103 research outputs found
Elevated turbulent and double-diffusive nutrient flux in the Kuroshio over the Izu Ridge and in the Kuroshio Extension
No full textWhile the Kuroshio is known to be a nutrient stream, as these nutrients are in dark subsurface layers, they are not immediately available for photosynthesis unless they are supplied to the sunlit surface layers. Recent microstructure observations have revealed that strong diapycnal mixing caused by the Kuroshio flowing over topographic features and double diffusion in the subsurface layers of the Kuroshio. However, it is still unclear how much nutrient flux can be provided by these microscale mixing processes. In this study, using an autonomous microstructure float and nutrient samplings, nutrient flux caused by the Kuroshio over the Izu Ridge, and that caused by double diffusion in the Kuroshio Extension are quantified. The nitrate diffusive flux is estimated to be >1mmolNm−2day−1 over a distance, 20–30 km near the Izu Ridge and >0.1mmolNm−2day−1, which persists further downstream direction over 100 km along the Kuroshio, increasing the subsurface chlorophyll-a concentration in the region 200 km downstream. The double-diffusion-induced nitrate flux is estimated to be 1-10mmolNm−2day−1 in the pycnostad 26–26.5kgm−3 of the Kuroshio Extension, suggesting that whether this double-diffusion-induced nutrient flux in the subsurface layers can ultimately contribute to surface primary production depends on additional eddy up- and northward fluxes
Factors controlling the geographical distribution of fluorescent dissolved organic matter in the surface waters of the Pacific Ocean
No full textDissolved organic matter (DOM) in the epipelagic ocean is produced by marine organisms and consumed by microbes. Thus, the distributional patterns of DOM quantity and quality in surface waters are possibly related to marine ecological provinces. In this study, surface waters collected throughout the Pacific Ocean were used to investigate the geographical distributions of fluorescent DOM (FDOM) quantity and quality. An excitation-emission matrix and parallel factor analysis revealed two humic-like and two protein-like components. The levels of humic-like components showed clear meridional trends with some zonal variability and were highest and lowest in the northern high-latitude and the subtropical provinces, respectively. The photochemical reactivity, determined by the ratio of two humic-like components, was found to be lowest in the subtropical provinces, implying that the major factor controlling the geographical distribution of humic-like components is the degree of photobleaching. The distributional patterns differed between levels of two protein-like components, i.e., tryptophan-like and tyrosine-like. The ratio of tyrosine-like to tryptophan-like components was established as a possible indicator of microbial degradability, and the highest ratio occurred in subtropical provinces. A negative correlation was found between this ratio and the chlorophyll a concentration. Such geographical distributions of protein-like components imply that relatively recalcitrant protein-like components are distributed uniformly throughout the surface waters, but substantial contributions of reactive fractions occur in regions characterized by high biological production. Cluster analysis with the FDOM composition clarified that the diagenetic states of DOM were similar and variable in the northern high-latitude and the subtropical provinces, respectively
Assessment of potential phagotrophy by pico- and nanophytoplankton in the North Pacific Ocean using flow cytometry
No full textMixotrophic phytoplankton are an important component of the microbial communities of the open ocean surface. They can control the metabolic balance and biogeochemical cycling of biogenic materials. To identify environmental factors that control the ecological prosperity of mixotrophs over obligate autotrophs in the open ocean, we quantified potential mixotrophic picoand nanophytoplankton throughout a wide area of the North Pacific Ocean on latitudinal and longitudinal transects, using a combination of flow cytometry and the fluorescent probe LysoTracker Green. The proportions of potential mixotrophic pico- and nanophytoplankton were higher in the subtropical gyre and Bering Sea than in surrounding areas, and even higher in the western area of the subtropical gyre. When the proportion of potential mixotrophs within the surface mixed layer was regressed against various environmental parameters, soluble reactive phosphorus concentration showed a significant negative partial regression coefficient. This result reflected the east-west gradient in the subtropical gyre and is consistent with previous implications that low nutrient availability can facilitate phagotrophy by mixotrophic phytoplankton. Unexpectedly, water stability did not show a significant correlation with the proportion of potential mixotrophs, which suggests that instantaneous physical parameters do not significantly affect their nutritional mode. When the data sets from water below the mixed layer were included in the multiple regression analysis, the significant regression against soluble reactive phosphorus concentration disappeared. This result may suggest a complex response of phytoplankton in the deeper ocean environment. The present results reinforce the importance of phosphorus availability as a control factor of phytoplankton nutritional physiology in the subtropical North Pacific Ocean.</p
Automated colorimetric determination of nanomolar urea in seawater by gas-segmented continuous flow analysis using a liquid waveguide capillary cell
No full textNutrient availability controls phytoplankton populations and their nutritional strategy in the eastern Indian Ocean
No full textThe pico- and nanophytoplankton communities in the eastern Indian Ocean during the fall–winter inter-monsoon season were analyzed using flow cytometry to clarify the environmental factors that control the horizontal and vertical distributions of phytoplankton. The average Synechococcus abundance within the surface mixed layer showed a significant positive correlation with the temperature and nitrate + nitrite (N + N) concentration. Similarly, the cell concentration of eukaryotic phytoplankton in the surface mixed layer was correlated with temperature but did not decrease with decreasing N + N availability. Instead, the proportion of potentially phagotrophic eukaryotic phytoplankton, assessed using a fluorescent probe, increased with decreasing N + N concentrations in the surface mixed layer. This suggested that nitrogen uptake from particles can compensate for the decrease in inorganic nitrogen nutrients in the mixed layer, which may help eukaryotic phytoplankton maintain their biomass in oligotrophic areas. Phagotrophy by eukaryotic phytoplankton in this area may facilitate their growth, with photosynthesis driven by high irradiance within the surface mixed layer, which is depleted of nitrogen. Inter-provincial variations in cell concentrations at the subsurface peak were smaller than those within the surface mixed layer. The cell concentration of Synechococcus at the peak was positively correlated with temperature. By contrast, the peak cell concentration of eukaryotes was positively correlated with light intensity at that depth, suggesting a potential light limitation. The lower potential phagotrophy in eukaryotic phytoplankton with depth suggested that they do not use phagotrophy to compensate for diminished photosynthetic carbon acquisition.Progress in Oceanography, 240, art. no. 103628; 2025journal articl
Phosphorus stress of microphytoplankton community in the western subtropical North Pacific
No full textAbundance and calculated biomass of diazotrophs and their nitrogen fixation in the tropical and subtropical western North Pacific
No full textRichelia reported as number of heterocysts, converted to number of cells on 1:5 basis
Nitrogen fixation rate of diazotrophs and their nitrogen fixation in the tropical and subtropical western North Pacific
No full textNitrogen fixation rate of diazotrophs and their nitrogen fixation in the tropical and subtropical western North Pacifi
Stable isotopic evidence for the differential contribution of diazotrophs to the epipelagic grazing food chain in the mid‐Pacific Ocean
No full textSensitive determination of total particulate phosphorus and particulate inorganic phosphorus in seawater using liquid waveguide spectrophotometry
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