Changes in climatic and other drivers in wetlands are generally reflected in a trend in vegetation development. Likewise, in a given environment, plant fractionation of carbon isotopes is imprinted in δ13C values that differ between C3 and C4 species, and also vary in soil organic matter derived from them. In the prairies and marshes of the Everglades, where soil organic matter (SOM) is derived from both plants and periphyton, spatial variation in vegetation composition along the hydrologic gradient correlates with changes in the relative abundance of C3 and C4 species: C4 species are prevalent in shorter hydroperiod marshes. In addition, periphyton productivity in the southern Everglades is high, and varies substantially, depending on substrate type, hydroperiod and water depth. The objective of this study was to examine the carbon isotopic signatures of SOM in surface soils and sediment cores, and to use the signatures in the cores to infer the past hydrologic conditions and related vegetation dynamics. The δ 13C values of SOM at different depths in the sediment core are expected to reflect temporal changes in proportions of C3 and C4 species related to hydrologic shifts. We collected vegetation and surface soil samples at 24 sites, and obtained 14 sediment cores from Southern Everglades marl prairies. δ 13C values were determined for vegetation, and SOM in surface soil and 5-21 subsamples, taken from different depths, in the sediment cores. The relationship between δ 13C values of surface SOM and the proportion of C3 and C4species along a hydrologic gradient was used to infer the past vegetation pattern and hydrologic conditions.
The δ 13C values in surface soil ranged between -10.4‰ and -28.3‰. Both the relative proportion of C3 and C4 plant species and the δ 13C values of SOM in surface soil were significantly correlated with hydroperiod and mean annual water depth, two commonly used metrics of hydrologic conditions in wetlands. The δ 13C values in the sediment cores ranged between -15.4‰ and -30.2‰, and the values varied among and within the cores. Values less than -23‰ along the whole length of 50% of the cores suggested relatively wet conditions in the past throughout a large area that is currently marl prairie landscape. However, a mixed pattern of δ 13C values in the remaining cores suggested spatial variation in temporal changes in hydrologic conditions, followed by a historical shift in vegetation composition in the Everglades marl prairies.