Vegetation change along salinity gradients in the tidal marshes of the upper Savannah River estuary

The tidal freshwater-oligohaline marsh interface was investigated in the upper Savannah River estuary. Tidal marsh vegetation, tide stages, and salinity were monitored from October 1997 through November 2001. Permanent belt transects for vegetation monitoring were established at ten locations chosen to bracket the salinity gradient between tidal freshwater and subsaline conditions. Marsh vegetation was monitored six times between November 1997 and October 2001, and data were collected on frequency and percent cover of each species. Automatic datalogging stations were used to continuously monitor tide stage and salinity at 12 locations. Tide stages were monitored both within tidal creeks and within the interiors of adjacent marshes. Salinity was monitored in tidal creeks, in high-tide surface waters within adjacent marshes, and in marsh sediments. High tides were shown to flood the marshes between 33.8 and 100% of the time, depending on location. Mean salinity in marsh sediments ranged from a low of 0.4 ± 0.3‰ at the site farthest upriver to a high of 8.1 ± 4.0‰ at the site farthest downriver. However, between October 1997 and October 2001, salinity within marsh sediments rose at all sites, a trend that was attributed to a 3-year drought in the Savannah River basin. Ordination of vegetation data defined the vegetation assemblages of each belt transect and separated them along two major gradients. The primary gradient was salinity; however, the secondary gradient remained undefined, possibly indicating an influence of sediment consolidation and differences in elevation. Comparison of belt transects over the six separate sample periods indicated a subtle shift to more saline vegetation assemblages at some sites, a result that was again attributed to the drought conditions. Salinity distribution across the tidal marshes was determined to have a strong spatial component associated with proximity to an extensive network of tidal creeks, the remnants of agricultural water management systems constructed for the tidewater rice industry in the eighteenth and nineteenth centuries. The influence of the tidal creek system on the salinity distribution was determined to have potential use in river management actions that could preserve or increase tidal freshwater marsh habitat.