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

English Abstract:

Gas exchange between the water and the atmosphere strongly influences the δ¹³C signature of the inorganic carbon pool in large rivers. Because the δ¹³C signature of DIC at isotopic equilibrium with the atmosphere (δ¹³C-DIC_equilibrium) is dependent on pH and temperature, we were interested in determining if these physicochemical variables could be used to generate an isoscape, or a map of the stable isotope variation in DIC, in rivers. We conducted a literature survey to examine the relationship between δ¹³C-DIC and δ¹³C-DIC_equilibrium in rivers throughout the world. We then used generalized additive mixed models to investigate if deviations from δ¹³C-DIC_equilibrium (δ¹³C-DIC_{disequilibrium}) and δ¹³C-DIC in lotic ecosystems were related to ecological variables including the partial pressure of dissolved CO₂, elevation, and Strahler order. To evaluate if carbonate weathering or groundwater inflow was responsible for longitudinal variation in δ¹³C-DIC, we compared within-river trends in weathering products and groundwater inflow with within-river trends in δ¹³C-DIC in the main stem of 18 of the rivers. For more than 99% of the samples, DIC was more depleted in ¹³C than δ¹³C-DIC_equilibrium. The GAMMs revealed that, whereas morphometric variables (elevation, Strahler order) explained a large fraction of the variation in δ¹³C-DIC_{disequilibrium}, morphometric and geochemical variables (DIC, pH, pCO₂) explained a large fraction of the variation in δ¹³C-DIC. In all rivers with limited groundwater inputs, headwaters were isotopically heavier than downstream reaches, suggesting greater inputs of DIC from groundwater than from terrestrial ecosystems. Rivers with groundwater inflow in both upper and lower reaches had higher rates of change in δ¹³C-DIC than rivers where groundwater inflow was limited to lower reaches, indicating that rivers with groundwater inputs in headwaters will be more suitable for use of δ¹³C in analyses of the spatial ecology of consumers.