (depth-balanced) variability makes it possible to measure spatial and (or) temporal differences on the order of 20% whereas unbalanced sampling designs may lead to erroneous conclusions. Significant reductions in metal concentrations in plant tissue were observed both between 1994 and 1996 (Fe, Mn, Pb, Zn, N) and between 1976 and 1996 (Cd, Cr, Pb, Zn, N). Particular care must be given to sampling design if V. americana is to be used as a biological indicator of further long-term reductions in metal concentrations.