Tracking tidal inundation in a coastal salt marsh with Helikite airphotos: Influence of hydrology on ecological zonation at Crab Haul Creek, South Carolina

Ecological zones in a salt marsh are influenced by many factors, including hydroperiod, soil salinity, groundwater flow, and nutrient/oxygen availability. In this project, high-resolution aerial images taken by a Helikite helium balloon-kite at 100 m altitude were used to develop a detailed flooding model and derive a spatially comprehensive map of ecological zones at the headwaters of a small tidal marsh creek at a forest-marsh boundary in North Inlet estuary, South Carolina. We constructed photomosaics from Helikite imagery over a 150 × 100 m area to identify ecological zones using an automated, maximum likelihood classification. In the same area, we developed a digital terrain model with ± 2 cm overall vertical accuracy, primarily from waterlines at known tide height tracked by Helikite imagery and GPS. These are low-cost, easily implemented methods for studying marsh processes where access is often challenging. Quantifying frequency of inundation, an approximation of hydroperiod, using our terrain model and comparing it to ecological zones revealed that there is not a linear relationship between elevation and inundation above mean high tide. Ecological zones exhibit complex spatial patterns that neither elevation nor hydroperiod alone can explain. In the northwestern basin, adjacent to a forest-marsh boundary, inflow of low salinity groundwater from a large freshwater lens beneath the forest prevents the development of Salicornia virginica zones and allows the high-marsh plant Juncus roemarianus to be present at middle-to-low marsh elevations. Conversely, in the southwestern portion of the basin adjacent to a relict beach ridge underlain by a smaller freshwater lens, saltwater infiltration during spring tides and evapotranspiration during neap tides allows Salicornia virginica zones to develop in hypersaline soils. Many of the current numerical models for predicting salt marsh ecological zonation consider hydroperiod as the main factor; however, the asymmetry of the basin in both ecological zones and freshened groundwater discharge imply groundwater and surface-water inundation should both be considered in the distribution of ecological zones in a coastal salt marsh.