Tidal Wetland Community Response to Varying Levels of Flooding by Saline Water

A long-term data base (2000–2009) was used to evaluate tidal floodwater salinity and the resulting soil biogeochemical setting (methanogenic or sulfate reducing) at 54 substations, which included a variety of marsh and swamp community types along the freshwater/saltwater boundary of the Cape Fear River/Estuary, North Carolina. During this decade, a variety of extreme climatic events, i.e. floods and droughts, occurred, but overall data reflected long-term, natural conditions. At sites flooded by \u3e1 ppt saline water more than 25 % of the time, wetlands consisted of varying types of tidal marsh. Temperate, tidal swamps were present at sites flooded by this level of saline water less than 12 % of high tides. Flooding \u3e25 % of tides by \u3e1 ppt seawater converted soils in those wetlands from methanogenic into sulfate reducing conditions \u3e50 % of the time. The point along an estuarine gradient where adjacent wetlands are flooded by \u3e1 ppt saline water less than 25 %, but more than 12 % is the zone of transition. The conversion of a tidal swamp to tidal marsh is not only caused by salt water itself, but by the sulfate constituent in seawater. Once a sufficient concentration of sulfate enters soils, sulfate reducing bacteria become active reducing the sulfate into hydrogen sulfide, which is toxic to any wetland plant species not adapted to this toxic substance. The incidence of flooding by high tides containing \u3e1 ppt salinity is an accurate predictor of functional change in adjacent wetlands. Wetlands receiving intermediate levels of saline water, i.e. 12-25 % flooding with 1 ppt floodwater, were in some state of transition from swamp to marsh. Once trees in tidal swamps are killed, the wetland moves inevitably towards a tidal marsh dominated by species of herbaceous vascular plants with varying tolerance to saline water