Long-term effects of low cadmium concentrations on a natural phytoplankton community

Experimental additions of cadmium (Cd) raised concentrations of Cd in the epilimnion of Lake 382 from background levels (0.0016 μg·L n1 ) to averages as high as 0.185 μg·L n1 for 150 days from 1987 to 1993. The effects on the algal community were assessed on the basis of 9 years of premanipulation data and 7 years of experimental data on phytoplankton population, phytoplankton photosynthesis, and epilimnetic chemistry. In addition, trends in the Lake 382 data were compared with those from two reference lakes over the full 16-year period. Cd additions to Lake 382 had no significant effects on the phytoplankton community. Phytoplankton biomass was relatively stable, and small changes were related to nutrient inputs. Species diversity and community size structure were not affected. Species described in the literature as being sensitive to Cd did not respond significantly in Lake 382. Cd additions did not significantly affect phytoplankton photosynthesis. Changes in photosynthesis were related to nutrient inputs and phytoplankton biomass. Resume : Des apports expOrimentaux de cadmium (Cd) ont augmentO la concentration de ce mOtal dans liOpilimnion du lac 382 jusqui‡ des moyennes atteignant 0,185 μg·L n1 pour 150 journOes de la pOriode 1987n1993, la concentration naturelle Otant de 0,0016 μg·L n1 . Nous avons OvaluO les effets de cette augmentation sur les communautOs dialgues, en utilisant 9 annOes de donnOes antOrieures au traitement et 7 annOes de donnOes recueillies pendant le traitement; ces donnOes portaient sur la population de phytoplancton, la photosynthse phytoplanctonique et la chimie de lieau Opilimnique. Nous avons Ogalement comparO les tendances de ces donnOes ‡ celles caractOrisant deux lacs de rOfOrence pour la mŒme pOriode de 16 annOes. Les apports de Cd dans le lac 382 niont eu aucun effet significatif sur la communautO de phytoplancton. La biomasse phytoplanctonique est demeurOe relativement stable, les lOgers changements Otant reliOs aux apports de nutriments. La diversitO en espces ainsi que la rOpartition des classes de taille dans la communautO niont pas OtO touchOes. Les espces mentionnOes dans les publications comme Otant sensibles au Cd niont pas rOagi de faAon significative dans le cas du lac 382. De mŒme, les apports de Cd niont pas modifiO de manire significative la photosynthse phytoplanctonique; les changements observOs ‡ ce titre Otaient reliOs aux apports de nutriments et ‡ la biomasse phytoplanctonique. (Traduit par la ROdaction)