Controls of longitudinal variation in δ¹³C-DIC in rivers: A global meta-analysis

Abstract. We conducted a literature survey to investigate controls and spatial and temporal patterns of δ¹³C-DIC and deviations between δ¹³C-DIC and the δ¹³C signature of DIC at isotopic equilibrium with the atmosphere (Δδ¹³C-DIC) in streams and rivers throughout the world. We used generalized additive mixed models to relate Δδ¹³C-DIC and δ¹³C-DIC in lotic ecosystems to ecological variables including elevation, Strahler order, and partial pressure of dissolved CO₂ (pCO₂), and to examin e seasonal shifts in Δδ¹³C-DIC and δ¹³C-DIC over a range in latitude. Elevation, Strahler order, and DIC concentrations explained a large fraction of the variation in Δδ¹³C-DIC, and these variables plus pH and pCO₂ explained much of the variation in δ¹³C-DIC. Seasonal fluctuations in δ¹³C-DIC were most apparent in rivers located in temperate regions with seasonal snow cover. Small streams tended to have lower δ¹³C-DIC values than large rivers. Overall, our analysis indicates that processes that add CO₂ to the water column, including groundwater inputs, decomposition, and respiration, should have a greater influence on δ¹³C-DIC than processes that remove CO₂. Both physical (gas exchange with the atmosphere, weathering, ice cover) and biolog ical (respiration in regions with high C₄ grass abundance, photosynthesis by cyanobacteria) processes appear to control δ¹³C-DIC in streams and rivers, but the relative importance of these processes shifts from upstream to downstream.