Fjords Are Potential Hotspots of Refractory Dissolved Organic Matter Production: Insights From 1 Year of Weekly Time‐Series Observations of Fluorescent Dissolved Organic Matter in the Bedford Basin, a Northwestern Atlantic Fjord.

Fjords are representative marine ecosystems that play an important role in regulating coastal carbon cycling. As fingerprints of the dissolved organic matter (DOM) pool, fluorescent DOM (FDOM) is widely used to study the ocean carbon cycle. Here, we report on 1‐year of weekly observations of FDOM (from May 2018 to May 2019) in the Bedford Basin, a well‐studied northwestern Atlantic fjord. Results showed that differences in FDOM indices (biological index, fluorescence index, and humification indices) between the euphotic and aphotic layers changed during the investigation period, suggesting that classical definitions of these indices may not apply consistently. Spearman rank correlations showed that the surface FDOM was affected by physical and biological factors, while water mass renewal was the major control for the deeper layer. Humic‐like FDOM (FDOMH) changed following the Atlantic deep‐water intrusion, indicating that FDOMH may be used to trace water mass movements in this region. FDOMH shows a significantly positive linear correlation with apparent oxygen utilization (AOU). Their slopes were higher than those from the open ocean, suggesting that the Bedford Basin is a hotspot of RDOM production. Hence, we proposed a possible carbon sequestration mechanism that fjords play as "RDOM producing hot spots" and that the produced RDOM is then transported to the open ocean via water mass movement and finally stored in the ocean for extended periods of time. The proposed biogeochemical processes may also occur, more generally, in the global coastal ocean. Plain Language Summary: Fjords are representative marine systems and play an important role in regulating coastal carbon cycling; however, few high‐frequency long‐term time series observations of dissolved organic matter (DOM) have been reported to date. This study reports a 1‐year weekly observation of fluorescent DOM (FDOM) in the Bedford Basin, a 70‐m deep fjord located in Nova Scotia, Canada. The results suggested that FDOM indices are not reliable indicators as proposed originally, and the control factors of FDOM behaviors were different in the surface and bottom water, as well that humic‐like FDOM (FDOMH) have the potential to trace water mass movements. Significant positive linear relationships between apparent oxygen utilization and FDOMH were observed in the aphotic zone, and their slopes were higher than those from the open ocean, suggesting that the Bedford Basin is a hotspot of RDOM production. Similar results are widely observed in dark marginal seas. Therefore, we propose a possible coastal carbon sequestration mechanism in which fjord systems (even coastal oceans) act as RDOM‐producing factories, and the produced RDOM is then transported to the open ocean via water mass movement and finally stored in the ocean at longer timescales. Key Points: Weekly time series observations of fluorescent dissolved organic matter (FDOM) in the Bedford Basin are presentedFDOM signals trace deep seawater intrusionsFjords are potential hot spots of refractory dissolved organic matter (RDOM) production and serve as RDOM producing factories [ABSTRACT FROM AUTHOR]