The past, present, and future of Hong Kong corals: hope for marine ecosystems found in an unlikely place

Abstract

Coral ecosystems are degrading worldwide. Regional anthropogenic and global climate stressors are decreasing both live coral cover and coral diversity at an alarming rate. Without assisted migration and/ or restoration, certain coral ecosystems may be lost forever. But what goals do we set for ecosystem restoration? Can natural ecosystem functioning be restored or will we simply return it to a previously degraded state? Kidwell et al. 2015 has shown that restoration efforts benefit from historical baselines that illustrate the diversity and persistence of an ecosystem through time. In light of this recommendation, we present the first paleoecological study to investigate coral assemblages in southeast China, focused on Hong Kong during the Holocene. Results show that coral composition has shifted from an Acroporiid-dominated community in the past to a modern dominance of massive morphology with significant species diversity decline in southern Hong Kong. Water parameters in this southern region are dominated by the outflow of the Pearl River, the most rapidly developing river in the world. Conversely, eastern Hong Kong communities constitute an oceanic- rather than river-driven water system and exhibit greater resilience to these diversity shifts through time. We discovered that in the modern era, coral community composition is driven by local anthropogenic stressors derived from highly degraded water quality. Furthermore, our study highlights potential historical impacts dating back hundreds to thousands of years, uncovered by various archaeological efforts. For centuries, local Hong Kong miners dredged surrounding waterways for both live and dead coral to produce exportable slaked lime. We discuss the effects that this ancient lime industry had on local assemblages and together, our findings constitute the first known quantitative comparison of modern and historical baselines of a coral ecosystem in this region. This story illustrates that the marginal coral communities of Hong Kong, impacted by the physical extraction of mining, eutrophication, high sediment loads and turbidity for hundreds if not thousands of years, constitute useful analogs for the futures of other similarly impacted reef ecosystems. Our story concludes with our present efforts to restore these communities, utilizing our “inferred restoration” methods in conjunction with local government. Through our historical-modern community comparisons, we identify what species have been lost and with population genetics coupled with coral fragmentation, have implanted new coral communities. With local government efforts to improve water quality, we show improved success for coral restoration. This model of inferred restoration gives hope in a frequently depressing outlook for the future of our coral reefs.