Using benthic species abundance records (2003–2009) collected by volunteer divers through ‘Seasearch’, coastal biodiversity patterns around the United Kingdom and Ireland were investigated. The first aim was to assess the utility of volunteer data as a baseline for monitoring environmental change. Separation of the influences of within- and between-surveyor variation from broadscale (year, latitude, longitude) and finer scale (month, depth, local habitat) biogoegraphical factors was achieved using a multilevel mixed effects framework. A high degree of consistency within surveyors was evident and, by modelling between-surveyor variations efficiently, expected trends in species richness and taxonomic distinctness were recovered. Moving from patterns to processes, the second aim was to test the effects of sea surface temperature on prevalence, on a species by species basis. This approach allowed identification of species likely to display range shifts in direct response to future warming trends. Within limitations, volunteer data can provide a valuable contribution to understanding of biodiversity, climate change and aquatic ecosystem health.

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