Evaluation of lacustrine floc Fe, Pb, and Cd biogeochemistry over seasonal (summer, winter) and water column depth (metalimnetic, hypolimnetic) scales reveals depth-independent seasonally significant differences in floc Fe biominerals and trace element (TE: Pb, Cd) sequestration, driven by floc microbial community shifts. Winter floc [TE] were significantly lower than summer [TE], driven by declining abundance and reactivity of floc amorphous Fe((III))-(oxy)hydroxide (FeOOH) phases under ice ([FeOOH](summer) = 37-77 mgg(-1) vs [FeOOH](winter) = 0.3-7 mgg(-1)). Further, while high summer floc [FeOOH] was observed at both water column depths, winter floc was dominated by Fe((II)) phases. However, the observed seasonal change in the nature and concentrations of floc Fe-phases was independent of water column [Fe], O2, and pH and, instead, significantly correlated to floc bacterial community membership. Bioinformatic modeling (Unifrac, PCA analyses) of in situ and experimental microcosm results identified a temperature-driven seasonal turnover of floc microbial communities, shifting from dominantly putative Fe metabolisms within summer floc to wintertime ancillary Fe reducing and S metabolizing bacteria. This seasonal shift of floc microbial community functioning, significantly the wintertime loss of microbial Fe((II))-oxidizing capability and concomitant increases of sulfur-reducing bacteria, alters dominant floc Fe minerals from Fe((III)) to Fe((II)) phases. This resulted in decreased winter floc [TE], not predicted by water column geochemistry.