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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Species of the genus Euphausia dominate the euphausiid biomass of the Southern Ocean, the three largest being Euphausia superba, E. triacantha and E. crystallorophias. We measured a number of morphological features to identify differences between, and within, these species to obtain ecological insights. Interspecifically, the greatest difference was carapace size, with that of E. superba being by far the largest and most variable. This likely reflects its prolific spawning capacity compared with other euphausiid species. E. triacantha exhibited an extended sixth abdominal segment that could facilitate greater levels of thrust in the tail flip escape response. The pleopods, which provide propulsion in forward swimming, were more than 50% larger in E. superba, indicating a greater capacity for directional movement at high velocities. E. crystallorophias had eyes that were 2 almost double the size of those in E. superba and E. triacantha, which may help retain visual resolution within its under-ice habitat. Intraspecifically, we found the above morphological features differed little between sexes and developmental stages in E. crystallorophias and E. triacantha, but differed significantly in E. superba. Compared to females and juveniles, male E. superba had significantly larger eyes and pleopods, while the carapace in males became shorter as a proportion of body length during growth. These features indicate a greater capacity for searching and swimming in males, which, we hypothesise, increases their ability to locate and fertilise females. This morphological specialisation in male E. superba is indicative of comparatively greater inter-male competition resulting from its tendency to form large, dense swarms.

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A general pattern of vertical zooplankton distribution is described, including 22 MOCNESS tows with a 48-h cycle of stratified zooplankton sampling; it showed no day-night differences in the vertical distribution of zooplankton, except for euphausiids. Most zooplankton groups sampled between 500 m and 0 m migrate only across the upper 100 m. Zooplankton carbon was three times higher in the upper 100 m than in the 100–500 m layer of the water column sampled. A difference of 1/24 between the shallower layer (0–50 m) and the 150–200 m layer was found. Euphausiids migrated into the Oxygen Minimum Zone (OMZ) during the day. Zooplankton migration was restrained to the upper 100-m layer by the shallow OMZ. Twenty-four zooplankton groups were studied, showing a ~ 50% reduction in the number and abundance of these groups inside the OMZ. These findings support the Habitat Compression Hypothesis (HCH). A Principal Component Analysis showed that vertical zooplankton distribution is limited by oxygen, concentrated mostly within the upper 100 m. The two most abundant euphausiid species were the vertical migrator Euphausia lamelligera (49.3%) and the non-migrator Stylocheiron affine (39.8%). Most zooplankton assemblages do not migrate across the oxycline.