Carbon Cycling in Floodplain Ecosystems: Out-Gassing and Photosynthesis Transmit Soil δ¹³C Gradient Through Stream Food Webs

Natural braided river floodplains typically possess high groundwater—surface water exchange, which is vital to the overall function and structure of these complex ecosystems. Spring-fed streams on the floodplain are also hotspots of benthic invertebrate diversity and productivity. The sources of carbon that drive these productive spring-fed systems are not well-known. We conducted field assessments and a manipulation, modeling, and a laboratory experiment to address this issue. Initially δ¹³C values of both dissolved inorganic carbon (DIC) and food-web components of five springs were used to assess the sources of carbon to spring food webs. Partial pressures of CO₂ in upwelling water ranged from 2 to 7 times atmospheric pressure, but rapidly approached equilibrium with the atmosphere downstream commensurate with ¹³C enrichment of DIC. Speciation modeling and a laboratory out-gassing experiment suggested that downstream changes in pH could be explained solely by CO₂ out-gassing. However, field results indicated that both out-gassing and photosynthetic drawdown by aquatic plants controlled the net flux of CO₂. A whole stream manipulation indicated out-gassing was the primary effect at the spring source, which was confirmed by invariant diel pH. At 1296 m downstream from the spring source a large diel shift in pH indicated a plant effect on CO₂ concentration which would contribute to the overall downstream gradient in δ¹³C DIC. Within the first 1296 m the gradient in δ¹³ DIC was transmitted through three trophic levels of the spring food web. These findings indicate dependency on groundwater inorganic carbon by spring stream food webs and strong hydrologically mediated linkages connecting terrestrial, subsurface, and aquatic components of the floodplain.