Macroinvertebrate responses along the recovery gradient of a regulated river (Spain) receiving an industrial effluent

Abstract

Macroinvertebrate responses were examined along the recovery gradient of the regulated Río Duratón (northern Spain) receiving an industrial effluent. Hypolimnial releases from Burgomillodo Dam caused short-term flow fluctuations and low concentrations of dissolved oxygen at S-2, S-3, S-4, and S-5 sampling sites (0.2, 0.4, 2.5, and 7.6 km below the dam, respectively) compared with the upstream reference station (S-1). The industrial effluent caused a significant (P<0.05) increase in the fluoride concentration at S-3, S-4, and S-5 stations. Moreover, because of the differential discharge of hypolimnial waters from the dam, fluoride concentrations experienced a temporal variation over a one-day period at S-3, S-4, and S-5. The suspended inorganic matter (SIM) only increased significantly (P<0.05) just below the effluent (S-3), settling on the stream bottom of this sampling site. Tricladida, Amphipoda, Plecoptera and Coleoptera were the major macroinvertebrate groups most adversely affected. Total density (N), total biomass (B) and family diversity (D′) were higher at S-1 than at downstream sampling sites, their values being lowest at S-3. The family uniformity (U′) had its highest value at S-3. The family dominance (d′) exhibited lower values at S-2, S-3, and S-4 than at S-1 and S-5. Shredders and collector-gatherers were the functional feeding groups most adversely affected, predators and filter-feeders increasing their abundances downstream from the dam. According to the River Continuum Concept, these changes in functional feeding groups reflect a potamonization (enhancement of potamic conditions) in the trophic structure of the macrobenthic community. The highest values of environmental impact (EI) were estimated at S-3 and the lowest values at S-5. The lowest value of percentage similarity (PS) was estimated between S-1 and S-3 and the highest value between S-4 and S-5. It is concluded that the major abiotic factors responsible for macroinvertebrate responses were short-term flow fluctuations and dissolved O₂ deficit at S-2, S-4, and S-5 sampling sites, and the siltation of SIM just below the industrial effluent (S-3). The fluoride pollution was a minor factor. In this sense, reductions in hypolimnial release and improvements in waste treatment are recommended in order to mitigate the adverse effects of the industrial effluent and Burgomillodo Dam on the structure of the macrobenthic community.