Stable N isotopes reveal the sources of nitrogen affecting Hong Kong’s coral communities

Abstract

Marine ecosystems have experienced dramatic changes since the 1850s in response to human activities. This epoch, often referred to as the Anthropocene, has witnessed the punctuated loss of reef-building corals worldwide. Coral reef degradation may induce subtle changes which remain un-witnessed; this is particularly true when the water quality is altered. Deteriorated water quality, in particular due to nitrogen enrichment, hampers coral larvae recruitment, amplifies bio-erosion, favors coral diseases and reduces the threshold of thermal bleaching, leading to local or regional coral species extinctions. Hong Kong SAR waters host more than 80 hard coral species which occur under N concentrations above the eutrophication threshold of ~1 µmol.l-1. A GIS-based study revealed that the hard coral species richness distribution followed the nutrient concentrations (nitrogen and phosphorus). N & P concentration appeared to be the main drivers of coral species richness. Identifying the nutrients sources affecting HK’s communities is thus crucial to implement N-mitigation policies in order to preserve hard coral biodiversity. The δ15N signature of hard coral tissues collected over 30 sites identified two main N-sources. Some 15N-depleted N is coming from the Pearl River delta area, possibly reflecting fertilizers-derived-N, whereas 15N-enriched N is found on eastern coral communities, suggesting a more local N source: most likely sewage. The N-sources identified in the present study provide a useful baseline for the conservation and the management of local coral communities but also of eutrophied coral ecosystems worldwide.