BACTERIALLY-MEDIATED CARBON DYNAMICS IN A HIGHLY IMPACTED RIVER NETWORK
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Inland freshwater ecosystems, though comprising a small portion of the earth’s surface, are thought to be important in the global carbon (C) cycle. Carbon processing by heterotrophic microbes (bacteria) is a critical process, contributing considerably to overall ecosystem production and processing of dissolved organic carbon (DOC). This study assesses spatial variation in C processing by heterotrophic bacterioplankton in a semi-arid river network: the Rio Grande/Rio Bravo del Norte in Texas, USA. I examined how bacterial metabolism and C processing varied with spatial differences in physicochemical conditions and patterns in DOC lability in this highly impacted riverine network. Physicochemical and biological data were collected at 14 sites from March - December of 2010. I additionally analyzed phytoplankton biomass, bacterial density, bacterial community metabolic rates [bacterial respiration (BR), bacterial productivity (BP), and bacterial growth efficiency (BGE)], and C quality parameters at a subset of nine sites within this drainage. Across the drainage, hydrology and landscape position (i.e., biogeoclimatic conditions, presence of reservoirs, and groundwater contribution to flow) substantially influenced in-stream physicochemical conditions, leading to spatial patterns in bacterial density, phytoplankton biomass, and bacterial metabolism. Bacterial C metabolism was influenced by both physicochemical and C quality – quantity gradients present within the drainage. Bacterial production and BR responded to different environmental gradients, with BP being driven by C quality and inorganic nutrients. This resulted in a negative correlation between BGE and the bacterial respiration of refractory C. Results from this study indicate that natural variation and anthropogenic impacts influence the physicochemical and biotic conditions across the Rio Grande/Rio Bravo del Norte drainage and these effects have implications for C sequestration, transformation, and transport, as well as for organic matter (OM) delivery to the Gulf of Mexico.