In many countries of the Global South, aquatic ecosystems such as streams, rivers, lakes, and wetlands are severely impacted by several simultaneous environmental stressors, associated with accelerated urban development, and extreme climate. However, this problem receives little attention. Applying a DPSIR approach (Drivers, Pressures, State, Impacts, Responses), we analyzed the environmental impacts and their effects on urban hydrosystems (including stagnant waters), and suggest possible solutions from a series of case studies worldwide. We find that rivers in the Global South, with their distinctive geographical and socio-political setting, display significant differences from the Urban Stream Syndrome described so far in temperate zones. We introduce the term of ‘Southern Urban Hydrosystem Syndrome’ for the biophysical problems as well as the social interactions, including the perception of water bodies by the urbanites, the interactions of actors (e.g., top-down, bottom-up), and the motivations that drive urban hydrosystem restoration projects of the Global South. Supported by a synthesis of case studies (with a focus on Brazilian restoration projects), this paper summarizes the state of the art, highlights the currently existing lacunae for research, and delivers examples of practical solutions that may inform UNESCO’s North–South–South dialogue to solve these urgent problems. Two elements appear to be specifically important for the success of restoration projects in the Global South, namely the broad acceptance and commitment of local populations beyond merely ‘ecological’ justifications, e.g., healthy living environments and ecosystems with cultural linkages (‘River Culture’). To make it possible implementable/practical solutions must be extended to (often poor) people having settled along river banks and wetlands.

Identifying nonpoint phosphorus (P) sources in a watershed is essential for addressing cultural eutrophication and for proposing best-management solutions. The oxygen isotope ratio of phosphate (δ18OPO4 ) can shed light on P sources and P cycling in ecosystems. This is the first assessment of the δ18OPO4 distribution in a whole catchment, namely, the Yasu River Watershed in Japan. The observed δ18OPO4 values in the river water varied spatially from 10.3‰ to 17.6‰. To identify P sources in the watershed, we used an isoscape approach involving a multiple-linear-regression model based on land use and lithological types. We constructed two isoscape models, one using data only from the whole watershed and the other using data from the small tributaries. The model results explain 69% and 96% of the spatial variation in the river water δ18OPO4 . The lower R 2 value for the whole watershed model is attributed to the relatively large travel time for P in the main stream of the lower catchment that can result in cumulative biological P recycling. Isoscape maps and a correlation analysis reveal the relative importance of P loading from paddy fields and bedrock. This work demonstrates the utility of δ18OPO4 isoscape models for assessing nonpoint P sources in watershed ecosystems.

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Agriculture is one of the most critical anthropogenic disturbances to freshwater ecosystems globally.In monsoon Asia, rice paddies provide habitats for aquatic species, but how their associated irrigation systems can affect downstream ecosystems is poorly understood. Here, we used structural equation modeling (SEM) to estimate seasonal variations in benthic macroinvertebrate diversity and environments and quantified indirect effects of land use, especially of rice paddy areas, on benthic macroinvertebrate diversity through local environmental alteration. Our study site was the Yasu River tributary of Lake Biwa, Japan, and we performed our investigation during both irrigation and nonirrigation periods, representing different seasons. Seasonal variations in physical and chemical variables and diversity were observed. Seasonal particulate phosphorus concentrations to a large extent determined the abundance and community composition of benthic macroinvertebrates. SEM revealed that the proportions of forest, urban, and rice paddy areas in the catchment had significant indirect effects on biodiversity indices of local benthic macroinvertebrate communities. The proportion of forest area had robust negative effects on water temperature, but the overall indirect effects on the macroinvertebrate diversity contrasted between the two seasonal periods. The proportion of rice paddy area had a strongly negative indirect effect on the diversity through increased particulate phosphorous loading during the irrigation period, while the proportion of urban area had a significantly negative indirect effect during the nonirrigation period only. The seasonal negative impacts of rice paddy irrigation on benthic macroinvertebrate communities were possibly due to siltation arising from rice paddy soils. Our results have implications for environmental restoration and biodiversity conservation in catchment management.

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