Temperature and Flow Control Organic Carbon Metabolism in Boreal Headwater Streams.

Metabolism in stream ecosystems alters the fate of organic carbon (OC) received from surrounding landscapes, but our understanding of in‐stream metabolic processes in boreal ecosystems remains limited. Determining the factors that regulate OC metabolism will help predict how the C balance of boreal streams may respond to future environmental change. In this study, we addressed the question: what controls OC metabolism in boreal headwater streams draining catchments with discontinuous permafrost? We hypothesized that metabolism is collectively regulated by OC reactivity, phosphorus availability, and temperature, with discharge modulating each of these conditions. We tested these hypotheses using a combination of laboratory experiments and whole‐stream ecosystem metabolism measurements throughout the Caribou‐Poker Creeks Research Watershed (CPCRW) in Interior Alaska, USA. In the laboratory experiments, respiration and dissolved OC (DOC) removal were both co‐limited by the supply of reactive C and phosphorus, but temperature and residence time acted as stronger controls of DOC removal. Ecosystem respiration (ER) was largely predicted by discharge and site, with some variance explained by gross primary production (GPP) and temperature. Both ER and GPP varied inversely with watershed permafrost extent, with an inverse relationship between temperature and permafrost extent providing one plausible explanation. Our results provide some of the first evidence of a functional response to permafrost thaw in stream ecosystems and suggest the role of metabolism in landscape C cycling may increase as climate change progresses. Plain Language Summary: Rivers and streams can transform organic matter received from the surrounding landscape through the metabolic activity of microbes and other organisms. In boreal streams, little is known about ecosystem metabolism and whether respiration rates might change as warming‐induced permafrost thaw progresses. To understand if and how permafrost thaw is affecting stream microbial activity, we compared respiration rates in streams draining areas with different amounts of permafrost and used lab experiments to test which factors are currently limiting microbial activity. We found that metabolic rates were greatest in streams where the most permafrost thaw had occurred. This pattern could be explained by warmer water temperature promoting faster rates of microbial activity and less flashy flow conditions allowing more time for microbes to break down organic matter before it is flushed downstream. This newly documented relationship between permafrost thaw and stream respiration should be considered in efforts to better incorporate in‐stream processes into broader carbon cycling models. Key Points: Stream ecosystem respiration increased with permafrost thaw, likely due to warmer temperature and a more stable flow regimeWithin individual streams, ecosystem respiration was positively correlated with both discharge and gross primary productionRespiration was most limited by temperature and residence time in lab experiments, with additional co‐limitation by carbon and phosphorus [ABSTRACT FROM AUTHOR]