Tropical and subtropical streams:A synthesis

Tropical and sub-tropical streams harbor great biodiversity and are responsible for crucial ecosystem services.Intending to contribute to the understanding of how anthropogenic stressors and pressures alter functional or taxonomic diversity in biotic assemblages or processes in these systems,Water Biology and Security published eight papers.The research originated from Cameroon(Central Africa),southeastern Brazil,eastern Amazonia,and eastern China,representing eight river basins:Huai,Sui and Yishusi from east Asia;São Francisco,Doce,Jequitinhonha,and Amazon from South America;and Congo from Africa.The last two are the most biodiverse on the planet.The studied biomes included tropical rainforest and savanna,neotropical savanna and rainforest,neotropical savanna mountains,and subtropical monsoon forest.The response variables included fish assemblages(3 papers),entire macroinvertebrate assemblages or sub-groups(4 papers),and stream physical habitat structure(1 paper).

Natural variation of physical-habitat conditions among least-disturbed streams of a neotropical river basin in Brazil

The determination of natural variation in physical habitat conditions and the establishment of least-disturbed reference standards is important for ecosystem conservation.Tropical regions hold most of the global freshwater diversity,nevertheless little is known about the natural conditions of their physical habitats.We calculated 255 physical habitat metrics for 31 stream sites in five protected areas of the Brazilian S~ao Francisco River basin to identify the most variable characteristics among tropical streams in least-disturbed areas.We performed principal components analyses(PCA)to find the most relevant metrics for ordination of streams in the following categories:water quality(2);substrate(7);fish shelter(6);riparian vegetation(9);and channel morphology(7).We used distance-based linear models(DISTLM)to test how much of the variation of these metrics could be explained by geographic position,average elevation,and average discharge of stream sites.The best statistically significant models explained 42%of substrate(33%geographic position;9%average elevation),26%of channel morphology(22%geographic position;4%average discharge),23%of water quality(14%geographic position;9%average discharge),17%of fish shelter(9%geographic position;8%average elevation),and 14%of riparian vegetation(7%geographic position;7%average elevation)variation.Thus,regional features(e.g.geology and climate)related to the geographic position are of greatest importance,followed by average elevation and discharge for determining physical habitat characteristics.Therefore,we suggest that reference sites should be set for tropical streams in the same regional landscape and with similar average elevation and discharge.