Influence of Microbial Activities on Fluorescent Dissolved Organic Matter in the Dark Canada Basin Waters

The fluorescence characteristics of dissolved organic matter (DOM) were measured to determine the distribution and drivers of DOM in the dark Canada Basin waters. We studied the relationship between fluorescent DOM (FDOM) and apparent oxygen utilization (AOU) in the whole water column as well as in the main water masses (Pacific Winter Water, Atlantic halocline (AH), Atlantic water—Fram Strait and Barents Sea branches, Deep Temperature Minimum, and Canada Basin deep water). The relative water mass fractions of different water masses were estimated using a five end‐members mixing model. The distribution of water mass fractions was found to be in good agreement with the distribution based on the traditional temperature‐salinity diagram. The fractions of AH waters were linked with AOU, highlighting the significant role played by mineralization in shaping AH humic‐like levels. The slope of the relationship between in situ FDOM—AOU was greater than those reported in the other oceans worldwide. This suggests that the AH is a hot spot for microbial mineralization of humic‐like DOM. On the other hand, the relationships with the humic‐like intensities differed among the four humic‐like components in the >125 m waters, indicating distinct environmental dynamics and biogeochemical roles for each humic‐like component. Plain Language Summary Fluorescent marine dissolved organic matter in the dark ocean is primarily produced by the microbial decomposition of organic material. In this study, we examine the relationship between fluorescent dissolved organic matter (FDOM) and remineralization‐related variables such as apparent oxygen utilization in the main six water masses in the dark Canada Basin waters. Our study reveals a strong influence of in situ microbial processes on the humic‐like FDOM levels measured by the in situ sensor in the Atlantic halocline waters. We identified four more humic‐like and protein‐like FDOM components in samples collected by the CTD‐rosette that behaved differently. To better understand their sensitivity to in situ mineralization in the dark ocean, future studies should concentrate on in situ monitoring of the blue‐shifted humic‐like and protein‐like FDOM. Key Points The high in situ fluorescent dissolved organic matter (FDOM) levels were linked to the high Pacific winter water mass fractions The slopes of the apparent oxygen utilization–FDOM correlations varied for the four humic‐like components The Atlantic halocline serves as a hot spot f