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Industrial activities have increased the supply of iron to the ocean, but the magnitude of anthropogenic input and its ecological consequences are not well-constrained by observations. Across four expeditions to the North Pacific transition zone, we document a repeated supply of isotopically light iron from an atmospheric source in spring, reflecting an estimated 39 ± 9 % anthropogenic contribution to the surface ocean iron budget. Expression of iron-stress genes in metatranscriptomes, and evidence for colimitation of ecosystem productivity by iron and nitrogen, indicates that enhanced iron supply should spur spring phytoplankton blooms, accelerating the seasonal drawdown of nitrate delivered by winter mixing. This effect is consistent with regional trends in satellite ocean color, which show a shorter, more intense spring bloom period, followed by an earlier arrival of oligotrophic conditions in summer. Continued iron emissions may contribute to poleward shifts in transitional marine ecosystems, compounding the anticipated impacts from ocean warming and stratification. 

Abstract The Bering Sea is a highly productive subarctic ecosystem with some of the most valuable commercial fisheries in the United States. This seasonally ice‐covered sea has been rapidly changing due to ocean warming with impacts to the ecosystem structure and fisheries. The long‐term effects of these shifts on primary producers, however, are still unknown. Continuous monitoring of primary productivity in the Bering Sea is critical, yet observations that capture the ephemeral nature of plankton are challenging to sustain. To address this gap, high temporal resolution primary productivity rates were quantified at a mooring site (M2) in the southeastern Bering Sea in 2021. From a suite of sensors at M2 (fluorescence, dissolved oxygen, temperature, and total dissolved gas pressure), we calculated gross primary productivity (GPP), net primary productivity (NPP), and net community productivity (NCP), the latter based on net biological oxygen saturation using dissolved oxygen/nitrogen (O₂/N₂) ratios. These estimates elucidate weekly patterns from the spring bloom through fall, when the water column becomes well‐mixed. In 2021, we observed average productivity during the spring bloom, yet wind patterns and mixing dynamics during the spring contributed to low productivity during summer and fall. The 2021 productivity metrics (GPP, NPP, and NCP) were compared across the growing season and contrasted with seasonal productivity estimates at M2 in previous years with consideration given to variability of ice conditions (warm/cold years) and wind stress. 

Abstract Siderophores are strong iron‐binding molecules produced and utilized by microbes to acquire the limiting nutrient iron (Fe) from their surroundings. Despite their importance as a component of the iron‐binding ligand pool in seawater, data on the distribution of siderophores and the microbes that use them are limited. Here, we measured the concentrations and types of dissolved siderophores during two cruises in April 2016 and June 2017 that transited from the iron‐replete, low‐macronutrient North Pacific Subtropical Gyre through the North Pacific Transition Zone (NPTZ) to the iron‐deplete, high‐macronutrient North Pacific Subarctic Frontal Zone (SAFZ). Surface siderophore concentrations in 2017 were higher in the NPTZ (4.0–13.9 pM) than the SAFZ (1.2–5.1 pM), which may be partly attributed to stimulated siderophore production by environmental factors such as dust‐derived iron concentrations (up to 0.51 nM). Multiple types of siderophores were identified on both cruises, including ferrioxamines, amphibactins, and iron‐free forms of photoreactive siderophores, which suggest active production and use of diverse siderophores across latitude and depth. Siderophore biosynthesis and uptake genes and transcripts were widespread across latitude, and higher abundances of these genes and transcripts at higher latitudes may reflect active siderophore‐mediated iron uptake by the local bacterial community across the North Pacific. The variability in the taxonomic composition of bacterial communities that transcribe putative ferrioxamine, amphibactin, and salmochelin transporter genes at different latitudes further suggests that the microbial groups involved in active siderophore production and usage change depending on local conditions.