Sorption and Sedimentation of Zn and Cd by Seston in Southern Lake Michigan

Recent estimates suggest that inputs of Zn and Cd to southern Lake Michigan exceed losses. We investigated the sorption and sedimentation of Zn and Cd by southern Lake Michigan seston particles using 65Zn and 109Cd in 21 radiotracer experiments. The time-series sorption by total seston >0.45 μm was asymptotic with apparent equilibria of ∼100–200 ng Zn/L and ∼1.0–2.0 ng Cd/L, occurring in ∼24–30h during the summer phytoplankton bloom. Studies of the sorption by different particle size fractions showed that the abundance of phytoplankton and detritus control the concentrations of particle-bound Zn and Cd. The seasonal maximum concentrations of particle-bound Zn and Cd correlated well with the development of both the summer and fall phytoplankton blooms. Serial additions of Zn and Cd in combination with the radiotracers showed that these additions inhibit the sorption of both metals as their toxic effects are expressed. Coupling of the particle-bound Zn and Cd estimates for the period May to December with data on the seasonal variations in the net settling velocity in southern Lake Michigan provided annual sedimentation rates of ∼9 μg Zn/cm2.yr and ∼0.1 μg Cd/cm2 yr that are in excellent agreement with similar estimates by other investigators. Likewise, dividing our particle-bound estimates by the dry weight of total seston in the samples provided sediment concentration estimates of ∼38–185 ng Zn/mg dry weight (ppm) and 0.5–2.3 ng Cd/mg dry weight that agree with measured levels from the surficial Lake Michigan sediment. The results of these experiments suggest that, if the dissolved concentrations of Zn and Cd do increase to toxic levels, the impacts on phytoplankton may potentiate further increases in the dissolved levels by reducing plankton sorption and removal by sedimentation.