Sensitivity of phytoplankton to copper in Lake Superior

Relatively high and constant concentrations of total dissolved Cu (≈12 nM) throughout the surface water of Lake Superior and lower yet more variable concentrations of total dissolved Mn (≈1 nM) present the scenario wherein Cu may inhibit primary productivity by reducing Mn uptake by phytoplankton. Cu enrichment experiments were conducted in the field on water sampled from pelagic and coastal stations in western Lake Superior. Phytoplankton collected from near the metalimnetic chlorophyll maximum in September 2000 and incubated at 15°C were sensitive to Cu enrichments as low as 5 nM, whereas phytoplankton from the same station and a coastal station sampled in May 2001 showed little Cu sensitivity. The lack of a marked response in May is attributed to the ambient 3°C water that slowed productivity. Laboratory experiments using the chlorophyte Chlorella and the cyanobacterium Microcystis demonstrated the wide range in phylogenetic responses to a range of Cu2+ at levels of Mn2+ (0.08 and 4.8 nM) naturally present in the water column. Chlorella was resistant to Cu2+ at higher photon flux density (319 vs 54 μmol photons/m2/s), whereas Microcystis was sensitive to 0.1 nM Cu2+ at low and high irradiance regardless of the concentration of Mn2+. Chlorella growth was enhanced at higher Mn concentrations at both high and low light yet Microcystis growth was not affected by the different concentrations of Mn assayed. Cu was toxic for both organisms at a [Cu 2+] = 0.1 nM at 54 μmol photons/m2/s, regardless of the [Mn2+]. These results suggest that light levels may play a role in determining phytoplankton response to Cu in Lake Superior