RESISTANCE, RESILIENCE, AND VARIABILITY IN TROPICAL STREAM CHEMISTRY: IMPLICATIONS FOR WATERSHED FUNCTION AND BIOGEOCHEMICAL THEORY (Invited Presentation)

Understanding spatial and temporal variability in stream chemistry is an important goal in environmental sciences. This task is particularly urgent for tropical watersheds, with their global significance and relatively sparse representation in the literature. In the Luquillo Mountains of Puerto Rico, 17 streams have been studied with a minimum of one year of weekly samples. Most of these streams have been sampled for over 20 years. This rich dataset allows the opportunity to address in a quantitative framework the spatial and temporal variability in stream chemistry. Such an assessment shows that these streams are consistently high in SiO2 and base cations derived from weathering reactions, with dilution at high flow; that they are relatively low in DOC and DON, and that N in streams reflects the legacy of land use from decades past, as well as more current hurricane regime. Fine-scale (weekly) variability in stream chemistry differs among watersheds, and appears to be an intrinsic watershed property associated with geomorphic conditions. Decadal scale variation in stream nitrate chemistry is driven by response to disturbance events (hurricanes). A formal assessment of variability in stream chemistry within and among biomes is required to assess the generality of these findings.