Controls of streamwater dissolved inorganic carbon dynamics in a forested watershed

Iinvestigated controls of stream dissolved inorganic carbon (DIC) sources andcycling along a stream size and productivity gradient in a temperate forestedwatershed 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 M) and wasnegatively related to discharge. Effects of algal photosynthesis on CO_2(aq) were minimal and stream CO_{2 (aq)} was primarily controlledby groundwater CO_{2 (aq)} inputs and degassing losses to theatmosphere. In contrast to the small streams, CO_{2 (aq)} in larger,open-canopied streams was often below atmospheric levels at midday duringbaseflow and was positively related to discharge. Here, stream CO_2(aq) was strongly influenced by the balance between autotrophic andheterotrophic processes. Dynamics of HCO₃ ^– werelesscomplex. HCO₃ ^– and Ca²⁺ were positivelycorrelated, negatively related to discharge, and showed no pattern with streamsize. Stable carbon isotope ratios of DIC (i.e. ¹³C DIC)increased with stream size and discharge, indicating contrasting sources of DICto streams and rivers. During summer baseflows, ¹³C DIC were¹³C-depleted in the smallest streams (minimum of–17.7) due to the influence of CO_{2 (aq)} derived frommicrobialrespiration and HCO₃ ^– derived from carbonateweathering. ¹³C DIC were higher (up to –6.6)inthe larger streams and rivers due to invasion of atmospheric CO₂enhanced by algal CO_{2 (aq)} uptake. While small streams wereinfluenced by groundwater inputs, patterns in CO_{2 (aq)} and evidencefrom stable isotopes demonstrate the strong influence of stream metabolism andCO₂ exchange with the atmosphere on stream and river carbon cycles.