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 d13C
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 d13C DIC. Within
the first 1296 m the gradient in d13 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.