Impact of Water Mass Mixing on the Composition and Mineralization of Dissolved Organic Matter in the Deep Labrador Sea

Abstract

The Labrador Sea is a region of the North Atlantic known for its strong ocean currents and deep water formation, which contribute to the transport and mixing of water masses throughout the region. The absorbing and fluorescing properties of dissolved organic matter (DOM) were assessed to track the water masses and the in situ production in the Labrador Sea (>200 m). No significant differences in DOM composition were found in the mesopelagic waters (200–1,000 m). In the bathypelagic waters (1,000–4,000 m), the estimated dissolved organic carbon (DOC) and dissolved lignin concentrations, as well as humic‐like fluorescence intensities, allowed the discrimination of North East Atlantic Deep Water versus Denmark Strait overflow water. The humic‐like intensities were significantly different between upper Labrador Sea water (uLSW) and deep LSW (dLSW) suggesting their applications as tracers of deep winter mixing. The significant correlations with apparent oxygen utilization support the in situ production of humic‐like fluorescence and the net microbial consumption of DOC and lignin in the dark Labrador Sea. We also demonstrated that microbial activities play a role in the production of humic‐like compounds in the dLSW that experiences deep convection mixing.