Seasonality and local nutrient loading drive changes in organic carbon in seagrass ecosystems in Hong Kong

Natural variation in environmental conditions (e.g., seasonality) modulates the ecological dynamics and functioning of seagrass ecosystems. However, these characteristics can be altered by anthropogenic-driven pressures resulting from coastal urbanization (e.g., nutrient pollution), with significant effects on ecosystem service provision, including carbon sequestration and storage. Understanding these effects is complex as seagrasses exhibit different sensitives to environmental stress and change, generating species-specific responses that vary through temporal and spatial scales. Here, we tested this hypothesis by quantifying the seasonal variation in total carbon (C%; upper sediment layer, 15 cm), total nitrogen (N%), and stable isotopes (δ¹³C and δ¹⁵N) in two seagrass species (i.e., Halophila ovalis and H. beccarii) in Hong Kong, one of the most urbanized coastal areas globally. The C:N ratios and δ¹⁵N values indicated seasonal differences in local nitrogen source and accumulation, resulting in increased total seagrass biomass in the wet season. Isotope results suggested belowground seagrass tissues significantly contributed towards sediment carbon, particularly in the wet season, characterized by increased nitrogen loading.