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 100μM 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±0.80μM) had high Mn(III) (6.98±0.63μM) whereas those with high H₂S (38.37±1.70μM) had low Mn(III) (1.12±0.17μM) indicating that Mn(III) is kinetically stabilized in situ by strong ligands so reduction to Mn(II) is not complete. Assays for MnO_x 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.