Controls of organic carbon processing in boreal headwater streams

Abstract

Carbon emissions from headwater streams are derived from both terrestrial inputs and in-stream microbial processing of organic carbon, but the relative importance of metabolic processes in boreal streams remains uncertain. Determining the factors that regulate organic carbon processing will aid in predicting how the carbon balance of boreal streams will respond to future environmental change. In this study, I addressed the question: what controls organic carbon uptake in boreal headwater streams draining catchments with discontinuous permafrost? I hypothesized that organic carbon uptake is collectively regulated by organic carbon lability, phosphorus availability, and temperature, with discharge modulating each of these conditions. I tested these hypotheses using a combination of laboratory resource manipulation experiments and ecosystem metabolism measurements throughout the Caribou-Poker Creeks Research Watershed (CPCRW) in Interior Alaska, USA. In the laboratory experiments, respiration and dissolved organic carbon (DOC) loss were both co-limited by the supply of labile carbon and phosphorus, but temperature and residence time acted as stronger controls of DOC loss. Ecosystem respiration (ER) was largely predicted by discharge and between-site variance, although some within-site variance was explained by gross primary production (GPP) and temperature. Between sites, both ER and GPP were inversely related to watershed permafrost extent, with an inverse relationship between temperature and permafrost extent providing the most plausible explanation for this pattern. These results provide some of the first evidence of a functional response to permafrost thaw in stream ecosystems and suggest that the contribution of metabolism to stream carbon emissions may increase as climate change progresses.