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Abstract. The western tropical South Pacific (WTSP) Ocean has been recognized as a global hot spot of dinitrogen (N₂) fixation. Here, as in other marine environments across the oceans, N₂ fixation studies have focused on the sunlit layer. However, studies have confirmed the importance of aphotic N₂ fixation activity, although until now only one had been performed in the WTSP. In order to increase our knowledge of aphotic N₂ fixation in the WTSP, we measured N₂ fixation rates and identified diazotrophic phylotypes in the mesopelagic layer along a transect spanning from New Caledonia to French Polynesia. Because non-cyanobacterial diazotrophs presumably need external dissolved organic matter (DOM) sources for their nutrition, we also identified DOM compounds using Fourier transform ion cyclotron resonance mass spectrometry (FTICRMS) with the aim of searching for relationships between the composition of DOM and non-cyanobacterial N₂ fixation in the aphotic ocean. N₂ fixation rates were low (average 0.63±0.07nmolNL⁻¹d⁻¹) but consistently detected across all depths and stations, representing ∼ 6–88% of photic N₂ fixation. N₂ fixation rates were not significantly correlated with DOM compounds. The analysis of nifH gene amplicons revealed a wide diversity of non-cyanobacterial diazotrophs, mostly matching clusters 1 and 3. Interestingly, a distinct phylotype from the major nifH subcluster 1G dominated at 650dbar, coinciding with the oxygenated Subantarctic Mode Water (SAMW). This consistent pattern suggests that the distribution of aphotic diazotroph communities is to some extent controlled by water mass structure. While the data available are still too scarce to elucidate the distribution and controls of mesopelagic non-cyanobacterial diazotrophs in the WTSP, their prevalence in the mesopelagic layer and the consistent detection of active N₂ fixation activity at all depths sampled during our study suggest that aphotic N₂ fixation may contribute significantly to fixed nitrogen inputs in this area and/or areas downstream of water mass circulation. 

Abstract The Thomas Fire began on December 4, 2017 and burned 281,893 acres over a 40‐day period in Ventura and Santa Barbara Counties, making it one of California's most destructive wildfires to date. A major rainstorm then caused a flash flood event, which led to the containment of the fire. Both airborne ash from the fire and the runoff from the flash flood entered into the Santa Barbara Basin (SBB). Here, we present the results from aerosol, river, and seawater studies of black carbon and metal delivery to the SBB associated with the fire and subsequent flash flood. On day 11 of the Thomas Fire, aerosols sampled under the smoke plume were associated with high levels of PM_2.5, levoglucosan, and black carbon (average: 49 μg/m³, 1.05 μg/m³, and 14.93 μg/m³, respectively) and aerosol metal concentrations were consistent with a forest fire signature. Metal concentrations in SBB surface seawater were higher closer to the coastal perimeter of the fire (including 2.22 nM Fe) than further off the coast, suggesting a dependence on continental proximity rather than fire inputs. On days 37–40 of the fire, before, during, and after the flash flood in the Ventura River, dissolved organic carbon, dissolved black carbon, and dissolved metal concentrations were positively correlated with discharge allowing us to estimate the input of fire products into the coastal ocean. We estimated rapid aerosol delivery during the fire event to be the larger share of fire‐derived metal transport compared to runoff from the Ventura River during the flood event.