Chemical and carbon isotopic evidence for the source and fate of dissolved organic matter in the northern Everglades

Surface waters in the Florida Everglades contain high levels of dissolved organic carbon (DOC) compounds. delta(13)C values of DOC samples collected from the northern Everglades indicate that less than about 23% of the DOC was derived from sugarcane (the dominant agricultural crop in the area), and the amount of DOC from sugarcane was greater during the dry period. Most of the DOC (>50%) in the northern Everglades was in the low molecular weight (< 1000 Dalton) fraction (LMW-DOC). The relative amount of high molecular weight DOC (HMW-DOC) was higher in the wet period than in the dry period. Radiocarbon ages of the DOC ranged from > modern to about 2400 years B.P., indicating that DOC was derived from both historic peat deposits and modern vegetation. At each site, the HMW-DOC had older radiocarbon ages than the LMW-DOC, and therefore contained a greater fraction of DOC derived from the historic peat deposits. It appears that at least some of the old DOC compounds from the historic peat deposits were decomposed during their residence in the surface water system in the northern Everglades, and the LMW-DOC was more microbially labile than the HMW-DOC. Our analysis suggests that accelerated decomposition of organic matter in the historic peat deposits (due to land-use change) could be a significant source of DOC and nutrients in the northern Everglades. Our data also suggest that the radiocarbon signature of DOC could be used as a sensitive indicator of the overall effectiveness of a wetland restoration project.