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| 夏季北黄海养马岛养殖区溶解甲烷浓度、影响因素及海气交换通量 |
| Distributions, Influencing Factors and Fluxes of Dissolved Methane in the Coastal Waters Adjacent to Yangma Island Aquaculture Area, North Yellow Sea in Summer |
| 投稿时间：2024-07-25 修订日期：2024-11-08 |
| DOI： |
| 中文关键词: 养马岛 甲烷 水产养殖 海-气交换通量 |
| 英文关键词: Yangma Island adjacent coastal area Methane Aquaculture Air-sea exchange flux |
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| 中文摘要: |
| 养殖海区是向大气中释放甲烷（CH4）的热点区域。本研究于2023年6月底、8月初和8月底对北黄海养马岛养殖区海域进行了三个航次调查，采集了表、底层海水样品，利用吹扫捕集-气相色谱法对样品进行分析测定，以认识夏季养殖区海水中溶解CH4的分布特征并估算其海-气交换通量。结果表明，夏季养马岛养殖区海域表、底层溶解甲烷浓度范围分别为3.32~25.29 和4.13~33.29 nmol?L-1，受沉积物释放的影响，底层溶解CH4浓度整体高于表层。受陆源输入、物理过程、生物活动等因素的影响，CH4的水平分布具有明显的空间差异性，其中近岸河口处CH4浓度明显高于远岸海域，贝类养殖区明显高于非养殖区。室内受控培养实验和现场调查结果表明，浮游植物产CH4等有氧产生过程是夏季该海域表层富氧水体中过剩甲烷ΔCH4的来源。夏季三个航次的表层海水CH4的平均饱和度分别为(364±201)%、(499±212)% 和(402±134)%，均处于过饱和状态，同时利用W2014公式估算出养马岛养殖区海域夏季表层海水CH4的海气交换通量为(18.87±28.82) μmol·m-2·d-1，表明夏季该海域是大气CH4的净源。 |
| 英文摘要: |
| The oceans represent a natural source of atmospheric methane (CH4)，with estuaries, shelf areas, and near-shore seas, collectively accounting for only 16% of the global ocean area but contributing approximately 75% of the total annual CH4 release. The release of CH4 from estuaries and near-shore areas is influenced by human activities, such as sewage discharge and aquaculture. Hence, it is a scientific priority to study the production and release of CH4 in near-shore aquaculture areas and understand the factors influencing its distribution.
In this study, three cruises were conducted in the Yangma Island aquaculture area of North Yellow Sea at the end of June, early August and late August 2023. Surface and bottom seawater samples were collected to understand the distribution characteristics of dissolved CH4 in the region, and sea-air fluxes were estimated based on the concentration of CH4 in the surface water and the wind speed. The dissolved CH4 in surface waters of the three cruises in summer were (8.88±4.99) nmol/L, (11.30±4.81) nmol/L, and (9.10±3.03) nmol/L(mean±SD), and the dissolved CH4 in bottom seawaters was (14.25±7.99) nmol/L, (16.15±5.93) nmol/L, and (10.88±4.08) nmol/L. Because of the CH4 realse from the sediments, CH4 in the bottom water is significantly higher than those in the surface. The bottom CH4 concentration was 2-6 nmol/L higher than that of the surface water at most stations at the end of June and the beginning of August, due to the presence of water column stratification, which effectively impeded the transport of methane produced by the sediment to the upper seawater. By the end of August, the water column stratification had dissipated, resulting in a notable reduction in the discrepancy between the surface and bottom CH4 concentrations. The distribution of dissolved CH4 concentrations in the Yangma Island aquaculture area are predominantly influenced by river inputs, aquaculture activities, and algal and microbial processes. High CH4 concentrations were observed in the nearshore estuaries (Xin 'an River, Yuniao River and Qinshui River) throughout the summer due to river input.Dissolved CH4 content in the water body is strongly influenced by aquaculture activities (mainly bivalve shellfish), which provide favorable conditions for the production of CH4 in the water column by affecting the content of suspended particulate matter and organic matter in the water column and the sediments. Concurrently, the anaerobic microenvironment of the shellfish intestinal tract and excreta also represents an optimal setting for anaerobic CH4 production. Consequently, the concentration of dissolved CH4 in seawater within the aquaculture zone is markedly elevated comparing to those in non-farming regions. The phytoplankton abundance was high in this area in summer, with the Bacillariophyta and Pyrrophyta as the main groups. Previous studies have demonstrated that the dominant algal species in this area, Leptocylindrus danicus, can directly produce CH4 at a rate of 0.0136μmol·CH4·g ·dry weight-1· h-1. Meanwhile, in this study, another dominant algal species, Pseudo-nitzschia, was subjected to laboratory controlled culture experiments, which demonstrated that it can also produce CH4 at a rate of 46.59 ag·cell-1·d-1. In addition, some Bacillariophyta and Pyrrophyta could indirectly produce CH4, through the degrdation of dimethylsulfoniopropionate (DMSP) released from the algae. At the same time, this area showed phosphorus limitation at the end of June and the beginning of August, and the high content of DOC in the aquaculture area provided rich C-P bonds, so that microorganisms could degrade organic phosphorus compounds to produce CH4. In early August, the concentration of ?CH4 in the surface layer has a significant positive correlation with Chl-a, verifying that aerobic processes such as phytoplankton production and methyl compound degradation can be the sources of ΔCH4 in aerobic surface waters in summer.
There are spatiotemporal variations in CH4 saturations and air-sea fluxes in surface seawater, with a trend of aquaculture area > non-aquaculture area, and bay area > coastal shelf area. The average saturation of CH4 in the surface water of the three cruises in the coastal waters adjacent to Yangma Island aquaculture area in summer was (377±209)%, (527±224)% and (391±130)%, respectively, and all were oversaturated with respect to atmospheric CH4. The air-sea exchange flux (estimated using the W2014 relationship) of CH4 in the surface water in summer was (18.87±28.82) μmol·m-2·d-1. Overall, we estimated the annual CH4 emission from the coastal waters adjacent to Yangma Island aquaculture area to be about 5.87×10-5 Tg·yr-1, indicating that this region is a net source of atmospheric CH4. |
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