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1. The dissolved organic matter (DOM) biogeochemistry of intermittent rivers is affected by periods of disconnection and drying after cessation of surface flow, with a pronounced concentra tion of solutes as pools evaporate and contract. However, inputs of alluvial ground water to stream pools may offset the effects of evaporation and influence the quantity and composition of DOM. 2. We used δ18 O and δℑ²H values of surface water and alluvial ground water (AW) together with DOM fluorescence excitation-emission spectroscopy to investigate changes in DOM biogeochemistry of pools from four intermittent streams in semi-arid north-west Australia. Our objective was to deter mine whether the fluorescence characteristics of DOM in pools are driven by their connectivity to alluvial ground water. 3. Pool water δ18O values ranged from -8 to 13.1‰ & and showed progressive enrichment as pools became disconnected from AW and subsequently evaporated. Parallel factor analysis of DOM fluo rescence revealed that DOM in pools was generally dominated by humic-like components, with les ser contributions from tryptophan-like and tyrosine-like components. 4. DOC concentrations were higher in more evaporated pools. Humic-like and tryptophan-like DOM were also highest in pools isolated from AW, suggesting that inputs of AW had a dilution effect while also moderating evaporative concentration of solutes. In contrast, tyrosine-like DOM was only weakly correlated with the hydrological status of pools. 5. The proportion that each component contributed to total DOM fluorescence remained mostly consistent across both AW-connected and evaporating pools. However, some pools with AW through-flow had higher contributions from tyrosine-like fluorescence and lower contributions from humic-like fluorescence. 6. Disconnection from AW and subsequent evaporative concentration of solutes is an important control on DOM fluorescence characteristics in these intermittent streams. However, production and removal of DOM were only rarely influenced by AW connectivity, likely reflecting the high hydrological variability of intermittent rivers, even following cessation of surface flow. [ABSTRACT FROM AUTHOR]

We used a complete block design to experimentally study direct and indirect interactions in a three trophic-level freshwater system consisting of a top predator, the green sunfish, Lepomis cyanellus, an intermediate predator, small-mouthed salamander larvae, Ambystoma barbouri, and prey, hatchling isopods, Lirceus fontinalis. This system occurs naturally in small stream pools in central Kentucky; experiments were done in laboratory pools. Salamander larvae ate isopods and thus had a direct, negative effect on isopod survival. Accordingly, isopods responded to the presence of salamander larvae by increasing their tendency to bury themselves in the sand substrate. Fish ate salamanders and thus had a direct, negative effect on salamander survival. Salamanders responded to fish presence by increasing their time spent under plexiglass plates that simulate refuge rocks. The overall effect of fish on isopods depended on the presence of salamanders. In the absence of salamanders, fish predation on isopods had a direct, negative effect on isopod survival; isopods thus responded to the presence of fish by burying themselves in the sand. With salamanders present, fish had a positive overall effect on isopod survival; i.e., direct, negative effects of fish on isopods were outweighed by indirect, positive effects. Indirect positive effects of fish on isopods came through a reduction in salamander predation rates on isopods in the presence of fish. The mechanism involved both a decrease in the number of salamanders (a trophic-linkage indirect effect; cf. Miller and Kerfoot 1987) and a reduction in the feeding rate of individual salamanders on isopods (a behavioral indirect effect). The decrease in individual salamander feeding rates on isopods was due to reductions in both salamander activity and in spatial overlap between salamanders and isopods in the presence of fish. The latter effect reflected the fact that salamanders and isopods used different refuges from fish; salamanders went under refuge plates, whereas isopods primarily buried themselves in sand. Estimates of the relative importance of various direct and indirect effects of sunfish on isopods suggested that positive, behavioral indirect effects were of roughly the same magnitude as direct, negative effects, both of which were more important than were trophic-linkage indirect effects. Contrary to expectations, the presence of isopods did not affect the refuge use or survival of salamanders in the presence of fish. [ABSTRACT FROM AUTHOR]