Patterns of Wetland Hydrologic Connectivity Across Coastal-Plain Wetlandscapes

Depressional wetlands influence the functions of wetlandscapes by storing and releasing water, providing critical habitat, amplifying carbon and nutrient cycling, and influencing microclimate. Despite persistent subsurface connectivity, depressional wetlands are surrounded by uplands so only sporadically connect via surface pathways. However, the frequency, duration, and relative importance of surface connectivity in depressional wetlands remains poorly understood, limiting quantification of their landscape functions. Using multiple years of stage variation in 67 depressional wetlands across four contrasting wetlandscapes, we observed wetland spill elevation is exceeded 10%-40% of the time, with substantial variation within and between wetlandscapes. Moreover, surface connectivity increased water loss rates by 200%-350% on a depth basis and 350%-850% on a volumetric basis compared with subsurface water loss rates. This temporally disproportionate water export suggests that short-lived surface connectivity is crucial for aggregate landscape export of water-borne materials and numerous hydrologic and habitat services. Contrasting water loss rates above and below spill thresholds create homeostatic feedback that stabilizes water levels near the thresholds. We explored geomorphic, climatic, and vegetative influences on hydrologic connectivity, quantified as nighttime recession rates below the spill threshold for groundwater connectivity and percent time above thresholds for surface connectivity. Groundwater connectivity was consistently greater in deeper wetlands and wetlandscape identity was the primary factor explaining variation in surface and subsurface connectivity. Our results highlight the critical role of surface connectivity in coastal plain wetlands, illustrate the heterogeneity of those wetland functions within and across wetlandscapes, and provide hydrologic benchmarks for evaluating restoration of aggregate landscape functions.