Natural variation in environmental conditions (e.g., seasonality) modulates the ecological dynamics and functioning of seagrass ecosystems. These characteristics, however, can be altered by anthropogenic-driven pressures resulting from coastal urbanization (e.g., nutrient pollution), with significant impacts on key ecosystem services such as carbon sequestration and storage. Here, we assessed this framework by quantifying the seasonal variation in total carbon (C%) and nitrogen (N%) and stable isotopes (δ¹³C and δ¹⁵N) in two seagrass species, (i.e., Halophila ovalis and Halophila beccarii) in Hong Kong, one of the most urbanized coastal areas in the world. The C:N ratios and δ¹⁵N values indicated seasonal differences in N input, resulting in an increased total seagrass biomass in the wet season. During this period, we observed that both species had depleted δ¹³C, suggesting they changed their source of C  from dissolved inorganic carbon to CO₂. Isotope modelling indicated a significant contribution of below-ground seagrass tissues towards sediment carbon, particularly in the wet season characterized by enhanced N loading. Considering that the 16.41 ha of seagrass (H. ovalis + H. beccarii) meadows of Hong Kong can store 1752.99 ton of CO₂,  this potential carbon stored in these ecosystems can be included in Nationally Determined Contributions of Hong Kong under the IPCC Tier-II assessments. These small seagrass ecosystems can act as nature-based solution towards climate change mitigation combined with mangroves and saltmarsh ecosystems of Hong Kong and the Gray Bay Area.

seagrass meadows, seagrass bed ecosystem, carbon storage capacity, sediment organic carbon, carbon sink