Ecological stoichiometry examines how the mass balance of elements and energy affects interactions among organisms, between organisms and their environment, and provides a framework for examining linkages between elemental cycles. We tested the importance of temperature, absolute and relative concentrations of stream nutrients, and benthic biofilm stoichiometry in driving nutrient uptake in headwater streams located along natural environmental gradients in two climate regimes, a temperate gradient in the Colorado Rockies and a tropical gradient in the Ecuadorian Andes. We were particularly interested in examining how temperature effects on nutrient uptake vary between these tropical and temperate systems. Because organisms in the tropics experience less seasonal temperature variation than temperate organisms and because temperature is more stable within most tropical streams, we expected that temperature would be a better predictor of nutrient use rates across our tropical sites. We also compared the effects of temperature to effects of both nutrient requirement and differences in the relative availability of nutrients on nutrient uptake. We measured nutrient uptake in ~10 streams from each climate. We also measured a broad range of environmental variables, including temperature, canopy cover, water column dissolved organic carbon, inorganic nitrogen, and phosphorus (SRP) and carbon:nitrogen:phosphorus stoichiometry of benthic biofilms.
As expected, we found that nitrogen uptake was positively correlated with elevation and negatively correlated with temperature in both our tropical Andean and our temperate Rockies sites, with a stronger relationship in the Andes. The percent carbon and nitrogen of biofilms were relatively uniform in both our temperate and our tropical streams, while biofilm percent phosphorus content was more variable in our tropical streams. Nitrate uptake was positively correlated with dissolved organic carbon concentration in both our tropical and our temperate streams, but the relationship was stronger in our tropical streams. With the exception of dissolved organic carbon, which was more variable across our temperate streams, water and benthic biofilm stoichiometry varied more in the tropics. Informing generalizable models about global nutrient fluxes requires fine scale testing and intimate understanding of differences between climates. This research addresses previously unexplored relationships between climate, ecosystem stoichiometry, and ecosystem functioning in highland streams.