A Lagrangian Model‐Based Analysis of Protist Plankton Variability and Its Impact on Organic Matter Dynamics Along Transit Pathways Through the Fram Strait.

The Arctic Ocean is characterized by substantial seasonal and inter‐annual variability, of which the sources and impacts are not yet fully understood. Here, we analyze how much of the variability found in in situ observations of biogeochemical and ecological variables collected at the Long‐Term Ecological Research Observatory HAUSGARTEN can be explained by differences in the physical conditions in the water masses passing through the Fram Strait (FS). Employing a size‐based plankton ecosystem model with nine distinct size classes of protist phyto‐ and zooplankton, we simulate standing stocks and fluxes within the nutrient, phytoplankton, zooplankton, and detritus pools in water parcels that follow trajectories tracing the opposing East‐Greenland and West‐Spitsbergen currents through the FS. Our model results agree with in situ observations of biogeochemical tracers, plankton size measurements, climatological data, and remote sensing observations. They show distinct temporal developments in plankton size composition, growth, and export in trajectory ensembles, highlighting how variable physical conditions affect the communities' specific growth histories. Our study indicates that 10%–72% of the variability in upper water column tracer concentrations observed in the FS can be attributed to differences in water parcel trajectories. The maxima of net primary production and vertical export along the trajectories occurred in some (spatial and temporal) distance upstream of the sites of in situ sampling. This study shows that Lagrangian modeling helps clarify complex biogeochemical‐ecological relationships in highly dynamic systems such as the FS, which is urgently needed to understand the role of climate change in the Arctic carbon cycle. Plain Language Summary: To study sources of biological and chemical variability in the Fram Strait (FS) (Arctic Ocean), this study uses a model that follows water parcels as they move along the currents and simulates the growth of plankton populations, their influence on nutrient concentrations and the production of dead, dissolved, and decaying organic matter. By simulating many different pathways, the model estimates how variability in the environmental conditions along those pathways contributes to the variability of the plankton ecosystem in the upper water column. When compared to real data measured from ships and observed by satellites, we find that the model successfully reproduces changes in the plankton size structure and estimates productivity and export close to observations from the region. Our study finds that a substantial portion (10%–72%) of the observed variability in biogeochemical tracers can be explained by differences in the origins and pathways of water parcels. Nonetheless, a large amount of variability remains unexplained, indicating the need for further research to better understand essential processes. This research helps us understand the sources of variability in the FS and make better predictions about elemental cycling, productivity and export, which is especially important in the face of climate change. Key Points: Local variability in tracer concentrations in Fram Strait (FS)'s upper 100 m is linked to differences in water parcel trajectoriesTrajectories differ in their origin and timing of passage, leading to differences in plankton size structure, growth, and exportAt the time of in situ observations in the FS, the maxima in net primary production and vertical export have already occurred [ABSTRACT FROM AUTHOR]