Seasonal changes in the forms and species of iron and manganese in a seasonally-inundated floodplain swamp

The forms and species in which Fe and Mn occurred in a seasonally-inundated Southeastern United States floodplain swamp were examined. The analytical scheme provided operational estimates of the concentrations of Fe(II), Fe(III) and Mn in particulate (> 0.45 μm), colloidal (0.45-0.0032 μm) and ultrafilterable (<0.0032 μm) size fractions. The procedure also provided a differentiation between forms of Fe that were readily reactive to bathophenanthroline (BPN) and those forms that were unreactive to BPN until undergoing acid extraction. Further, the concentrations of Fe and Mn associated with suspended particulate matter in ion-exchangeable, reducible, and oxidizable forms and as inert mineral were determined using an operational classification scheme.The ferric species predominated due to the oxygenated conditions of the swamp water. Concentrations of colloidal-sized, readily-reducible Fe and of ultrafilterable Fe(II) and Fe(III) were low (<0.25 mg 1⁻¹) and fairly constant throughout the study; more Fe occurred in the colloidal than ultrafilterable size range. The majority of the colloidal-sized Fe was in an acid-extracted form. Almost all ultrafilterable Fe was ferric, although high Fe(II) concentrations occurred in floodplain soil pore water. Total Fe, mainly particulate-associated, varied with periods of flooding and intense bioturbation. Manganese occurred in low concentrations, primarily in the ultrafilterable fraction, although colloidal-sized Mn forms were important at times. Most of the Fe and Mn associated with suspended particulates occurred in the inert mineral form and to a lesser extent in a reducible form. Ion-exchangeable and oxidizable particulate Fe and Mn were relatively unimportant.