Bacterially mediated carbon and nutrient dynamics in a highly impacted river system

Background/Question/Methods

Rivers are critical in the transport and transformation of carbon (C) and inorganic nutrients exported from terrestrial landscapes, with suspended bacteria potentially playing a critical role. Suspended bacteria actively participate in organic carbon cycling and reallocation. Differential bacterial mediation of carbon occurs when labile carbon is preferentially incorporated over refractory carbon. It is hypothesized that more productive systems are characterized by higher levels of labile carbon. This study evaluated carbon (C) and inorganic nutrient dynamics in the Rio Grande network (in Texas, USA) and examined trends in bacterial C use along a downstream gradient of highly arid to semi-tropical landscapes. Bacterial production (BP), bacterial respiration (BR), water color, dissolved organic carbon (DOC), carbon lability parameters and inorganic nutrients were measured at 9 sites in the Rio Grande and two of its major riverine tributaries (the Pecos and Devils Rivers). Sampling was conducted seasonally over a nine month period.

Results/Conclusions

BR varied significantly by season while BP varied significantly between watersheds. BGE and k (the constant of aerobic decomposition) covaried, displaying an expected positive relationship between bacterial C growth efficiency and the decomposition rate of labile carbon (OC_L). Across all sites and all seasons, the spring-influenced sites (DR and PR4) are characterized by higher BP, low Chla, and low turbidity while URG sites show the greatest variability in both abiotic and biotic parameters. These findings indicate that, unlike many river systems, the more upstream sites on the Rio Grande are more impacted and these impacts may be mediated by increased discharge, presence of reservoirs, and substantial ground water inputs downstream. BGE and OC_L exhibit a weak inverse relationship, in accordance with other studies. This study indicates that an understanding of bottom-up controls on C availability is essential to the creation and implementation of conservation efforts in highly impacted river systems.