Temporal dynamics of carbon sequestration in coastal North Atlantic fjord system by dissolved organic matter characterisation

The fjord systems in higher latitudes are unique coastal water ecosystems that facilitate the study of dissolved organic matter (DOM) dynamics from surface to deep waters. The current work undertaken in Trondheim fjord of coastal North Atlantic waters, compared DOM fractions from surface (3 m), intermediate (225 m) and deep (440 m) during four seasons from late spring to winter in 2017. The high-resolution mass spectrometry data showed that DOM composition varies significantly in different seasons than in different depths in the fjord systems. The bacterial community composition was comparable to each other except in spring surface and summer intermediate depths. Even in the sufficient availability of inorganic nutrients bacterial production was minimal below the euphotic layer. Compared to aphotic zone, bacterial production rate in the surface waters was about 7 times and 50 times higher in the winter and the summer seasons respectively. The surface heterotrophic microbial communities might have rapidly consumed the available labile DOM with production of more refractory DOM limiting bacterial production in aphotic layers. The higher number of CRAM-like formulas determined in the surface waters compared to other depths also support our hypothesis. The refractory DOM sequestered in the water column may either be exported into sediments attached to particulate matter and marine gels or may escape into the atmosphere as carbon dioxide/monoxide during the photochemical oxidation pathways, thus relevant in climate change scenarios.

高纬度峡湾系统是一类独特的近岸水域生态系统，为开展从表层到深层水体的溶解有机物（dissolved organic matter, DOM）动态研究提供了理想平台。本研究以北大西洋近岸的特隆赫姆峡湾为研究对象，于2017年晚春至冬季的四个季节中，分别采集表层（3米）、中层（225米）与深层（440米）水体的DOM组分进行对比分析。高分辨质谱数据显示，峡湾系统中DOM组成的季节差异显著大于深度差异。除春季表层与夏季中层水体外，其余采样点的细菌群落组成无显著差异。即便无机养分供应充足，透光层以下的细菌生产力仍处于极低水平。与无光带相比，冬季和夏季表层水体的细菌生产力分别约为其7倍和50倍。表层异养微生物群落可能快速消耗了可利用的易变性DOM，并产生更多难降解DOM，进而限制了无光带内的细菌生产力。相较于其他深度，表层水体中检出的类CRAM分子式数量更多，该结果佐证了本研究的假说。储存在水柱中的难降解DOM，既可通过附着于颗粒物与海洋凝胶的形式被输运至沉积物，也可在光化学氧化过程中以二氧化碳/一氧化碳的形式释放至大气，因此在气候变化情景中具备重要研究价值。