Riparian land use and stream habitat regulate water quality

Riparian land use is a key driver of stream ecosystem processes but its effects on water quality are still a matter of debate when proposing measures to improve freshwater quality. The aim of this study was to examine the influence of riparian land use on stream habitat and water chemistry, and to assess in what extent stream habitat also affects water quality. To that end, we selected eight reaches in the Ave River basin (northwestern Portugal) and compared longitudinal variations in water chemistry and stream habitat between reaches with different land use (urban, agricultural and natural), and between reaches with natural riparian areas and different habitats. Stream habitat was assessed using the Fluvial Functional Index, the HABSCORE, and the Riparian Forest Quality Index. Longitudinal variations in water chemistry were determined measuring differences in concentrations of ammonium, nitrate, phosphate and oxygen, and conductivity, pH and temperature between the downstream and the upstream ends of each reach. Nitrate concentration tended to decrease along reaches with more natural riparian areas and to increase along reaches with more urban and agricultural land uses. Longitudinal variations in water chemistry also differed between reaches with natural riparian areas, suggesting that water quality also depends on stream habitat. Moreover, longitudinal variation in water chemistry was proven a simple, useful and low-cost approach to assess the influence of land cover and stream habitat on water quality. Overall results demonstrated that both riparian land use and stream habitat influence water quality and that riparian forests are essential to reduce nutrient export to downstream ecosystems.This study was supported by the CLIMALERT project (ERA4CS) funded by JPI-CLIMATE, the EU Commission and the Portuguese Foundation for Science and Technology (FCT), the EcoAgriFood project (NORTE-01-0145-FEDER-000009) supported by Norte Portugal Regional Operational Programme (NORTE 2020), under the PORTUGAL 2020 Partnership Agreement, through the European Regional Development Fund (ERDF), and the strategic programme UID/BIA/04050/2019 (POCI-01-0145-FEDER-007569), funded by national funds through FCT, and by the ERDF through the COMPETE2020 -Programa Operacional Competitividade e Internacionalizacao (POCI). The authors thank to Francisco Carvalho, Eva Lima-Fernandes, Rocco Scolozzi, Paulo Geraldes, Isabel Fernandes, Jose Trabulo, Bruno Antunes, Ana Pereira, and Sofia Duarte for field and laboratory assistance, and to the Portuguese Environment Agency (APA) for providing GIS data for the Ave River Basin