Abstract:An important driver of coastal marine ecosystem processes involves seaweed photosynthesis converting atmospheric CO2 into organic carbon and internally storing it to achieve carbon storage, which is of great significance for the realization of the goal of carbon neutrality and carbon peak in China. Seaweed fields transport a large amount of organic detritus to the ocean during the peak period of seaweed withering, providing primary productivity for the surrounding area and realizing the function of a carbon sink after settlement. The transport and settlement processes of seaweed detritus is one of the key dynamic processes connecting the circulation of important biogenic elements in different coastal habitats. It not only affects the primary productivity of the sea area around the seaweed field, but also affects the temporal and spatial distribution pattern of its carbon sink function. In this study, a three-dimensional Ocean numerical model (Estuarine Coastal Ocean Model, semi-implicit, ECOM-si) coupled to a Lagrange particle tracking module were combined with tidal elevation, current velocity, and drift buoy trajectory to investigate the dynamic mechanism and carbon sink function of organic detritus transport and settlement processes in a natural seaweed field on the northwest of Gouqi Island. The model revealed the spatial distribution pattern of the organic detritus contributing to the carbon sink, which presents a high value area centered on the seaweed field and decreases in the northwest-southeast direction. The difference in the hydrodynamic conditions and particle size of the detritus leads to significant variation in the spatial distribution of the settlement. During spring tides, the highest carbon sink was 699 g C/(hm2·d) and the carbon sink contribution of seaweed detritus was not limited to the adjacent sea area. The smaller the particle size of the detritus, the larger its contribution range, contributing approximately 7.5~33.7 kg C of organic carbon daily to the sea area 5 km away from the center of the seaweed area. During a neap tide (with lower flow velocity) the contribution of the seaweed area to the carbon sink was concentrated in the coastal area. The radiation range was relatively small, but the carbon sink intensity was high, reaching approximately 817 g C/(hm2·d). The tidal current direction when seaweed detritus drops and the geographical location of the seaweed site are also important factors affecting the contribution of the seaweed field to the carbon sink and ecological radiation function. When the seaweed was located in relatively open terrain with less shielding by the tidal current, the settlement and distribution range of the organic detritus was wider and provided a greater contribution to the carbon sink in the surrounding sea area. The Lagrange method is feasible for investigating the offshore transportation and settlement mechanism of organic detritus from a seaweed field. This method can effectively evaluate the contribution of a typical nearshore seaweed field to the carbon sink in the surrounding sea area and the role of the main dynamic factors. The research results reveal that the temporal and spatial characteristics of seaweed detritus transport and settlement and the carbon sink function are affected by many factors, such as tidal current magnitude, seaweed particle size (with different settlement velocities), detritus shedding time, and the geography of the location of the seaweed field. This research provides an important scientific reference for investigating seaweed fields and evaluating ecological radiation range in China.
