Impact of agricultural land use on aquatic insect assemblages in the Garonne river catchment (SW France)

The impact of agricultural land use on the composition and structure of aquatic insect assemblages (i.e., taxa of Ephemeroptera, Plecoptera, Trichoptera, and Coleoptera (EPTC)) was investigated in tributary streams of the Garonne river basin, southern France. The self-organizing map (SOM) method was applied to compare both instream environmental conditions and EPTC assemblages between forest and agricultural streams. According to the SOM model, the study sites were classified into three main clusters corresponding to distinct EPTC assemblages. The SOM cluster associated with most of the agricultural sites had lower EPTC species richness and diversity. This cluster was also characterized by high levels of total dissolved solids, nitrate (NO₃), and chemical oxygen demand. Overall, our study shows that agricultural streams when compared with forest streams had lower biological integrity. In accordance with the European Water Framework Directive, our results indicate that the sites most impacted by agricultural land use should be restored and that the least-impacted forest sites could serve as reference conditions.