Does dissolved phosphorus concentration affect leaf breakdown in lowland neotropical streams?

M. Ardón¹, C.M. Pringle¹, L.A. Stallcup¹, and K. Maynard². ¹Institute of Ecology, University of Georgia, Athens, GA 30602, ²Department of Ecology and Evolutionary Biology, Yale University, New Haven, CT 06520

We tested the hypothesis that high concentrations of dissolved phosphorus increase leaf breakdown rates in tropical streams draining Costa Rica’s Caribbean slope. In addition we wanted to examine potential interactions between dissolved phosphorus and leaf chemistry. Principal Component Analysis (PCA) was used to select three common riparian tree species (Trema integerrima, Castilla elastica and Zygia longifolia ) that had similar initial carbon and nitrogen concentrations but different chemical composition in regards to secondary (phenolics and tannins) and structural (cellulose, hemicellulose and lignin) compounds. During the rainy season, leaves were incubated in six streams that had different concentrations of dissolved phosphorus: three streams had low natural concentrations (less than 15 μg/L), two had high natural concentrations due to geothermal inputs (100 μg/L) and a whole stream phosphorus enrichment experiment (more than 200 μg/L) was used to isolate phosphorus effects from other geothermal input solutes. Significant differences in leaf breakdown rates among species suggest the importance of initial leaf chemistry. Differences among sites could not be explained by ambient stream phosphorus concentration. Results suggest that physical abiotic factors (e.g. sediment deposition associated with high discharges during the rainy season) were more important than previously reported phosphorus effects in determining leaf breakdown.