Coral isoscapes reveal varied nitrogen sources and dynamics in Hong Kong’s coastal environment

Abstract

Hong Kong's marine ecosystem health is under threat due to large inputs of dissolved inorganic nitrogen (DIN) from various sources. We used stable isotope analysis to trace major DIN source(s) affecting corals and examined their spatial and temporal variations in coastal environments. We collected corals along coastlines in a wet and dry season and analyzed the δ15N signals of their Symbiodinium (δ15Ns). Spatially, the mean δ15Ns value was significantly lower in the Southern zone–SHK (5.6±1.5‰) than Northeastern zones–PS (8.0±1.1‰) and MB (8.8±1.3‰) in the wet season. In the dry season, there was no significant difference in the mean δ15Ns values among zones. When compared between seasons, the mean δ15Ns value in SHK in the dry season (6.9±0.7‰) was significantly higher than the wet season. The spatial and seasonal differences in the δ15Ns values highlighted the influence of the Pearl River (PR), which supplies Hong Kong with a high concentration of 14N compared to N derived from oceanic water. The dominance of N from PR could contribute to the low δ15Ns values in SHK in the wet season as it is in close proximity to PR. The seasonal difference, i.e. higher δ15Ns values in SHK in the dry than wet season, can be explained by the decreasing influence of PR as the dry season accounts for ~20% of the annual discharge. These data suggested PR has a major influence to the DIN dynamics and its significance is subject to spatial and seasonal changes.