Coastal Wetland Geomorphic and Vegetative Change: Effects of Sea-Level Rise and Water Management on Brackish Marshes

Abstract

Conservation and management of coastal ecosystems require an understanding of how accelerated sea-level rise (SLR) and altered hydrology impact community shifts over time. This study evaluates the response of tidal wetlands of the Ten Thousand Islands, Collier County, Florida, to SLR and water management, with a focus on the development and distribution of tidal ponds across a wetland landscape. Sediment cores collected from marshes, mangroves, and tidal ponds reveal a clear transgressive stratigraphy. Facies analyses demonstrate that ponds originate from the surface downward through the degradation of marsh peat. Analyses of 1953 and 2009 aerial imagery using ArcGIS® software clearly identified tidal pond initiation, growth, and merger over time. Wetlands west of the Faka Union Canal, which have limited freshwater sheet flow due to canalization, are experiencing a greater increase in pond count, pond density (p = 0.0038), and mean pond area (p < 0.0001). Qualitative observations also recognize a relatively larger influence of mangrove envelopment over time in western sites compared with those retaining near-natural flows. Future land management plans must account for the expected submergence of inland marsh ecosystems driven by SLR and accelerated by hydrologic alteration. Continued restoration of freshwater sheet flow is necessary for slowing the regional transition (and loss) of graminoid marshes to either mangrove or pond environments. Without such action, a complete loss of these biologically diverse marsh ecosystems as mangrove forests encroach and marsh surfaces submerge is probable in the short term.