Agricultural impacts on streams near Nitrate Vulnerable Zones: a case study in the Ebro basin, northern Spain

Abstract

Agricultural intensification during the last century has produced river degradation across Europe. From the wide range of pressures derived from agricultural activities that impact rivers, diffuse agricultural pollution has received most of the attention from managers and scientists. The aim of this study was to determine the main pressures exerted by intensive agriculture around Nitrate Vulnerable Zones (NVZs), which are areas of land that drain into waters polluted by nitrates according to the European Nitrate Directive (91/676/EEC). The study area was located in the NW of La Rioja (Northern Spain), which has the highest levels of nitrate concentrations within the Ebro basin. The relationships between forty environmental variables and the taxonomic and functional characteristics of macroinvertebrate assemblages (which are good indicators of water quality) were analyzed in 11 stream reaches differentially affected by upstream agricultural activity. The streams affected by a high percentage of agricultural area had significantly greater nitrate concentrations and distinct macroinvertebrate assemblages dominated by pollution tolerant taxa. Hydromorphological alteration (i.e. channel simplification, riparian forest degradation and sediment inputs), which is closely linked to agricultural practices, was the main factor affecting macroinvertebrate assemblages. Good agricultural practices should be implemented in streams affected by NVZs to reverse stream degradation, in consonance with the European Water Framework Directive (WFD). Management actions in these areas should not focus exclusively on nitrate reduction, but also on restoring riparian and aquatic habitats.