Isotopic evidence for the source and fate of phosphorus in Everglades wetland ecosystems Xin Li a,b , Yang Wang b,⇑ , Jennifer Stern b,c , Binhe Gu d a Institute of Hydrobiology, Jinan University, Guangzhou, Guangdong 510632, China b Department of Earth, Ocean and Atmospheric Science, Florida State University & National High Magnetic Field Laboratory, Tallahassee, FL 32306-4100, USA c NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA d Everglades Division, South Florida Water Management District, West Palm Beach, FL 33406, USA article info Article history: Received 26 November 2009 Accepted 11 January 2011 Available online 15 January 2011 Editorial handling by W.B. Lyons abstract 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 com- munities. 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 deter- mined. 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 tempo- ral variations in the d 18 O signature of DIP, reflecting the source and the degree of cycling of P. The d 18 O values of DIP collected from the Everglades National Park were close or equal to the predicted d 18 O 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 d 18 O values that deviated from the predicted equilib- rium DIP–d 18 O values based on the d 18 O of water and water temperature, suggesting that biological cycling of P was not rapid enough to remove the fertilizer d 18 O signature in the DIP pool from areas receiving high P loading. The d 18 O 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 con- trolled 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, fer- tilizers 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 Ever- glades National Park was dominated by biologically cycled P. The study shows that O isotopic measure- ments of dissolved PO 3 4 can be a useful tool for tracing the fertilizer P inputs to freshwater ecosystems. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Nutrient loading in wetlands is the subject of much concern due to human dependence on the benefits offered by a healthy wet- land. Wetlands serve to store and filter water, provide habitats for a diverse array of species, and support recreational activities such as fishing. Many coastal and freshwater wetlands have expe- rienced eutrophication due to poor land-use and the contributions of excessive nutrients from agricultural or urban runoff upstream (Carpenter et al., 1998). Eutrophication of freshwater wetlands is problematic as it causes a reduction of biodiversity and the inva- sion of exotic species (Sklar et al., 2005). While the sources of these excess nutrients can sometimes be traced, the fate of these nutri- ents upon entering the wetland system is poorly understood. Phosphorus is a limiting nutrient in most freshwater ecosystems (Jickells, 1998). Unlike C and N, P added to an aquatic ecosystem or released during the decomposition of organic matter usually stays within the system, resulting in an enrichment of P in detritus and surface soil/sediment (Reddy et al., 1999). Phosphorus is present in the water column as particulate organic P (POP), dissolved organ- ic P (DOP), and dissolved inorganic P (DIP). DIP is the most bioavail- able form of P and may be quickly taken up by organisms. It may also be sequestered in sediments/soils by adsorption to or reaction with Fe, Al, Ca and Mg minerals. Soil P undergoes various transfor- mations as it cycles through inorganic P pools (associated with Fe, Al, Ca and Mg minerals) and organic P pools (including plants, animals, microbes, and soil organic matter) (Richardson, 1999). Microbes transform organic P from decaying organic matter into bioavailable inorganic P. These processes are largely governed by nutrient content (C, N and P) of the soil and water as well as by the size of the microbial pool (Richardson, 1999; Reddy et al., 1999). 0883-2927/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2011.01.027 ⇑ Corresponding author. Fax: +1 850 644 0827. E-mail address: ywang@magnet.fsu.edu (Y. Wang). Applied Geochemistry 26 (2011) 688–695 Contents lists available at ScienceDirect Applied Geochemistry journal homepage: www.elsevier.com/locate/apgeochem