Compound specific δD and δ13C analyses as a tool for the assessment of hydrological change in a subtropical wetland

Abstract

Compound specific carbon and deuterium stable isotope values (δ13C and δD) and the relative abundance of mid-chain n-alkanes (Paq) were determined for a series of dominant wetland plants, a surface slough-to-ridge soil transect, and slough and ridge soil cores, to assess historical vegetation successions induced by hydrological modification in an anthropogenically impacted, subtropical wetland, the Florida Everglades, USA. A difference of as much as 3.6 and 130 ‰ in their δ13C and δD values was observed between the two most abundant emergent macrophyte species (Cladium and Eleocharis), respectively. A clear n-alkane δD value depletion (−130 to −167 ‰) and decreasing Paq was observed along the slough-to-ridge soil transect, likely the result of an eco-hydrological transition from slough-to-ridge dominated vegetation (Eleocharis to Cladium). In agreement with the relatively constant Paq values, the lack of significant changes in the δD depth profile for the slough core, suggest a consistent slough type of vegetation composition over time at that location. In contrast, changes of both n-alkane δ13C and δD values for the ridge core, especially after ~1960 AD, coincide with the expected plant successions from historically long hydroperiod (>8 months), slough type plants (Eleocharis, Utricularia, Nymphaea) to present day, shorter hydroperiod (<8 months), ridge type plants (Cladium). These δ13C and δD changes seem to be driven by vegetation shifts associated with hydrological change. The application of the compound-specific stable isotope determinations may strongly complement the biomarker approach for paleo-hydrological assessments in wetland ecosystems.