| 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3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。结果显示，夏季养马岛养殖区海域表、底层溶解CH4浓度范围分别为3.32~25.29 nmol/L和4.13~33.29 nmol/L，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显示，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3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/(m2·d)，表明夏季该海域是大气CH4的净源。 |
| 关键词: 养马岛养殖区 甲烷 水产养殖 海–气交换通量 |
| DOI：10.19663/j.issn2095-9869.20240725001 |
| 分类号: |
| 基金项目:国家自然科学基金(42430405; 42076035; 42176016; 42176044)资助 |
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| Distributions, influencing factors and fluxes of dissolved methane in the coastal waters adjacent to Yangma Island aquaculture area, North Yellow Sea in summer |
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ZHOU Chenxia1,2, WEN Jianwen1,2, ZHAI Fangguo3, SONG Changyuan3, SONG Guodong1,2, ZHANG Guiling1,2
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1.Frontiers Science Center for Deep Ocean Multispheres and Earth System, Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, Ocean University of China, Qingdao 266100, China;2.Laboratory for Marine Ecology and Environmental Science, Qingdao Marine Science and Technology Center, Qingdao 266237, China;3.College of Oceanic and Atmospheric Sciences, Ocean University of China, Qingdao 266237, China
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| Abstract: |
| The oceans represent a natural source of atmospheric methane (CH4), with estuaries, shelf areas, and near-shore seas collectively accounting for 16% of the global ocean area and contributing approximately 75% of the total annual CH4 release. The release of CH4 from estuaries and near-shore areas is influenced by human activity 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.
Three cruises were conducted in the Yangma Island aquaculture area of the North Yellow Sea at the end of June, and in early and late August 2023. Surface and bottom seawater samples were collected to understand regional dissolved CH4 distribution characteristics, and sea-air fluxes were estimated based on the CH4 concentrations in surface water and wind speeds. The dissolved CH4 concentrations in surface waters obtained during the three cruises during summer were (8.88±4.99), (11.30±4.81), and (9.10±3.03) nmol/L (Mean±SD), and the dissolved CH4 concentrations in bottom seawaters were (14.25±7.99), (16.15±5.93), and (10.88±4.08) nmol/L, respectively. The CH4 concentrations in the bottom water were significantly higher than those at the surface, because of CH4 release from the sediments. The bottom CH4 concentrations were 2~6 nmol/L higher than those in the surface water at most stations at the end of June and beginning of August, owing to the presence of water column stratification, which effectively impeded the transportation of CH4 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 was predominantly influenced by river inputs, aquaculture activity, and algal and microbial processes. High CH4 concentrations were observed in the nearshore estuaries (Xin'an, Yuniao and Qinshui Rivers) throughout summer due to river input. Dissolved CH4 content in the water body was strongly influenced by aquaculture activity (primarily bivalve shellfish), which provided favorable conditions for CH4 production in the water column, thereby affecting the suspended particulate and organic matter contents in the water column and the sediments. The anaerobic microenvironments of the intestinal tract and excreta of shellfish also represent an optimal setting for anaerobic CH4 production. Consequently, the dissolved CH4 concentration in seawater within the aquaculture zone was markedly elevated compared to that in the non-farming regions. Phytoplankton abundance was high in this area during summer, with Bacillariophyta and Pyrrophyta being the primary groups. Previous studies have demonstrated that the dominant algal species in this area, Leptocylindrus danicus, can directly produce CH4 at a rate of 0.0136 μmol·CH4/(g·dry weight∙h). Moreover, 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∙d). In addition, some Bacillariophyta and Pyrrophyta indirectly produce CH4 through the degradation of dimethyl sulfoniopropionate (DMSP) released from the algae. Simultaneously, this area showed phosphorus limitation at the end of June and beginning of August, and the high DOC content in the aquaculture area provided rich C-P bonds, allowing microorganisms to degrade organic phosphorus compounds to produce CH4. In early August, the ∆CH4 concentration in the surface layer had a significantly positive correlation with Chl-a, verifying that aerobic processes such as phytoplankton production and methyl compound degradation can provide ΔCH4 sources in aerobic surface waters during summer.
Spatiotemporal variations in CH4 saturation and air-sea fluxes in the surface seawater showed trends of aquaculture area > non-aquaculture area and bay area > coastal shelf area. The CH4 saturation in the surface water obtained during the three cruises in the coastal waters adjacent to Yangma Island aquaculture area during summer were (377±209)%, (527±224)% and (391±130)%, respectively, and were all oversaturated with respect to atmospheric CH4. The air-sea exchange flux (estimated using the W2014 relationship) of CH4 in the surface water during summer was (18.87±28.82) μmol/(m2∙d). Overall, we estimated the annual CH4 emissions from the coastal waters adjacent to Yangma Island aquaculture area to be approximately 5.87×10-5 Tg/yr, indicating that this region is a net source of atmospheric CH4. |
| Key words: Yangma Island adjacent coastal area Methane Aquaculture Air-sea exchange flux |
