Controls of streamwater dissolved inorganic carbon dynamics in a forested watershed

I investigated controls of stream dissolved inorganic carbon (DIC) sources and cycling along a stream size and productivity gradient in a temperate forested watershed in northern California. Dissolved CO₂ (CO_{2 (aq)}) dynamics in heavily shaded streams contrasted strongly with those of larger, open canopied sites. In streams with canopy cover > 97%, CO_{2 (aq)} was highest during baseflow periods (up to 540 μM) and was negatively related to discharge. Effects of algal photosynthesis on CO_{2 (aq)} were minimal and stream CO_{2 (aq)} was primarily controlled by groundwater CO_{2 (aq)} inputs and degassing losses to the atmosphere. In contrast to the small streams. CO_{2 (aq)} in larger, open-canopied streams was often below atmospheric levels at midday during baseflow and was positively related to discharge. Here, stream CO_{2 (aq)} was strongly influenced by the balance between autotrophic and heterotrophic processes. Dynamics of HCO₃^- were less complex. HCO₃^- and Ca²⁺ were positively correlated, negatively related to discharge, and showed no pattern with stream size. Stable carbon isotope ratios of DIC (i.e. δ¹³C DIC) increased with stream size and discharge, indicating contrasting sources of DIC to streams and rivers. During summer baseflows, δ¹³C DIC were ¹³C-depleted in the smallest streams (minimum of -17.7‰) due to the influence of CO_{2 (aq)} derived from microbial respiration and HCO₃^- derived from carbonate weathering. δ¹³C DIC were higher (up to -6.6‰) in the larger streams and rivers due to invasion of atmospheric CO₂ enhanced by algal CO_{2 (aq)} uptake. While small streams were influenced by groundwater inputs, patterns in CO_{2 (aq)} and evidence from stable isotopes demonstrate the strong influence of stream metabolism and CO₂ exchange with the atmosphere on stream and river carbon cycles.