Small-scale experiments and theory suggest that ecological functions provided by communities become more stable with increased species richness. Whether these patterns manifest at regional spatial scales and within species-rich communities (e.g., coral reefs) is largely unknown. We quantified five biogeochemical processes, and an aggregate measure of multifunctionality, in species-rich coastal fish communities to test three questions: (1) Do previously predicted biodiversity-ecosystem-function relationships hold across large spatial scales and in highly diverse communities? (2) Can additional covariates of community structure improve these relationships? (3) What is the role of community biomass and functional group diversity in maintaining biogeochemical processes under various scenarios of species loss across ecosystem types? These questions were tested across a large regional gradient of coral reef, mangrove and seagrass ecosystems. Statistical models demonstrated that species richness and the mean maximum body size per species strongly predicted biogeochemical processes in all ecosystem types, but functional group diversity was only a weak predictor. Simulating three scenarios of species loss demonstrated that conserving community biomass alone increased the ability for communities to maintain ecosystem processes. Multifunctionality of biogeochemical processes was maintained least in simulations that conserved biomass and community structure, underscoring the relative lack of importance of community structure in maintaining multiple simultaneous ecosystem functions in this system. Findings suggest that conserving community biomass alone may be sufficient to sustain certain biogeochemical processes, but when considering conservation of multiple simultaneous biogeochemical processes, management efforts should focus first on species richness
