Rapid decline of a relatively high latitude coral assemblage at Weizhou Island, northern South China Sea

“Refuge theory” suggests that global warming would be beneficial to corals in high latitude waters. In theory, then, the Weizhou Island reef (21°00′–21°10′N, 109°00′–109°15′E), which is located in a relatively high latitude area in the northern South China Sea, is a refuge for corals under global warming. Yet, the corals here have degenerated significantly. We investigated the ecological status of the Weizhou Island reef in 2015 and recorded 11 families, 22 genera, and 41 coral species. The mean living coral cover has decreased from ~ 42% in 1984 to ~ 10% in 2015 and there are many dead Acropora in the study area, especially at the reef flat. Coral assemblage structure has undergone degradation with the dominant group shifting from high complexity branching, foliaceous and massive colonies to a simpler group of massive morphologies. The only sign indicating the corals here benefiting from global warming is the occurrence of a large amount of juvenile Porites lutea (31.91% of the total population), which represents the recovery potential of the Weizhou Island reefs. Further analysis concludes that the main reason for the rapid degeneration is escalating anthropogenic impact, such as seawater pollution, unsustainable tourism activities, ongoing overfishing, all of which degrade the local ecological environment. It seems that intensive anthropogenic activities have weakened the “refuge” function significantly. Decreasing living coral cover as well as degraded assemblage structure all suggest that Weizhou Island offers limited potential as refuge habitat for corals in the context of global warming and intensive human activities.