Contrasting Net Community Production in the Oligotrophic Western North Pacific Subtropical Gyre During the Late Summers of 2020 and 2022: The Roles of Multiple Environmental Drivers

초록

Although the North Pacific Subtropical Gyre (NPSG) is among the most oligotrophic regions, its vast expanse means that small productivity shifts can alter the global carbon budget. We quantified net community production (NCP) from high-resolution /Ar measurements and gross primary production (GPP) from O-18 incubations in the western NPSG boundary in September 2020 and 2022. NCP doubled from 3.3 +/- 1.9 mmol O(2 )m(-2) d(-1) in 2020 to 7.5 +/- 3.1 mmol O-2 m(-2) d(-1 )in 2022, accompanied by a threefold increase in GPP. This enhanced productivity was not driven by increased vertical nutrient supply, as water column stratification was stronger in 2022. Consistent with this, 1D biogeochemical models indicated that wind forcing did not alter nutrient or chlorophyll-a distributions; only unrealistically strong mean winds (>15 m ) s(-1))( )reproduced the productivity increase. In contrast, atmospheric inorganic nitrogen deposition was threefold higher in 2022 and could account for similar to 16% of the NCP increase. Reported nitrogen fixation rates suggest that diazotroph activity could explain all NCP in 2020 and roughly half in 2022, underscoring its role in driving the interannual differences in NCP. However, this inference remains uncertain because it relies on literature-derived upper estimates, highlighting the need for targeted observations. These findings indicate that the observed interannual differences in the western NPSG cannot be explained by a single factor but instead reflect multiple interacting physical and biogeochemical processes. By characterizing these year-to-year NCP differences in this underrepresented region, our findings refine the role of subtropical gyres in the global carbon budget.

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