The Effect of Changing Water Chemistry on Rates of Manganese Oxidation in Surface Sediments of a Temperate Saltmarsh and a Tropical Mangrove Estuary

Abstract

The influence of changing water chemistry on manganese oxidation in surface sediments of a Sapelo Island, Georgia saltmarsh and an Andros Island, Bahamas mangrove swamp was examined. Surface sediment interstitial water salinities for sample stations in the mangrove swamp, and for saltmarsh creek bank sample sites on Sapelo Island were similar to that of the overlying waters. Intertidal sediments from high marsh sites on Sapelo Island were frequently exposed to the atmosphere on low tides, resulting in enhanced evaporation, and interstitial water salinities up to 105 were observed. Rates of microbial Mn oxide production were determined using the oxidation of leuco crystal violet in sediment/water suspensions. Sodium azide was added to some incubations to measure rates of abiotic Mn oxide formation. Overall, rates of Mn oxide production were much higher in saltmarsh sediments than in mangrove sediments. Within the mangrove swamp estuary, sediments approximately 15 km inland exhibited the highest rates of microbial Mn oxide production (station salinities of 0 and 8, 50 to 119 pmol mg dwt -1h -1, respectively), compared with sediments from the mouth of the estuary (station salinities of 24 and 34, 3 to 16 pmol mg dwt -1h -1, respectively). Suspension of mangrove sediments in water from different parts of the estuary, and in coastal seawater, had no affect on rates of Mn oxidation. In the saltmarsh rates of Mn oxidation were much higher for creek bank sediments than for sediments from high marsh sites (2·31 ± 0·28 SD and 0·45 ± 0·14 SD nmol mg dwt -1h -1, respectively). Suspension of creek bank sediments in coastal seawater diluted with distilled water had no affect on rates of Mn oxidation when compared with suspensions in natural saltmarsh creek water. Sediments from high marsh sites exhibited short-term enhancement of Mn oxidation when the coastal seawater used for suspensions was experimentally diluted to 24% of full strength with distilled water (final salinity 8). High marsh sediments experimentally exposed to hypersaline conditions, demonstrated decreasing rates of Mn oxidation as sediment suspension salinity reached 102. This inhibition of Mn oxidation appears to have been due to the effects of the concentrated bulk salts present in the water. Differences in organic or other trace components of the creek water appear to have had a negligible effect on rates of Mn oxidation.