Relationship between hydrology and zonation of freshwater swale wetlands on lower hatteras island, North Carolina, USA

Abstract

Only a few relatively wide barrier islands support shallow freshwater aquifers. Rare, swale wetlands occur on islands where fresh water inundates, at least seasonally, low-lying troughs between interior dunes. Swale wetlands are dominated by emergent vegetation and submerged aquatic vegetation in the deepest areas and by woody shrubs in more shallow areas. On southern Hatteras Island, wetland shrubs have progressively invaded open water areas over the past 40 years, suggesting a change in hydrologic regime. To determine the relationship between vegetation cover type and length of saturation, water-level fluctuations over time were analyzed to tie boundaries of six wetland cover types to the duration of soil saturation at 20-cm depth. We found that areas dominated by herbaceous vegetation had significantly longer flooding regimes than areas dominated by shrubs (85–95% vs. 12–69% of the growing season, respectively). Only 22–25 cm elevation differences were found to separate emergent marsh from the various shrub cover types, suggesting that lowering the mean water level via drainage has likely been responsible for shrub swamps replacing emergent marsh in swales. Although succession from open water to shrub swamp probably occurs naturally in the absence of drainage through the accumulation of organic matter, natural disturbances such as wildfire and storm-driven surges of saline water would have periodically re-set succession. Therefore, managing for long-term maintenance of freshwater swale wetlands on barrier island should include (1) eliminating or controlling drainage through constructed ditches, (2) eliminating man-made barriers that prevent the transport of saline water into ponds during hurricanes and nor’easters, and (3) initiating a prescribed burning program to mimic the historic, natural fire regime.