The geochemical behavior of Co in aquatic systems has often been related to the presence of Fe and Mn particles. A few studies have shown that Co is exclusively associated with particulate Mn, but the dynamics of Co and Mn cycling have never been determined in real time under natural conditions. In this study, we used a combination of analytical techniques to study the temporal and spatial evolution of Mn microparticles (MnOx) over 2 weeks in the water column of a shallow stratified lake (Paul Lake, MI). We report a temporal accumulation of dissolved Mn at the oxic-anoxic transition, and we show that this accumulation is due to the reductive dissolution of Mn particles. The reductant has not been identified, but abiotic reduction by ΣH2S and ferrous iron is excluded because they are produced below the zone of MnOx reduction. Hybridization of RNA isolated from Paul Lake with oligonucleotide probes targeting the δ proteobacteria, which include metal-reducing species, suggests that their activity is greatest at and just below the oxic-anoxic transition, so that Mn reduction may be influenced by bacterial activity. Mn-oxidizing bacteria were isolated from this zone as well. We also demonstrate that the dynamic evolution of MnOx has a direct influence on the distribution of Co in the water column of this lake: dissolved Co is released during the reductive dissolution of MnOx and accumulates at the redox interface.