Concentrations of phosphate, an essential nutrient for phytoplankton growth, are depleted across a large extent of the western subtropical North Pacific. However, the drivers and consequences of this are still not well constrained. We investigated this with field and satellite observations, shipboard experiments, and model estimates of aerosol nutrient supply across a major phosphate gradient in this region. Collectively these data suggested that phosphate depletion was primarily driven by enhanced nitrogen fixation rates stimulated by aerosol iron, in turn supplying biologically available nitrogen without a corresponding supply of phosphate. The impact of observed phosphate depletion was enhanced microbial utilization of the more abundant dissolved organic phosphorus pool.

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Atmospheric aerosol deposition into the low latitude oligotrophic ocean is an important source of new nutrients for primary production. However, the resultant phytoplankton responses to aerosol deposition events, both in magnitude and changes in community composition, are poorly constrained. Here, we investigated this with 19 d of field and satellite observations for a site in the subtropical North Atlantic. During the observation period, surface dissolved aluminum concentrations alongside satellite‐derived aerosol and precipitation data demonstrated the occurrence of both a dry deposition event associated with a dust storm and a wet deposition event associated with strong rainfall. The dry deposition event did not lead to any observable phytoplankton response, whereas the wet deposition event led to an approximate doubling of chlorophyll a, with Prochlorococcus becoming more dominant at the expense of Synechococcus. Bioassay experiments showed that phytoplankton were nitrogen limited, suggesting that the wet deposition event likely provided substantial aerosol‐derived nitrogen, thereby alleviating the prevalent nutrient limitation and leading to the rapid observed phytoplankton response. These findings highlight the important role of wet deposition in driving rapid responses in both ocean productivity and phytoplankton community composition.