Survey of metabolically essential trace metals in inland lakes and reservoirs across Canada: What constitutes a low metal system?

Trace metals are metabolically essential with many proteins dependent on metals for proper functioning yet little is known about the influence of low concentrations on freshwater microbial productivity and diversity. Dissolved iron (Fe), manganese (Mn), zinc (Zn), molybdenum (Mo), nickel (Ni), cobalt (Co), copper (Cu) and vanadium (V) were surveyed in 39 lakes and reservoirs across Canada representing different geology, dominant land uses, lake depth, trophic status and climatic zones. Concentrations varied considerably and cross-Canada patterns were not uniform among the eight metals, but PCA analysis revealed two major patterns: Co, Cu, Ni and V in one group and Fe and Mn in a second group. Sub-nanomolar concentrations of Co and Mo were common while sub-nanomolar concentrations of Zn, V and Ni were less common. Fe and Mn accumulated in the hypolimnion of the six lakes and reservoirs deep enough to thermally stratify with Co and Zn accumulation less common. Mo, Zn and Fe occasionally exceeded Canadian guidelines for protection of aquatic biota. Genomics and Monod growth kinetics were explored for their potential in identifying low metal environments and metal limitation without using metal enrichment bioassays. Metal concentrations in the cross-Canada survey were probably not low enough to limit growth but the impact of low metals on microbial diversity is unknown.