Isotopic evidence for the source and fate of phosphorus in Everglades wetland ecosystems

0883-2927/$ see front matter 2011 Elsevier Ltd. A doi:10.1016/j.apgeochem.2011.01.027 ⇑ Corresponding author. Fax: +1 850 644 0827. E-mail address: [email protected] (Y. Wang) Phosphorus has historically been a limiting nutrient in the Florida Everglades. Increased P loading to the Everglades over the past several decades has led to significant changes in water quality and plant communities. Stormwater runoff that drains agricultural lands and enters the Water Conservation Areas (WCAs) are known to contain elevated levels of P, but the exact source of this P has not been fully determined. Here the results of an O isotope study of dissolved inorganic phosphate (DIP) in both polluted and relatively pristine (or reference) areas of the Everglades are reported. The data reveal spatial and temporal variations in the dO signature of DIP, reflecting the source and the degree of cycling of P. The dO values of DIP collected from the Everglades National Park were close or equal to the predicted dO values of DIP formed in situ in equilibrium with ambient water, indicating that P is quickly cycled in the water column in oligotrophic ecosystems with very low P concentrations. However, most DIP samples collected from areas impacted by agricultural runoff yielded dO values that deviated from the predicted equilibrium DIP–dO values based on the dO of water and water temperature, suggesting that biological cycling of P was not rapid enough to remove the fertilizer dO signature in the DIP pool from areas receiving high P loading. The dO signature of DIP in impacted areas reflects a mixing of fertilizer P and biologically cycled P, where the relative proportions of biologically cycled vs. fertilizer DIP are controlled by both biological (microbial activities and plant uptake) and hydrologic factors (loading rate and residence time). Using a two-end-member (i.e., fertilizer P and biologically cycled P) mixing model, fertilizers were estimated to contribute about 15–100% of the DIP pool in the highly impacted areas of the northern Everglades, whereas the DIP pool in the reference (i.e., relatively pristine) wetlands in the Everglades National Park was dominated by biologically cycled P. The study shows that O isotopic measurements of dissolved PO 4 can be a useful tool for tracing the fertilizer P inputs to freshwater ecosystems. 2011 Elsevier Ltd. All rights reserved.