Temporal functional changes in coral and fish communities on subtropical coastal coral reefs

Context. Global and local stressors can drive phase shifts from zooxanthellate scleractinian coral communities to macroalgae-dominated ecosystems. However, our understanding of altered ecosystem functioning, productivity and stability remains limited as pre-shift data are typically lacking for degraded coral-reef sites.Aims. Here, we assessed functional changes in fish communities in Nakagusuku Bay, Okinawa, Japan, over 45 years, by comparing pre-disturbance (1975) to post-disturbance (2018-2020) datasets, and identified possible drivers of changes.Methods. We analysed data for 393 fish species and 26 coral genera at 13 sites, measured at four-time points (1975, 2018, 2019 and 2020). Analyses were performed using a range of ordination techniques.Key results. We found reductions in functional richness and trait space contraction over time for fishes. Changes in coral functional groups over time correlated with changes in the functional diversity of reef-fish communities; a reduction in branching corals reduced habitat availability for coral-reliant fishes.Conclusions. Increasing sedimentation and eutrophication as a result of construction along the Nakagusuku Bay coast likely reduced living coral cover and fish diversity, and thermal stress likely facilitated the simplification and shifts of both coral communities and coral specialist fishes away from shore. Implications. Both global and local threats need to be considered when assessing functioning of coral-reef ecosystems for coral-reef conservation efforts.