RESUMEN
Freshwater ecosystems are severely threatened by urban development and agricultural intensification. Increased occurrence of algal blooms is a main issue, and the identification of local dynamics and drivers is hampered by a lack of field data. In this study, data from 13 cities (250 water bodies) were used to examine the capacity of trained community members to assess elevated phytoplankton densities in urban and peri-urban freshwater ecosystems. Coincident nutrient concentrations and land use observations were used to examine possible drivers of algal blooms. Measurements made by participants showed a good relationship to standard laboratory measurements of phytoplankton density, in particular in pond and lake ecosystems. Links between high phytoplankton density and nutrients (mainly phosphate) were observed. Microscale observations of pollution sources and catchment scale estimates of land cover both influenced the occurrence of algal blooms. The acquisition of environmental data by committed and trained community members represents a major opportunity to support agency monitoring programmes and to complement field campaigns in the study of catchment dynamics.