Hydrologic control of dissolved organic matter biogeochemistry in pools of a subtropical dryland river

Dryland rivers function as strongly linked ecologic-hydrologic systems, including both extended periods of drought and episodic flooding events. However, few studies have combined hydrologic and biogeochemical measurements to better understand the ecology of pools within dryland rivers. We used delta H-2 and delta O-18 values of pool water, rainfall, and groundwater combined with pool water measurements of C, N, and P and dissolved organic matter (DOM) fluorescence characteristics to determine (1) the concentration and chemical composition of DOM and (2) the origin of surface water in 16 pools of a dryland river in northern Western Australia. Parallel factor analysis of excitation-emission matrices showed that humic-like components derived mainly from terrestrial plant material dominated total DOM fluorescence for all pools. Evaporation models using delta H-2 and delta O-18 showed a variety of pool hydrologic regimes, including pools with moderate to high evaporative water loss that were largely isolated from shallow alluvium water inputs and pools with consistent alluvium water throughflow and low evaporation. Concentrations of C, N, and P as well as total DOM fluorescence were generally greater in pools with high evaporative loss and lower in pools with alluvium water inputs. Pool delta H-2 and d18O values were also significantly correlated with DOM fluorescence characteristics and C, N, and P concentrations, providing quantitative evidence of the hydrologic influence on DOM biogeochemistry. Taken together, our findings suggest that individual pools function as distinct ecosystems within the riverine environment.