Exploring longitudinal trends and recovery gradients in macroinvertebrate communities and biomonitoring tools along regulated rivers.

Flow regime alteration by dams has been recognized as a major impact factor for aquatic communities. Spain is currently the member state of the EU with the largest number of large reservoirs. With the broad objective of diminishing the ongoing river degradation trend through the management of environmental flows and the use of biomonitoring tools, we investigated the effects of dams on stream macroinvertebrates in several regulated rivers in Spain with contrasting environmental settings. Specifically, we studied longitudinal trends in macroinvertebrate communities to test: i) if currently used biomonitoring tools and multivariate community analyses can detect hydrological impact responses and biological recovery; ii) if an applicable quantification of the recovery gradient, in terms of distance downstream from dams, can be obtained for Iberian fluvial systems; iii) if macroinvertebrate community structure respond different to flow regulation, depending on the contrasting environmental river typologies; and iv) if the type and intensity of hydrological alteration modulates the observed community responses/recovery. Biotic indices and metrics displayed a decrease in 5 out of 6 systems immediately downstream of infrastructure. Complete recovery could not be clearly detected, but some recovery patterns started at a distance >11km. Multivariate community patterns and biomonitoring metrics showed the most pronounced hydrological alteration impacts and weaker recovery of the downstream macroinvertebrate communities within dammed Mediterranean streams (comparing to other rivers with continental or oceanic climate influence). Finally, both the intensity and type of hydrological alteration (highlighting the alteration of the floods and droughts components) were related to changes in common biomonitoring metrics. Our results could help in recognizing heavily modified water bodies (sensu European Water Framework Directive) downstream of dams or the delineation of fluvial zones or reserves. Furthermore, applied research areas dealing with environmental flows or the bioassessment of hydrological impacts could benefit from our main findings.