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[1] 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 d 2 H and d 18 O 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 d H and d18 O 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 d H and d18 O 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.

Mining below groundwater tables is increasing globally, yet little is known of how associated large-scale modification of water tables impact functioning of surrounding ecosystems. We used measurements of foliage density (ρ F ) and sapwood-related sap flow (Q S ) to assess effects of depth to groundwater on Eucalyptus victrix, a tree species that is common in riparian zones in central and northern parts of Australia.Foliage density (ρ F ) varied with season and among sites. Of itself, ρ F provided a partial indicator of how trees responded to falling (more than 10 m) and rising (more than 9 m) water tables. Assessment of Q S was highly informative. Across all sites, Q S was least (90-130 l m À2 sapwood h À1) where groundwater was naturally deep (30 m) or had fallen substantially over the past 4 years (from 8 to 19 m). Fastest rates of Q S (>245 l m À2 sapwood h À1 ) were recorded where groundwater had risen to a depth similar to a site where depth to groundwater remained stable at 6-7 m. Our analyses of daytime and night-time Q S emphasize that water use by E. victrix is highly plastic and opportunistic. We discuss how empirical analysis of Q S , coupled with a sound understanding of local hydrogeology, can help assess responses in ecosystem function to large-scale modification of groundwater levels -an important issue globally, as well as in Australia.