Combined effects of hydrologic alteration and cyprinid fish in mediating biogeochemical processes in a Mediterranean stream

Rubio-Gracia, FrancescAlmeida, DavidBonet, BertaCasals, FredericEspinosa, CarmenFlecker, A. S.García-Berthou, EmiliMartí, EugèniaTuulaikhuu, Baigal-AmarVila-Gispert, AnnaZamora, LuisGuasch, Helena. Science of the Total Environment 601-602 : 1217-1225 (2017) DIGITAL CSIC

Flow regimes are important drivers of both stream community and biogeochemical processes. However, the interplay between community and biogeochemical responses under different flow regimes in streams is less understood. In this study, we investigated the structural and functional responses of periphyton and macroinvertebrates to different densities of the Mediterranean barbel (Barbus meridionalis, Cyprinidae) in two stream reaches differing in flow regime. The study was conducted in Llémena Stream, a small calcareous Mediterranean streamwith high nutrient levels.Weselected a reachwith permanent flow(permanent reach) and another subjected to flowregulation (regulated reach)with periods of flowintermittency. At each reach,we used in situ cages to generate 3 levels of fish density. Cages with 10 barbels were used to simulate high fish density (N7 indm−2); cages with open sides were used as controls (i.e. exposed to actual fish densities of each stream reach) thus having low fish density; and those with no fish were used to simulate the disappearance of fish that occurs with stream drying. Differences in fish density did not cause significant changes in periphyton biomass and macroinvertebrate density. However, phosphate uptake by periphyton was enhanced in treatments lacking fish in the regulated reachwith intermittent flow but not in the permanent reach, suggesting that hydrologic alteration hampers the ability of biotic communities to compensate for the absence of fish. This study indicates that fish density canmediate the effects of anthropogenic alterations such as flowintermittence derived from hydrologic regulation on stream benthic communities and associated biogeochemical processes, at least in eutrophic streams.