Light-mediated thresholds in stream-water nutrient composition in a river network.

The elemental composition of solutes transported by rivers reflects combined influences of surrounding watersheds and transformations within stream networks, yet comparatively little is known about downstream changes in effects of watershed loading vs. in-channel processes. In the forested watershed of a river under a mediterranean hydrologic regime, we examined the influence of longitudinal changes in environmental conditions on water-column nutrient composition during summer base flow across a network of sites ranging from strongly heterotrophic headwater streams to larger, more autotrophic sites downstream. Small streams (0.1-10 km2 watershed area) had longitudinally similar nutrient concentration and composition with low (approximately 2) dissolved nitrogen (N) to phosphorus (P) ratios. Abrupt deviations from this pattern were observed in larger streams with watershed areas > 100 km2 where insolation and algal abundance and production rapidly increased. Downstream, phosphorus and silica concentrations decreased by > 50% compared to headwater streams, and dissolved organic carbon and nitrogen increased by approximately 3-6 times. Decreasing dissolved P and increasing dissolved N raised stream-water N:P to 46 at the most downstream sites, suggesting a transition from N limitation in headwaters to potential P limitation in larger channels. We hypothesize that these changes were mediated by increasing algal photosynthesis and N fixation by benthic algal assemblages, which, in response to increasing light availability, strongly altered stream-water nutrient concentration and stoichiometry in larger streams and rivers.

Stoichiometric response of nitrogen-fixing and non-fixing dicots to manipulations of CO2, nitrogen, and diversity.

Human activities have resulted in increased nitrogen deposition and atmospheric CO(2) concentrations in the biosphere, potentially causing significant changes in many ecological processes. In addition to these ongoing perturbations of the abiotic environment, human-induced losses of biodiversity are also of major concern and may interact in important ways with biogeochemical perturbations to affect ecosystem structure and function. We have evaluated the effects of these perturbations on plant biomass stoichiometric composition (C:N:P ratios) within the framework of the BioCON experimental setup (biodiversity, CO(2), N) conducted at the Cedar Creek Natural History Area, Minnesota. Here we present data for five plant species: Solidago rigida, Achillea millefolium, Amorpha canescens, Lespedeza capitata, and Lupinus perennis. We found significantly higher C:N and C:P ratios under elevated CO(2) treatments, but species responded idiosyncratically to the treatment. Nitrogen addition decreased C:N ratios, but this response was greater in the ambient CO(2) treatments than under elevated CO(2). Higher plant species diversity generally lowered both C:N and C:P ratios. Importantly, increased diversity also led to a more modest increase in the C:N ratio with elevated CO(2) levels. In addition, legumes exhibited lower C:N and higher C:P and N:P ratios than non-legumes, highlighting the effect of physiological characteristics defining plant functional types. These data suggest that atmospheric CO(2) levels, N availability, and plant species diversity interact to affect both aboveground and belowground processes by altering plant elemental composition.