The biogeochemical manganese cycling in Oneida Lake, N.Y. is the focus of this dissertation. The involvement of manganese in biological, physical and chemical interactions determines the spatial and temporal distribution of this biologically active trace metal. In the aquatic environment, the most commonly found oxidation states of manganese were Mn(II), Mn(III) and Mn(IV). Evidence is presented implicating microbial rather than purely chemical mechanisms in oxidation-reduction conversions among manganese species.

The results from studies of the different aspects on manganese biogeochemistry provided persuasive evidence for the following scenario for the manganese cycle in Oneida Lake:

a) Soluble manganese Mn(II), was oxidized by certain cyanobacteria, particularly Microcystis aeruginosa and Anabaena circinalis. The quantitative oxidation of micromolar concentrations of reduced manganese was driven by the photosynthetically induced increases in local dissolved oxygen concentration and pH in the vicinity of cells and cyanobacterial aggregates.

b) Sedimentation of the manganese oxide-coated cyanobacteria rapidly transported particulate manganese to the sediment surface. Depositional zone sediments are anoxic within a few mm of the surface.

c) Manganese reducing bacteria metabolize the manganese oxides releasing soluble, reduced manganese. Microbially-mediated reduction by organisms such as Shewanella putrefaciens probably accounts for a large proportion of the manganese reduction that occurs in the sediments, chemical reduction is important late in the summer. The concomitant oxidation of organic matter couples the manganese and carbon cycles remineralizing the phytoplanktonic biomass.

d) During episodes of calm weather, transient stratification and subsequent development of anoxic conditions promoted diffusion of soluble manganese into the hypolimnion, with concentrations exceeding 20 uM in many instances.

e) Reoxidation of manganese by manganese oxidizing bacteria was widely present in oxic, non- depositional zones in shoals areas, adjacent to the reducing depositional zones of the hypolimnion.

f) Accretion of particulate manganese into nodules at the bottom of non-depositional zones.