Physicochemical and carbon quantity–quality gradients equally influence bacterial carbon metabolism across an arid riverscape

Abstract

Although freshwater systems comprise a small portion of land surface, they play a pivotal role in landscape carbon (C) cycling. Carbon processing by heterotrophic bacteria is critical, contributing to ecosystem production and dissolved organic carbon (DOC) processing. Riverine bacterial C metabolism is related to a diversity of factors, including (1) physiochemical conditions and inorganic nutrient concentrations and (2) DOC quality and quantity; however, the relative importance of these factors in influencing C metabolism across arid fluvial networks remains equivocal. This study examined C metabolism by heterotrophic bacterioplankton in the Rio Grande drainage, an arid river network in Texas, USA. We examined spatial variation in physicochemical and inorganic nutrient conditions, bacterial C metabolism, and DOC lability across the drainage and assessed whether variation in bacterial C metabolism was more related to physicochemical–inorganic nutrient conditions or DOC quantity and lability. Across the drainage, hydrology and landscape position influenced physicochemical conditions, bacterial abundance, phytoplankton biomass, and bacterial C metabolism and the proportion of variation in bacterial C metabolism explained by physicochemical/inorganic nutrient and DOC quality–quantity gradients was approximately equal. Bacterial abundance and production were associated with greater NH4+ and DOC concentrations and phytoplankton biomass, whereas bacterial respiration and growth efficiency were driven by DOC color and suspended particulate concentrations. Results indicate that aspects of bacterial C metabolism are influenced by different environmental factors related to autochthonous and allochthonous inputs to riverine systems, which has implications for C transformation, sequestration, and transport to the ocean.