Positive responses of benthic macroinvertebrates to spatial and temporal reductions in water pollution downstream from a trout farm outlet

Abstract

Inland trout farms can cause important adverse effects on freshwater communities due to the discharge of wastewater effluents into recipient rivers. In this research, responses of benthic macroinvertebrates to diminished water pollution downstream from a trout farm outlet were examined at spatial and temporal scales. Field studies were carried out in the Upper Tajuña River (Central Spain) during the springs of 2007 and 2015. Water pollution decreased, with increasing the downstream distance from the trout farm effluent, and also from 2007 to 2015 as a likely consequence of the lowered annual production of farmed rainbow trout following the 2008 economic recession. Reductions in water pollution resulted in increased concentrations of dissolved oxygen and decreased levels of turbidity and inorganic nutrients (ammonia and phosphate). Benthic macroinvertebrates responded positively to diminished water pollution by decreasing the relative abundance of collector-gatherers (oligochaetes and chironomids) and increasing the relative abundance of shredders and scrapers (ephemeropterans, plecopterans and trichopterans). In addition, values of taxonomic diversity and Biological Monitoring Water Quality biotic indices tended to increase. It is concluded that the wastewater treatment system of the trout farm should be improved to minimize the environmental impact on the recipient river.