Concentrations and composition of suspended particulate organic matter suggest distinct biological carbon pump regimes in Norwegian Trench waters.

Knowledge gaps in our understanding of processes that control transport and biogeochemical cycling of carbon in highly productive shelf seas like the North Sea restrict our ability to make accurate predictions of future environmental and climate change as shelf seas play a crucial role in global marine CO₂ uptake and long-term storage. The NoSE project aims to tackle these knowledge gaps by constraining the past, present and future exchange of carbon and other essential nutrients in the Norwegian Trench (NT), the main outflow route of water from the North Sea to the Atlantic Ocean. In spring 2023, water samples were collected with a CTD-rosette on board the RV Pelagia along four transects to quantify and characterize pelagic fluxes of carbon and nitrogen associated with suspended particulate organic matter (SPOM) as well as dissolved organic matter (DOM).

First results reveal variations in SPOM and DOM throughout the NT that are related to the presence of two oceanic currents: the southward flowing Atlantic Inflow Water (AIW) characterized by high temperatures and salinity and the northward flowing Norwegian Coastal Current (NCC) characterized by low temperatures and low salinity. DOC concentrations in the AIW and NCC range between approximately 53 to ±80 µM and ±70-128 µM, respectively. Surface POC concentrations vary between 0.04-0.20 mg/L in the AIW and 0.06-0.23 mg/L in the NCC. Moreover, the δ¹³C, δ¹⁵N and C:N signatures of the surface mixed-layer SPOM samples show increased trophic complexity and decreased bio-availability in the NCC. Likely, these two major currents in the NT are paralleled by distinct changes in Biological Carbon Pump regime, with different plankton communities, export fluxes and mechanisms. These results will allow for the determination of organic carbon export efficiencies and overall organic carbon fluxes throughout the NT.