基于拉格朗日法的海藻场有机碎屑离岸输运研究

Study on offshore transport of organic detritus in seaweed field based on Lagrange method

中文关键词: 拉格朗日质点追踪  天然海藻场  有机碎屑  海洋碳汇  数值模型

英文关键词: Lagrange particle tracking  Natural seaweed farm  Organic detritus  Marine carbon sink  Numerical model

中文摘要:

      海藻场在海藻凋落盛期向海洋输送大量有机碎屑，为海藻场外围海域提供大量初级生产力，并在沉降后实现碳汇功能。本研究采用具有拉格朗日质点追踪模块的三维海洋数值模式(Estuarine Coastal Ocean Model, semi-implicit, ECOM-si)，结合潮位、流速及漂流浮标轨迹跟踪观测，研究了枸杞岛西北侧天然海藻场有机碎屑输运与沉降的动力学机制及碳汇功能。结果显示，海藻场对周边海域碳汇贡献的空间分布格局呈现为以海藻场为中心的高值区，向西北–东南海域扩展分布并递减。水动力强弱和海藻碎屑粒径差异导致其沉降后的空间分布差异显著。大潮期间，海藻场对邻近海域碳汇贡献最高为699 g C/(hm2·d)。碎屑粒径越小、沉降速度越小的海藻碎屑对外围海域碳汇贡献范围越大。海藻场碳汇功能不局限于邻近海域，不同粒径海藻碎屑每天为海藻场5 km外海域提供有机碳约7.5~33.7 kg C；小潮期间水动力较弱，约90%以上的海藻碎屑沉降至5 km以内海域。海藻场碳汇功能集中于沿岸邻近海域，对邻近海域有碳汇贡献的区域范围小，但强度大，最高为817 g C/(hm2·d)。海藻碎屑脱落时的潮流方向和海藻场所处区位也是影响海藻场有机碎屑输运沉降和碳汇归趋的重要因素。赋予拉格朗日质点单位碳含量后，拉格朗日法在海藻场有机碎屑离岸输运沉降机制研究中具有可行性，可有效评估典型近岸海藻场对周边海域碳汇的贡献及主要动力学因子的作用。本研究可为我国海藻场本底调查及其生态辐射范围评估提供参考。

英文摘要:

      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.