Biological nitrogen fixation is increasingly recognized as an important source of new nitrogen in a warming ocean. However, the basin-scale spatiotemporal distribution of nitrogen-fixing organisms (diazotrophs) in the ocean and its controlling environmental factors remain unclear. Here we examined the basin-scale seasonal distribution patterns of major diazotrophs (filamentous cyanobacterial Trichodesmium, unicellular cyanobacterial UCYN-A1, and proteobacterial Gamma-A) in surface waters of the North Pacific from 2014 to 2016 with unprecedented coverage and resolution. In general, UCYN-A1, Trichodesmium, and Gamma-A were abundant during spring-autumn, summer-autumn, and spring respectively. Regarding latitudinal patterns of abundance, UCYN-A1 showed dome shape; Trichodesmium was gradually decreasing from low-to high-latitude regions; and Gamma-A did not show a clear pattern, which were coincident with the distinct correlations between the diazotrophs and temperature. All three diazotrophs were abundant (reached 10 6-10 7 nifH gene copy number L −1) in the North Pacific transition zone and subtropical gyre, where the cyanobacterial diazotrophs were more abundant in both the western and eastern North Pacific than in the central North Pacific. The diazotroph abundance in the western North Pacific was positively correlated with eddy kinetic energy and sea surface height anomaly, which implies an enhancement of diazotrophs in mesoscale eddies associated with the western boundary current Kuroshio and its extension. The cyanobacterial diazotrophs were positively correlated with wind stress curl, a measurable parameter of wind-driven upwelling, in the eastern North Pacific. Our study refines the biogeography of three major diazotrophs and highlights the importance of physical forcing in mediating their dynamics.

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Morphological differences associated with sex or stage, together with total lipids and carotenoids, were studied in Euphausia superba as possible indicators of physiological condition. E. superba displays sexual dimorphism during growth. A group of mature males, called Males II herein, has a greater abdominal length, suggesting that they are faster swimmers, a feature implying higher metabolic rates and a higher demand for protecting pigments like carotenoids. Mature Males II have proportionally lower lipids but higher total lipid-soluble carotenoids, a counterintuitive finding. Males II also have bigger eyes. Significant regressions with carotenoids were found for wet weight, abdominal length, and eye diameter. On a spatial analysis, population composition reflects reproductive activity. Males II would be in search of females for fecundation and, thus, are dominant in some areas. The PCA analysis of 10 allometric and biochemical variables show a distinct Males II group differing in morphology, carotenoids, and lipid contents. The carotenoid:lipid ratio was highest for Males II, supporting the hypothesis of the role of carotenoids in the activity of the species. Mature males may experience physiological stress during reproduction and probably die shortly afterwards. A relationship between activity, morphometrics, and carotenoid content seems evident, deserving further investigation.

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