Data from a ten-month monitoring study during 2007 in south Florida provide insight into the variation of δ18O, δD, and δ13C of DOC in surface water and shallow groundwater of the Everglades ecosystem. Bi-monthly samples were taken from surface water and time-averaged precipitation at Taylor Slough, and shallow groundwater from a well and a small cave within Palma Vista Hammock, an exposure of the Upper Pleistocene Miami Limestone.

δ18O and δD values in shallow groundwater from the well and cave remain near the mean of -2.4 ‰ and -12 ‰, respectively (VSMOW scale). 18O and D are enriched in surface water compared to shallow groundwater. δ18O and δD values in surface water fluctuate in sync with, but to a lesser amplitude than, those measured in rainfall. The local meteoric water line (LMWL) for precipitation is in close agreement to the global meteoric water line (GMWL); however, the local evaporation line (LEL) for surface water and shallow groundwater is δD = 5.6 δ18O + 1.5 (R2=0.97), a sign that these waters have experienced evaporation. The intercept of the LMWL and LEL indicates that the primary recharge to the Everglades occurs primarily from tropical or frontal sources. Local convection merely recycles available water.

Time-series of deuterium excess (Dex), clearly reveals two moisture sources for precipitation; an evaporation-dominated source with Dex>10 and a source significantly influenced by transpiration with Dex<10. Samples with higher Dex cluster in the fall and winter, and appear to be associated with maritime moisture carried along the Trade Winds. Samples with lower Dex cluster in the late spring and summer, and could reflect continental moisture carried by the Westerlies or local convection.

Values of δ13CDOC between -22.6 and -28.0‰ suggest C-3 vegetation as the primary source of DOC at all sample sites. C:N ratios of DOC averaging 20:1 at the cave indicate that organic matter originates from woody material, while an average of 15:1 at the well along with δ13CDOC similar to the cave indicate further decomposition of the organic matter entering the cave. C:N ratios of DOC the slough averaged 15:1, and with δ13CDOC values, suggest sources of organic matter not present at the cave and well.


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