Nutrient enrichment intensifies the effects of warming on metabolic balance of stream ecosystems

Climate warming and eutrophication are leading drivers of environmental change, and both are likely to alter the role of freshwaters in the global carbon cycle. Recent studies demonstrate that warming of streams can increase freshwater contributions of CO2 to the atmosphere, yet little is known about how such contributions are modulated by the identity or supply of limiting nutrients. We quantified responses of ecosystem metabolism and metabolic balance to whole-stream enrichments of either nitrogen or phosphorus across a 5 degrees C range of ambient temperature. We show that nutrient enrichment drove large reductions in net ecosystem production, and interactions with temperature amplified these effects in warm N-enriched streams. Partitioning of autotrophic and heterotrophic respiration revealed that these responses were driven by increased respiration of microbial heterotrophs. Our study provides direct experimental evidence that warming-induced increases in CO2 emissions to the atmosphere are likely intensified in stream ecosystems subject to eutrophication.