Abstract
Fluctuations in sea level throughout the Holocene have driven changes in vegetative communities along Southwest Florida’s coastline. Predicted rates of current sea-level rise (SLR) for this region––indicated at 30cm/ 100y via tide gauge data (Maul and Martin 1993)—vastly exceed rates experienced over the last 5,000 years. Conservation and management of the Ten Thousand Islands (TTI) complex and larger Everglades system will thus require a detailed understanding of how local coastal communities will respond to accelerated SLR, and even more, how regional altered hydrology resulting from extensive freshwater drainage and diversion will alter, and potentially exacerbate, identified changes. This study quantifies the response of Southwest Florida’s coastal environments to accelerated SLR and the impact water management may have on these trends. Key objectives of the research were to identify a mechanism of environmental shifts in relation to accelerated SLR through stratigraphy, and to quantify the evolution in the distribution and aerial coverage of tidal ponds over time in response to SLR via spatial analyses. Thirty-five sediment cores crossing through numerous wetland types and ecotones were extracted along 5 transects parallel to the tidal gradient. Cores were predominantly extracted in pairs––one core within each distinct wetland community, and one within an adjacent tidal pond—at each unique vegetative environment along the five transects. Facies were described based on sedimentologic characteristics’ and their representative paleoenvironments were interpreted. Core analyses clearly indicate a transgressive stratigraphy among all marshes, mangrove communities, and tidal ponds. Wetland community transition has thus followed a clear and predictable pattern over time in response to accelerated regional SLR. Evaluation of three transects, where NAVD88 surface elevations provide vertical calibration of sediment cores, indicates that the onset of wetland facies becomes younger as relative distance from the shore increases. Radiocarbon dates further indicate that long hydroperiod surficial sediment is modern (post-1950), suggesting this facies, and thus the pond environments it represents, has developed on the landscape since the shift from a slow to relatively rapid rate in regional SLR. A greater thickness of the long hydroperiod facies within pond cores, when compared to neighboring marsh cores, is found in 12 of 14 marsh-pond pairs, indicating increased long hydroperiod wetland facies development since submersion and implicating deposition of this facies within the pond environment. On the other hand, relatively reduced thicknesses of peat facies within tidal pond cores among marsh-pond pairs imply marsh surface degradation occurred from the top down as tidal ponds developed. Qualitative assessment indicates that Spartina bakeri-dominated marshes are disparately impacted relative to other marsh types; the relative abundance of S. bakeri on the landscape and the potential for the shrinkage and oxidation of its well-developed peat during drying events are probable reasons for this degradation. ESRI, Inc. ArcGIS software was utilized to quantify the surface area and distribution of tidal ponds within 10 sites spanning the study region over time via analysis of aerial imagery from 1953 and 2009. Tidal pond initiation, growth, merger, and subsequent mangrove encroachment into pond-dense marshes were identified through both qualitative and quantitative analyses of aerial imagery. Although a transgressive stratigraphy was nearly universal, GIS analyses indicate hydrologically altered western sites are experiencing greater growth in the number of ponds, a higher increase in mean pond area, and a comparatively larger influence of, though lower overall percent loss due to, mangrove envelopment over time compared to eastern sites. Furthermore, although pond density significantly increased at each site over time (p = 0.0038), this increase occurred at a higher rate at sites closest to the Faka Union Canal. Even more, the rate of increase in pond density moving closer to the canal was higher at western sites (p = 0.0355). All else being equal, these data signify that ecosystem changes resulting from hydrologic alteration west of the canal have impacted the growth in pond number and area since its construction. GIS data have been compiled and included as supplemental materials (Appendix B). Globally and regionally, SLR is expected to continue at an accelerated rate into the foreseeable future (IPCC 2014). Future land management plans must account for the expected submergence of inland marsh ecosystems ensuing from landscape-wide changes driven by accelerated SLR. Continued restoration of freshwater sheet flow through the removal of canals and roads is necessary for slowing the transition 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.