Nitrogen isotopes as tracers of anthropogenic nitrogen input in coastal ecosystems : two case studies from Asia

Abstract

Marine ecosystem health is threatened by excessive inputs of nitrogen (N) from various anthropogenic sources and effective mitigation can result from source identification. Stable isotope analysis is a useful tool as a tracer for sources and sinks of N sources in the marine environment. This research consisted of two case studies; I conducted stable isotope analysis on various benthic organisms that served as bioindicators of anthropogenic N sources in two different coastal marine ecosystems, i.e., a megalopolis within the Pearl River Delta, China and a small village called Tan-awan in the Philippines. Additionally, I revealed the spatial and seasonal variation of N sources within the Pearl River Delta and conducted ecological assessment of the benthic communities in Tan-awan. In the first case study, I utilized the stable nitrogen isotope ratio of endosymbiotic dinoflagellate Symbiodinium spp. (δ15Nsym) associated with hard coral Porites to trace DIN sources in the Pearl River Delta. The mean δ15Nsym value found in the coastal waters of Hong Kong (7.4 ± 1.2‰), located on the eastern edge of the PRD, was +2.7‰ higher than at Dongsha Atoll (4.7 ± 0.4‰), a reference site unaffected by anthropogenic DIN. The isotope enrichment suggested a consistent dominance of DIN deriving from local and regional sewage discharges on the eastern edge of Hong Kong. Furthermore, the strong depletion of the summer δ15Nsym values (-1.6‰) observed in southern Hong Kong revealed that the Pearl River plume strongly modulates the coastal DIN pool. In the second case study, I conducted stable isotope analysis on three benthic organisms in the small village of Tan-awan, the Philippines where ecotourism centered around resident whale sharks has rapidly developed. I revealed an enriched mean δ15N value (~+1‰) in macroalga Padina sp. and gorgonian Rumphella sp. within near shore areas of the impacted site (where tourist activities and whale shark provisioning occur) relative to a reference site (without the aforementioned activities). This supported the hypothesis that inputs of N derived from anthropogenic activities were present within the tourism-impacted area. Also, based on the benthic community composition and health surveys, I revealed multiple signs of reef degradation at the impacted site, e.g., higher macroalgal cover, higher bleaching rates, and loss of biodiversity. In summary, I gave strong evidence of the impact of anthropogenic N inputs on the two studied environments. These data provide important baselines for comparison with any future changes in these ecosystems through long-term monitoring programs. Hence, management decisions and mitigation efforts can be evaluated. Nonetheless, this research offers directions to various stakeholders to contribute towards conserving and/or preserving the coastal marine environment under the imminent threat of eutrophication.