Reefs of the future: Water quality constrains coral biodiversity in an urbanized seascape

Abstract

Background/Question/Methods As foundational species, corals provide a complex habitat that hosts an immense diversity of marine life. Yet, globally corals are in a perilous decline leading to loss of biodiversity and ecosystem services. To date, the relative importance of global (e.g. climate change, acidification) versus local (e.g. overfishing, pollution) are still hotly debated. This debate is rife with shifting baselines, because reef degradation often preceded science and management efforts. Increasingly, studies are focusing on natural (e.g. CO2 seeps) or anthropogenic (e.g. power plant effluent) gradients to test the hypothesis that global change processes are decreasing coral diversity. Yet, few studies have examined how water quality affects coral species richness. Hong Kong is host to more than 80 hard coral species, which cling to the edges of strong water quality gradients across a limited spatial scale. Owing to more than 20 years of water quality and coral diversity monitoring, Hong Kong provides an interesting scenario to test the hypothesis that poor water quality reduces coral species richness. This GIS and field-based study investigates the effect of five water quality parameters (total inorganic nitrogen & phosphate, particulate suspended matter, salinity and dissolved oxygen) on hard coral species richness. Results/Conclusions Coral species richness was negatively correlated with surface productivity (chla). Both nitrogen and phosphorus were negatively correlated with coral diversity and had twice the relative importance as particulate suspended matter. These negative effects were ground-truthed in an explantation experiment along a water quality gradient emanating from a heavily urbanized watershed. Corals growing near sewage effluents suffered high mortality, whereas corals further away showed increasing survivorship and growth. Taken together, we can use these data to identify biodiversity hot-spots and water quality thresholds for hard coral diversity to better inform future MPA and restoration site selection.