Physical and ecological controls on freshwater floc trace metal dynamics

Significantly higher concentrations of Ag, As, Cu, Co, Ni, and Pb are found in suspended floc compared to surficial bed sediments for a freshwater beach in Lake Ontario. Contrasting observed element-specific bed sediment metal partitioning patterns, floc sequestration for all elements is dominated by one substrate: amorphous oxyhydroxides. More specifically, floc metal scavenging is controlled by floc biogeochemical architecture. Floc organics, largely living microbial cells and associated exopolymeric substances (EPS), act as scaffolds for the collection and/or templating of amorphous Fe oxyhydroxides. While interactions between floc organics and amorphous Fe oxyhydroxides affected floc sorption behavior, specific element affinities and competition for these limited substrates was important for overall floc partitioning. Further, assessment of metal dynamics during stormy conditions indicated energy-regime driven shifts in floc and bed sediment partitioning that were specifically linked to the exchange of floc and bed sedimentary materials. These novel results demonstrate that the microbial nature of floc formation exerts an important control on floc metal dynamics distinguishable from surficial bed sediments and that hydrologic energy-regime is an important factor to consider in overall floc metal behavior, especially in beach environments.