Summary Understanding mechanisms by which agricultural practices affect freshwater ecosystems helps to inform land‐use policies and management strategies aimed at mitigating effects of agriculture on biodiversity. Land‐use activities in the catchment, riparian and local scales likely influence stream fish communities via multiple pathways, for instance, by modifying the instream habitat. We investigated the mechanisms driving local stream fish taxonomic richness and functional diversity in South Brazilian grasslands by testing a theoretical path model in which we specified a priori relationships predicting how land use at multiple scales affects instream habitat and fish communities. Agricultural activities adjacent to streams (i.e. local impact on the streambank) and catchment‐scale cropland area were positively related to macrophyte cover and negatively associated with coarse particulate organic matter (CPOM, i.e. woody debris and leaf litter). Local impact also increased substrate siltation and homogenisation. Riparian vegetation in the upstream buffer ameliorated instream habitat condition by dampening macrophyte proliferation and providing CPOM. Fish species richness increased with both macrophyte cover and CPOM, revealing multiple pathways of agricultural influence. However, functional diversity decreased only with substrate siltation, revealing a response to agricultural activities adjacent to sample sites. Agricultural streams showed a replacement of benthic and lithophilic species by a larger number of morphologically similar and macrophyte‐associated nektonic fish. Our study indicates that land‐use adjacent to streams and upstream riparian zones is critical for maintaining taxonomically and functionally diverse fish communities due to their strong effects on instream habitat. Protection and recovery of riparian zones from land‐use change can mitigate the effects of agriculture on fish communities in South Brazilian grasslands.

Protecting riparian vegetation around streams is vital in reducing the detrimental effects of environmental change on freshwater ecosystems and in maintaining aquatic biodiversity. Thus, identifying ecological thresholds is useful for defining regulatory limits and for guiding the management of riparian zones towards the conservation of freshwater biota. Using nationwide data on fish and invertebrates occurring in small Brazilian streams, we estimated thresholds of native vegetation loss in which there are abrupt changes in the occurrence and abundance of freshwater bioindicators and tested whether there are congruent responses among different biomes, biological groups and riparian buffer sizes. Mean thresholds of native vegetation cover loss varied widely among biomes, buffer sizes and biological groups: ranging from 0.5% to 77.4% for fish, from 2.9% to 37.0% for aquatic invertebrates and from 3.8% to 43.2% for a subset of aquatic invertebrates. Confidence intervals for thresholds were wide, but the minimum values of these intervals were lower for the smaller riparian buffers (50 and 100 m) than larger ones (200 and 500 m), indicating that land use should be kept away from the streams. Also, thresholds occurred at a lower percentage of riparian vegetation loss in the smaller buffers, and were critically lower for invertebrates: reducing only 6.5% of native vegetation cover within a 50‐m riparian buffer is enough to cross thresholds for invertebrates. Synthesis and applications. The high variability in biodiversity responses to loss of native riparian vegetation suggests caution in the use of a single riparian width for conservation actions or policy definitions nationwide. The most sensitive bioindicators can be used as early warning signals of abrupt changes in freshwater biodiversity. In practice, maintaining at least 50‐m wide riparian reserves on each side of streams would be more effective to protect freshwater biodiversity in Brazil. However, incentives and conservation strategies to protect even wider riparian reserves (~100 m) and also taking into consideration the regional context will promote a greater benefit. This information should be used to set conservation goals and to create complementary mechanisms and policies to protect wider riparian reserves than those currently required by the federal law.

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Motivation: We compiled a global database of long-term riverine fish surveys from 46 regional and national monitoring programmes and from individual academic research efforts, with which numerous basic and applied questions in ecology and global change research can be explored. Such spatially and temporally extensive datasets have been lacking for freshwater systems in comparison to terrestrial ones. Main types of variables contained: The database includes 11,386 time-series of riverine fish community catch data, including 646,270 species-specific abundance records, together with metadata related to the geographical location and sampling methodology of each time-series. Spatial location and grain: The database contains 11,072 unique sampling locations (stream reach), spanning 19 countries, five biogeographical realms and 402 hydrographical basins worldwide. Time period and grain: The database encompasses the period 1951-2019. Each timeseries is composed of a minimum of two yearly surveys (mean = 8 years) and represents a minimum time span of 10 years (mean = 19 years). Major taxa and level of measurement: The database includes 944 species of rayfinned fishes (Class Actinopterygii). Software format: csv. Main conclusion: Our collective effort provides the most comprehensive long-term community database of riverine fishes to date. This unique database should interest ecologists who seek to understand the impacts of human activities on riverine fish biodiversity and to model and predict how fish communities will respond to future environmental change. Together, we hope it will promote advances in macroecological research in the freshwater realm.

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