New Insights On The Nitrogen Footprint Of A Coastal Megalopolis From Coral-Hosted Symbiodinium Delta N-15

The development of megalopolises in coastal areas is often linked with severe eutrophication, requiring mitigation of anthropogenic dissolved inorganic nitrogen (DIN) pollution. Yet, identifying the DIN-sources responsible for eutrophication is challenging, hampering mitigation efforts. Here, we utilize the stable nitrogen isotope ratio of endosymbiotic dinoflagellate Symbiodinium spp. (delta N-15(sym)) associated with the hard coral Porites to trace DIN sources in one of the most urbanized areas of the planet: the Pearl River Delta (PRD). The mean delta N-15(sym) value found in the coastal waters of Hong Kong (HK), located on the eastern edge of the PRD, (7.4 parts per thousand +/- 1.2 parts per thousand) was +2.7 parts per thousand higher than at Dongsha Atoll, a reference site unaffected by anthropogenic-DIN (4.7 parts per thousand +/- 0.4 parts per thousand). The isotopic enrichment suggested a consistent dominance of DIN deriving from local and regional sewage discharges on the eastern edge of HK. Furthermore, the strong depletion of the summer delta N-15(sym) value (-1.6 parts per thousand) observed in southern HK revealed that the Pearl River plume strongly modulates the coastal DIN pool. Our results revealed the value of benthic marine organisms' delta N-15 for deciphering the complex dynamics of coastal eutrophication and highlighted the pivotal role of transboundary coordination in DIN-pollution mitigation.