Water diversion and pollution interactively shape freshwater food webs through bottom-up mechanisms.

Water diversion and pollution are two pervasive stressors in river ecosystems that often co-occur. Individual effects of both stressors on basal resources available to stream communities have been described, with diversion reducing detritus standing stocks and pollution increasing biomass of primary producers. However, interactive effects of both stressors on the structure and trophic basis of food webs remain unknown. We hypothesized that the interaction between both stressors increases the contribution of the green pathway in stream food webs. Given the key role of the high-quality, but less abundant, primary producers, we also hypothesized an increase in food web complexity with larger trophic diversity in the presence of water diversion and pollution. To test these hypotheses, we selected four rivers in a range of pollution subject to similar water diversion schemes, and we compared food webs upstream and downstream of the diversion. We characterized food webs by means of stable isotope analysis. Both stressors directly changed the availability of basal resources, with water diversion affecting the brown food web by decreasing detritus stocks, and pollution enhancing the green food web by promoting biofilm production. The propagation of the effects at the base of the food web to higher trophic levels differed between stressors. Water diversion had little effect on the structure of food webs, but pollution increased food chain length and trophic diversity, and reduced trophic redundancy. The effects at higher trophic levels were exacerbated when combining both stressors, as the relative contribution of biofilm to the stock of basal resources increased even further. Overall, we conclude that moderate pollution increases food web complexity and that the interaction with water abstraction seems to amplify this effect. Our study shows the importance of assessing the interaction between stressors to create predictive tools for a proper management of ecosystems.

Interactive effects of discharge reduction and fine sediments on stream biofilm metabolism.

Discharge reduction, as caused by water diversion for hydropower, and fine sediments deposition, are prevalent stressors that may affect multiple ecosystem functions in streams. Periphytic biofilms play a key role in stream ecosystem functioning and are potentially affected by these stressors and their interaction. We experimentally assessed the interactive effects of discharge and fine sediments on biofilm metabolism in artificial indoor channels using a factorial split-plot design with two explanatory variables: water discharge (20, 39, 62, 141 and 174 cm3 s-1) and fine sediments (no sediment or 1100 mg L-1 of sediments). We incubated artificial tiles for 25 days in an unpolluted stream to allow biofilm colonization, and then placed them into the indoor channels for acclimation for 18 days. Subsequently, we manipulated water discharge and fine sediments and, after 17 days, we measured biofilm chlorophyll-a concentration and metabolism. Water velocity (range, 0.5 to 3.0 cm s-1) and sediment deposition (range, 6.1 to 16.6 mg cm-2) increased with discharge, the latter showing that the effect of increased inputs prevailed over sloughing. In the no-sediment treatments, discharge did not affect biofilm metabolism, but reduced chlorophyll-a. Sediments, probably as a consequence of nutrients released, promoted metabolism of biofilm and chlorophyll-a, which became independent of water discharge. Our results indicate that pulses of fine sediments can promote biofilm algal biomass and metabolism, but show interactive effects with discharge. Although discharge reduction can affect the abundance of basal resources for food webs, its complex interactions with fine sediments make it difficult to forecast the extent and direction of the changes.

Structural and functional recovery of macroinvertebrate communities and leaf litter decomposition after a marked drought: Does vegetation type matter?

Climate change and anthropogenic disturbances are expected to lead to more intense and frequent droughts, with potentially severe effects on structure and function of perennial temperate streams. However, more information is required on whether streams flowing through basins already affected by exotic plantations will respond to droughts in the same way as streams under native forests. The recolonisation dynamics of benthic macroinvertebrate communities and leaf litter decomposition rates were examined in nine streams of oceanic-temperate climate that differed in catchment vegetation (three streams draining native deciduous forest, three in pine plantations and three in eucalypt plantations) after a marked drought. In each stream, five benthic samples were collected three times (ca. 1.5months between sampling dates) after flow recovery, and the taxonomic and functional trait compositions of the macroinvertebrate communities were analysed. The decomposition rate of Alnus glutinosa was measured in fine- and coarse-mesh litter bags. Benthic macroinvertebrate density, richness and diversity increased with time after flow recovery but only richness and diversity differed among stream types, with eucalypt streams showing the lowest values. Both the taxonomic and functional compositions of the macroinvertebrate community were dependent on vegetation type and time, with the differences among stream types diminishing over time. While leaf-litter decomposition rate did not depend on catchment vegetation after drought, detritivore activity was the lowest under eucalypt streams and it was positively correlated to benthic shredder density. Our results indicated that in these perennial temperate streams the catchment vegetation influenced the recovery of benthic macroinvertebrate communities after a period of drought, although the decomposition rate of leaf litter was not strongly affected. Greater understanding of the structural and functional responses of stream ecosystems to different stressors is required before the effects of expected more intense and frequent hydrological changes caused by climate change can be adequately forecast.