Abstract:

Dongsha Atoll is one of the largest atolls in the northern South China Sea, featuring high coral cover and diversity, especially on its outer reef slopes. It lies in the path of some of the world’s largest internal solitary waves that propagate westward from the Luzon Strait. As these waves interact with underlying topography and break, they drive deeper, cooler, and more nutrient-rich waters onto the shallow parts of the reef. Such upwelling has been linked to reduced coral bleaching during thermal anomalies. However, the extent to which these oceanic inputs may nutritionally support Dongsha’s outer reef communities, and how spatial variability in internal-wave forcing shapes reef community structure, remain open questions. To investigate, we combined high-resolution time-series of temperature records across depths (between 10–25 m) at three outer-reef sites (north, east, south), together with tissue stable carbon (δ13C) and nitrogen (δ15N) isotopes measured in a range of reef organisms (i.e., fish, seagrass, crinoids, and corals) from the same sites. Our results demonstrate strong temperature variability that increases with increasing depth at all outer sites, consistent with internal-wave forcing across depth previously identified. Time-series of temperature-derived nitrate estimates indicate episodic delivery of oceanic nutrients onto the reef, corroborating the observed patterns of cooling of shallow reef slopes. Preliminary tissue isotope results suggest two potential nutrient pathways: (1) an oceanic pathway, with δ13C of small planktivorous damselfish, crinoids and some corals aligning with pelagic particulate organic matter (~ −20 to −18 ‰); and (2) a benthic or seagrass-influenced pathway, where some larger predators and corals presented more enriched δ13C (~−16 to −11 ‰) suggesting utilisation of seagrass or benthic sources. Further source elucidation using more source-specific carbon and nitrogen compound-specific isotope analysis of amino acid (CSIA-AA) is warranted to shed light on the influences of internal waves on the nutrient dynamics of communities around the outer reef slopes of remote Dongsha Atoll.