Investigating the use of macrophyte stable C and N isotopic ratios as indicators of wetland eutrophication: Patterns in the P-affected Everglades

We investigated the use of stable C and N isotopic ratios as indicators of shifts in nutrient limitation of aquatic macrophytes in native Typha (Typha domingensis Pers.) and Cladium (Cladium jamaicense Crantz) communities growing along the well-established phosphorus enrichment gradient of Water Conservation Area 2A of the Florida Everglades. Both Typha and Cladium had significantly different d15N (,4% and 6%, respectively) in affected areas, with live leaves of Typha showing elevated d15N up to 7 km from nutrient inflows. In contrast, changes in d13C were inconsistent, with an ,2% increase in Typha and a corresponding 2% decrease in Cladium of nutrient-affected areas. The isotopic patterns of live leaves were well represented in standing dead leaves of Cladium, but not for Typha, indicating a significant alteration of isotopic signature during senescence for this emergent species. Correlations of isotopic values with tissue nutrients (total C, N, and P) indicated a greater effect of P on the d13C of both plants and the d15N of Typha, and a greater importance of N content in determining d15N of Cladium. These results support the use of macrophyte biomass d13C and d15N as an indicator of eutrophication and shifts between N and P limitation. However, the results also highlight potential pitfalls arising from differences in species-specific response to nutrient enrichment. The process of wetland eutrophication, like that of any other aquatic system, often involves a transition in the limiting nutrient. Commonly, subtropical and tropical freshwater systems are limited by phosphorus (P) (Reddy et al. 1999). When P is added to these systems, productivity increases, and if continued, other nutrients may become limiting. Most frequently, nitrogen (N) is the element limiting freshwater wetland systems apart from P; therefore P inputs to a P-limited system are likely to result in a shift to increasing N limitation (Verhoeven et al. 1996). Such shifts in nutrient limitation can result in drastic changes in the functioning of a wetland, and thus, there has long been interest in identifying indicators of this transition. Macrophytes are generally the dominant producers in wetlands, and for this reason many approaches have used their biomass to assess N and P limitation. These methods have included direct measurements of biomass N and P concentrations (Gerloff and Krombholz 1966), stoichiometric ratios between carbon (C) and N (C : N) and N and P (N : P) (e.g., Gusewell et al. 2003), and time-consuming factorial fertilization experiments (Chapin et al. 1986). Natural abundance ratios of stable C and N isotopes (d13C and d15N) have also been proposed as indicators of nutrient limitation in aquatic macrophytes (McKee et al. 2002;