The effects of land use on downstream water quality and biodiversity in a changing tropical mosaic landscape

Abstract Context Land use and other human activities generally reduce downstream water quality, with consequences for ecosystem services and freshwater biodiversity. However, the relative effects of different types of land use and associated activities across large spatial scales can be difficult to understand, and thus to address, in heterogeneous landscapes. Objectives To assess how different types of landscapes affect the water quality and aquatic biodiversity downstream. To assess if coffee agroforestry landscapes are intermediate between forests and agriculture in terms of their effects on biodiversity in streams. Methods We measured water quality parameters and sampled stream macroinvertebrates in 46 catchments with perennial streams draining through mosaic landscapes managed by smallholder farmers in southwest Ethiopia. For each of the catchments, we estimated the proportion of each land use, including agriculture, coffee agroforestry and forests, estimated the settlement density and calculated an index representing the number of and distance to coffee washing stations. Results We found that water quality was better (e.g. high dissolved oxygen, low turbidity and low nutrient concentration) for streams draining through landscapes with high forest cover as compared to landscapes dominated by agroforestry or agriculture. Furthermore, we found elevated Escherichia coli ( E. coli ) concentrations in most streams, especially those draining agroforestry landscapes. The composition of macroinvertebrates varied much across streams with sensitive taxa dominating clean streams and vice versa. This pattern became even clearer when we analysed the direct effects of settlement density and coffee washing stations on the abundances of families with different sensitivity to poor water quality, but there were also inconsistent responses in some groups. Conclusion Our finding suggests that a single measurement of water chemistry alone inadequately captured land use effects on water quality, highlighting the importance of including biodiversity assessment. Since people frequently use these streams for various purposes and freshwater biodiversity is at stake, our findings highlight the urgent need of measures to reduce these negative effects. Such measures should focus on agriculture and agroforestry-dominated landscapes to primarily reduce the negative impacts of wastes from settlement areas and wet coffee washing stations.