Temporal dynamics of carbon sequestration in coastal North Atlantic fjord system by dissolved organic matter characterisation. Rapid carbon sequestration by heterotrophic microbes in surface waters

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.