Primary Production and Respiration in the Norwegian Sea Estimated From Biogeochemical Argo Floats

Biogeochemical (BGC) Argo floats were used in this study to investigate phytoplankton blooms. We assessed the seasonal and annual rates of net primary and community production, along with respiration in the Norwegian Sea. The years 2020 and 2021 were contrasted to illuminate similarities and differences. In both years the onset of the bloom occurred at the beginning of February, coinciding with a deep winter mixed layer. However, during spring and summer the biological production appeared to develop differently. In 2020 the mixed layer depth shoaled quickly in April due to surface heating, triggering a strong spring bloom event. In contrast, a significant surface cooling in April 2021 triggered a substantial vertical mixing, that delayed the mixed layer's shoaling. This delay initiated cascading effects impacting nitrate consumption and bloom development that resulted in increased respiration in 2021 compared to 2020. In both years, vertical nitrate mixing from deeper layers to the surface emerged as a pivotal factor determining primary production. Using a mixed layer model in combination with Argo observations, we found that the upward nutrient fluxes due to mixing doubled net community production. These findings underscore the capability of BGC‐Argo floats, operating at a 5‐day resolution, to unveil the intricate interplay between hydrography, physical drivers, and biogeochemical processes in shaping phytoplankton dynamics and overall ecosystem productivity in the Norwegian Sea. Plain Language Summary The magnitude of seasonal phytoplankton growth is complex but generally depends on several physical and chemical variables. Thus, estimating the annual phytoplankton production accurately is a challenging task as it requires measurements with high temporal resolution over a full season. In this study, we used data from two vertical biogeochemical profiling Argo floats, operating at 5 days resolution. Combined with a mixed layer model, we resolved the development of phytoplankton production and respiration in the Norwegian Sea. We found that the vertical mixing of nutrients from deeper layers to the surface appeared as a determining factor on phytoplankton production and contributed to 50% of the production. In 2021, an anomalous wind and cooling event in April caused deep mixing at the onset of the spring bloom. This caused a slower and delayed phytoplankton growth and nutrient consumption, resulting in higher grazing of phytoplankton and reduced phytoplank