Community structure and functional diversity of seagrass fish assemblages in an oceanic atoll system

Seagrass habitats in tropical mid-oceanic atolls are highly vulnerable to climate change and anthropogenic stressors. Yet the ecological function and structure of seagrass-associated fish assemblages remain poorly resolved. Here, we undertake the first published seagrass-associated fish surveys (n = 35 sample points) in the Maldives to determine the structure and functional diversity of fish assemblages. Specifically, we (i) assess the abundance and diversity of fish assemblages; (ii) detail the size distribution and maturity of individuals in seagrass habitat; and (iii) use a trait-based approach to understand the trait frequency and functional diversity of fish assemblages. By measuring 9,192 fish from 50 species, the results show seagrass meadows in this atoll setting can support a high abundance of juvenile fishes (up to 4,552 individuals per 1000 m 2 ) but demonstrate high variability between samples (mean 982 individuals per 1000 m 2 ±SD 955). Most notably, trait-based analysis revealed a high proportion of mobile invertivores and herbivores that will mature as habitat generalists as well as rocky rubble and coral reef inhabitants. We hypothesise that the seagrass nursery function may be particularly important in this atoll setting given the declines of other nursery habitats across the country (particularly coral). Collectively, the data demonstrate that seagrass meadows are a nursery habitat for fishes that perform influential functions across the wider oceanic atoll seascape as adults (e.g. invertivore predation and grazing on reef algae). As such, maintaining healthy seagrass habitats in atoll settings could buffer the effects of ecological decline on the wider seascape.