Environmental Controls on Carbon Cycling in a Small Forested Headwater Stream

Small streams play an important role in the global carbon (C) cycle, transporting C between terrestrial landscapes and regional watersheds and outgassing carbon dioxide (CO2) to the atmosphere. However, on a local scale, streamwater dissolved inorganic carbon (DIC) concentrations are sensitive to seasonal changes in environmental conditions and singular climate events. The primary focus of this project is to investigate spatial and temporal variation in C cycling in a small forested headwater stream in Hopkins Memorial Forest, Berkshire County, Massachusetts. We sampled stream water biweekly from 2019-2023. Additionally, we performed seasonal upstream transects (early summer, fall) over a 4800m reach from the headwaters in 2019 and 2021. Biweekly and transect samples were analyzed for δ13C and δ15N of particulate organic matter (POM), and the δ13C and concentration of DIC. Low winter DIC concentrations reflect a strong seasonal influence on C cycling, as less local biological decomposition and respiration lowers stream C concentration. Low DIC concentration during drought years (Summer 2022) also reflect the connection between local meteorological conditions and stream chemistry. Decreased plant respiration and reduced streamflow are the likely drivers for this C cycling reduction. Transect data revealed the δ13C of DIC was most depleted at the headwaters and increased with distance downstream. This change reflects a transition between biologically-dominated DIC sources and an increased contribution of carbonate bedrock to the total DIC pools. The data provide increased understanding of CO2 dynamics in the Birch Brook watershed, and more fully characterize how environmental change influences C cycling in small streams.