Headwater streams are abundant worldwide and important to global biogeochemical cycles, serving as critical processors and transporters of C. C spiraling is a useful way to understand the retention and mineralization of organic C (OC) in streams. However, analyses of seasonal and interannual variability in OC spiraling are currently limited. In this study, we aimed to understand the temporal patterns and driving mechanisms of OC spiraling, which will inform our understanding of future OC changes under climate change. We used 7 y of daily data in a small headwater stream (Walker Branch, Tennessee, USA) to assess seasonal and interannual variability in OC spiraling length (SOC) and mineralization velocity (vfOC), as well as their potential related variables. On average, SOC in Walker Branch was ~10× shorter than in previously studied small streams, indicating strong connections between the water column and the benthic environment where OC mineralization mostly takes place. OC spiraling was faster during the more biologically active periods of spring and autumn compared with more elongated OC spiraling in summer and winter, when OC retention was lower and downstream transport was higher. Gross primary production (GPP) was most strongly related to SOC and vfOC. Photosynthetically active radiation (PAR) and NO3− were also positively and negatively related to vfOC, respectively. Trends toward earlier and longer canopy cover and reduced GPP and PAR may result in longer SOC and slower vfOC, reducing localized instream processing of OC and potentially shunting more OC downstream. However, long-term observations indicate reduced NO3− at Walker Branch, suggesting opposing effects to those of GPP and PAR, leading to faster vfOC and greater OC retention. Time-series analyses of OC spiraling in streams can enhance our understanding of current and future responses of OC processing and downstream transport to climate change, as well as implications for downstream OC dynamics.
