Evidence for the presence of strong Mn(III)-binding ligands in the water column of the Chesapeake Bay

Soluble manganese speciation was determined in suboxic and anoxic waters of the Chesapeake Bay using a water soluble porphyrin ligand as spectrophotometric reagent. Initial addition of the reagent detected only Mn(II); on addition of 100 μM H2S (excess of the expected total dissolved Mn concentration) an increase in Mn occurred, indicating a reduction of Mn(III). Mn(III) comprised up to 54.21 ± 2.71% of the total dissolved Mn pool. Samples with low H2S (4.82 ± 0.80 μM) had high Mn(III) (6.98 ± 0.63 μM) whereas those with high H2S (38.37 ± 1.70 μM) had low Mn(III) (1.12 ± 0.17 μM) indicating that Mn(III) is kinetically stabilized in situ by strong ligands so reduction to Mn(II) is not complete. Assays for MnOx particles showed that these were negligible or not detected except near the oxic-anoxic interface, and Mn(III) depth profiles showed peaks below the oxic-anoxic interface. Sulfidic sediments were not a source of Mn(III) to overlying waters as no Mn(III) was detected in overlying waters. These Mn(III) profiles likely result from the one electron reductive dissolution of solid MnO2 particles formed at the oxic-anoxic interface, which then fall into the anoxic hydrogen sulfide rich zone. Laboratory experiments with known ligands bound to Mn(III) confirm that Mn(III)-L complexes do not completely react with H2S as these are concentration dependent reactions under kinetic control.