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The lateral transport of nutrient‐rich water masses from coastal areas to offshore areas is an important factor controlling the primary production of phytoplankton in offshore regions. If the coastal low‐salinity waters have high nutrient levels, lateral transport can be traced via salinity observations. However, if high level of nutrients in the waters are not coupled with low salinity, for example, input from coastal sediments, water transport cannot be traced via salinity observations. In this study, visible fluorescent organic matter (FOMvis) was monitored by an in situ sensor as a tracer for sediment‐affected water masses from coastal areas. The FOMvis distribution in the upper 500 m water layer in offshore Japan along the 143°E transect was generally controlled by the distributions of subtropical water (i.e., Kuroshio water and Tsugaru warm current water) and subarctic water (i.e., Oyashio water). However, anomalously high levels of FOMvis were observed at depths of 20–60 m at an observational station, which could not be explained by physical mixing. This likely originated from the lateral transport of water masses from coastal areas. In and around Sendai Bay, where observations were carried out as an example of the Japanese coast, high levels of FOMvis were observed, accompanied by negative N*, an indicator of denitrification, and phosphate and silicic acid inputs from anoxic sediments. These observational results imply that FOMvis observation via in situ sensors can be useful for tracing the input of nutrient‐rich sediment‐affected water masses from coastal areas to offshore oceans. Plain Language Summary: Phytoplankton activities in offshore oceans are often controlled by the supply of nutrients. Since the coastal oceans are generally characterized by high levels of nutrients, lateral transport of nutrients from the coastal ocean to the offshore ocean is likely an important nutrient source for phytoplankton, particularly in the boundary region. Rivers and sediments supply nutrients to coastal oceans. Nutrient transport accompanied by river water can be traced by salinity observations using conductivity sensors. However, the transport of sediment‐derived nutrients is difficult to trace via sensor‐based observations. The visible fluorescence of nonliving organic matter (traditionally defined as humic‐like fluorescence) has been suggested to be useful for tracing river water as well as coastal sediment‐affected water in marine environments. Here, we observed the simultaneous presence of sediment‐derived nutrients and visible fluorescent organic matter, which was detected by a sensor in the subsurface layer in the coastal area, that is, in and around Sendai Bay, Japan. A relatively high level of fluorescent organic matter was also observed in the Kuroshio‐Oyashio transition zone. These observational results indicate that fluorescent organic matter as detected by in situ sensors can be useful for tracing inputs of coastal sediment‐derived nutrients to offshore oceans. Key Points: A portion of the fluorescent organic matter in the Kuroshio‐Oyashio transition zone could be laterally transported from coastal areasHigh levels of fluorescent organic matter in subsurface layers in coastal regions are linked with sediment‐derived nutrientsSensor‐based observation of fluorescent organic matter can be used to effectively trace the export of coastal sediment‐derived nutrients [ABSTRACT FROM AUTHOR]

The marginal seas have been found to be important external sources of dissolved iron (Fe) in the North Pacific through circulation of intermediate water. Here, we show comprehensive spatial distributions of dissolved Fe concentrations and Fe(III) solubilities inferred from humic‐like fluorescent dissolved organic matter (FDOMH) over the North Pacific, including the marginal seas, the Sea of Okhotsk, and the Bering Sea. FDOMH was used as a proxy of chemical speciation of dissolved Fe in the intermediate and deep waters where liner relationships were previously observed between FDOMH and Fe(III) solubility. When dissolved Fe concentration exceeds Fe(III) solubility, Fe(III) solubility is assumed to be equivalent to concentration of FDOMH‐Fe complexes, and excess dissolved Fe concentration above the Fe(III) solubility is assumed to be colloidal Fe which is not complexed with FDOMH. In the intermediate water, the dissolved Fe concentration exceeded the Fe(III) solubility in the marginal seas, while excess Fe(III) solubility was evident downstream of the intermediate water circulation, suggesting that the major dissolved Fe chemical form derived from shelf and slope sediments in the marginal seas changed from colloidal Fe to FDOMH‐Fe complexes. With the previous findings, namely the dominance of labile particulate Fe in total Fe in the intermediate water of the Sea of Okhotsk and the Bering Sea, we hypothesized that the average size of sediment‐derived Fe decreases during transportation by intermediate water, most likely due to reversible scavenging with the highest removal rate for labile particulate Fe and the lowest removal rate for FDOMH‐Fe complexes. Plain Language Summary: Iron (Fe) supply from the exterior of the ocean is important in sustaining global primary productivity. However, the distribution/strength of external sources and the mechanism for long‐distance transport of dissolved Fe have not been well documented. We suggest that the major chemical form of marginal sea sediment‐derived dissolved Fe changed from Fe colloids to soluble Fe during long‐distance transport by the intermediate water of the North Pacific. The soluble Fe is likely dissolvable in seawater through forming complex with humic substances which are microbiologically and/or abiotically altered biogenic organic matter. This finding may help marine biogeochemical models better reproduce the Fe cycle. Key Points: Fe(III) solubility was inferred from humic‐like fluorescent dissolved organic matterDissolved Fe concentration exceeded Fe(III) solubility in the intermediate water in the marginal seasFe(III) solubility exceeded dissolved Fe concentration in the intermediate water away from the marginal seas [ABSTRACT FROM AUTHOR]

Interactions between chemodiverse dissolved organic matter (DOM) and biodiverse microbes are governed by a myriad of intrinsic and extrinsic factors which are not well understood. Here, we update and bridge the gap of this interdisciplinary theme comprehensively. At an ecosystem level, aquatic ecosystems dominated by algae-sourced DOM (e.g., eutrophic lake or coastal upwelling areas) harbor more biolabile DOM, such as directly assimilable monomers and readily hydrolysable biopolymers. However, other ecosystems prevailed by DOM supply from soil and vascular plants (e.g., river or wetland) have more biorefractory DOM, such as low molecular weight (LMW) residue of aliphatic C skeletons and geopolymers. A variety of heterotrophic bacteria, archaea, fungi, phagotrophic protists, and even photoautotrophic phytoplankton shows genomic and/or culturing experimental evidence of being able to process a diverse type of organics. The various biodegradable organics have different chemical structures and chemical bonds such as carbohydrates, amino acids, proteins, lignins, lipids, carboxylic acids, humic acids, hydrocarbons, and nanoplastics. Meanwhile, bio-production of metabolism intermediates and/or biorefractory organics (e.g., carboxyl-rich alicyclic molecules, CRAM) is observed despite general decay of bulk dissolved organic carbon (DOC) during bioassay experiments. In particular, emerging evidence shows that archaea contribute significantly to biomass in the marine mesopelagic zone and subsurface environments and their abundance often increases with depth in sediments. Furthermore, not only intrinsic factors (e.g., DOM composition and structure), but also extrinsic ones (e.g., sunlight and dissolved oxygen) play important roles in interplays between DOM and microbes. [ABSTRACT FROM AUTHOR]

An increasing body of work has shown the potential impacts of subglacial discharge from marine‐terminating glaciers on the marine environment around Greenland. Upwelling of nutrients associated with rising buoyant plumes near the front of marine‐terminating glaciers plays a key role in maintaining the high productivity of connected fjords. The response of protist communities to subglacial discharges into fjords nevertheless remains poorly understood. Here we show data of water properties, nutrients, and protist communities during two summers in 2018 and 2019 in a Greenlandic fjord system fed by marine‐terminating glaciers. This study included the period of intense summer melting of the Greenland Ice Sheet in 2019. The data revealed high nutrient concentrations in 2019 that were attributed to intensified upwelling of nutrients and dissolved iron into the subsurface layer. The source of the iron and the nutrients was subglacial discharge and deep fjord water, respectively. Intense glacial discharges in 2019 mitigated nitrate and phosphate limitations of phytoplankton in the fjord and resulted in an increase of chlorophyll a in the subsurface layer and growth of large diatoms. Heterotrophic protists such as dinoflagellates, tintinnids, and nanoflagellates were more abundant in the summer of 2019. We concluded that nutrient upwelling by subglacial discharges was the major driver of the structure of lower trophic levels in fjord ecosystems. We hypothesize that the more intense melting of glaciers and related increase in subglacial discharge will enhance nutrient upwelling, and increased summer productivity in fjords until the glaciers retreat and terminate on land. Key Points: Higher nutrient concentrations in a Greenlandic fjord with marine‐terminating glaciers were found in 2019 compared to 2018Intense summer melting during 2019 led to higher subglacial discharge and enhanced nutrient upwellingIntensified upwelling of nutrients into the euphotic zone increased productivity of large diatoms [ABSTRACT FROM AUTHOR]

A cruise observation of atmospheric aerosols and gaseous air/seawater compounds was performed in the western North Pacific during the spring pre‐bloom period. The aerosol size distribution and cloud condensation nuclei (CCN) activity were characterized according to the source type: terrestrial/pollution, marine, and mixed. The CCN activity was represented by the aerosol hygroscopicity parameter (ĸCCNC) from the activation diameter (dact) in water‐vapor supersaturation (SS), which is controlled by the composition of particles with sizes around dact. For marine type, whereas a high hygroscopicity parameter value of 0.86 for accumulation mode (ĸaccum) suggests a strong influence of sea salt and sulfate, a lower value of 0.49 in Aitken mode (ĸAit) suggests the contribution of biogenic organic matter (OM). Lower ĸAit and ĸaccum for terrestrial/pollution type (0.12 and 0.62, respectively) suggests greater contributions of OM. Under the influence of marine sources, ĸCCNC at a dact of ∼110 nm (low SS) was not responsive to the chlorophyll a (Chl‐a) concentration, whereas ĸCCNC at ∼50 nm (high SS) correlated negatively with Chl‐a. The hygroscopicity parameter of OM was estimated as 0.13–0.29 considering the aerosol composition. During a new particle formation event, the ĸCCNC at 0.60% SS increased; nucleated particles grew in size to become CCN. Sensitivity analysis indicated that changes in the size distribution and chemical composition of the aerosol contributed to the increase in CCN concentration. This study highlights the high variability in size distribution and CCN activity of off‐coast aerosols from different sources, including marine biological activities. Key Points: The size distribution and hygroscopicity parameter κ of aerosols were characterized by terrestrial and marine sourcesThe κ values of organic matter, presumably of marine origin, varied between 0.13 and 0.29New particle formation was a contributing factor to the high variability in the cloud condensation nuclei concentration [ABSTRACT FROM AUTHOR]

GEOTRACES is an international collaboration project aiming to unravel the distributions of trace elements and their isotopes in the global ocean. As part of the GEOTRACES‐Japan project, we carried out two surveys in the subarctic Pacific in 2012 and 2017. Here, we synthesize the outcomes of research completed so far associated with these surveys. Together our findings revealed that the Subarctic Pacific Intermediate Water is an efficient carrier of trace metals. Trace metals such as iron and manganese originating in coastal regions are transported far into the open ocean due to combinations of favorable conditions. First, the young upper Subarctic Pacific Intermediate Water receives fresh inputs of trace metals, which are stabilized by organic ligands, from terrestrial sources. Meanwhile, the lower Subarctic Pacific Intermediate Water has a relatively low dissolved oxygen concentration, which slows down scavenging processes and allows trace metals to persist in seawater. Decaying sinking organic matter also releases additional trace metals into the intermediate water. The Subarctic Pacific Intermediate Water has unusually high concentrations of dissolved zinc relative to silicate, even though zinc and silicate generally have strongly coupled distribution patterns in the global ocean. We hope that the datasets and insights generated by our surveys and associated studies can contribute to greater understanding of the subarctic Pacific and provide useful information for future studies. [ABSTRACT FROM AUTHOR]

One of the most important breakthroughs in oceanography in the last 30 years was the discovery that iron (Fe) controls biological production as a micronutrient, and our understanding of Fe and nutrient biogeochemical dynamics in the ocean has significantly advanced. In this review, we looked back both previous and updated knowledge of the natural Fe supply processes and nutrient dynamics in the subarctic Pacific and its impact on biological production. Although atmospheric dust has been considered to be the most important source of Fe affecting biological production in the subarctic Pacific, other oceanic sources of Fe have been discovered. We propose a coherent explanation for the biological response in subarctic Pacific high nutrient low chlorophyll (HNLC) waters that incorporates knowledge of both the atmospheric Fe supplies and the oceanic Fe supplies. Finally, we extract future directions for Fe oceanographic research in the subarctic Pacific and summarize the uncertain issues identified thus far. [ABSTRACT FROM AUTHOR]

A large part of marine dissolved organic matter (DOM) is considered to be recalcitrant DOM (RDOM) produced by marine bacteria. However, it is still unclear whether differences in bacterial species and/or physiology control the efficiency of RDOM production. Here, batch culture experiments with glucose as the sole carbon source were carried out using three model marine bacterial strains, namely, Alteromonas macleodii (Alt), Vibrio splendidus (Vib), and Phaeobacter gallaeciensis (Pha). Dissolved organic carbon (DOC) concentrations drastically decreased during the exponential growth phases of these bacteria due to the consumption of glucose. The efficiency of bacterial DOC production at the end of incubation was largely different among the strains and was higher for Vib (20%) than for the other two strains (Alt, 4%; Pha, 6%). All strains produced fluorescent DOM (FDOM), including humic-like FDOM which is considered as recalcitrant component in the ocean, even though the composition of bacterial FDOM was also different among the strains. The efficiency of humic-like FDOM production during the exponential growth phase was different among the bacterial strains; that is, Pha produced humic-like FDOM efficiently compared with the other two species. The efficiency of humic-like FDOM production with mineralization of organic matter was lower during the exponential growth phase than during the stationary phase of Alt and Pha. Four processes for the production of bacterially derived recalcitrant humic-like FDOM are suggested from this study: (1) production during active growing (in all strains), (2) production with the reutilization of bacterial DOM (Alt), (3) production with the consumption of cellular materials (Pha), and (4) release from lysis (Vib). Our results suggest that bacterial species and physiology can regulate RDOM production and accumulation in the ocean. [ABSTRACT FROM AUTHOR]

This study establishes an optical inversion scheme for deriving the absorption coefficient of colored (or chromophoric, depending on the literature) dissolved organic material (CDOM) at the 440 nm wavelength, which can be applied to global water masses with near-equal efficacy. The approach uses a ratio of diffuse attenuation coefficient spectral end-members, i.e., a short- and long-wavelength pair. The global perspective is established by sampling "extremely" clear water plus a generalized extent in turbidity and optical properties that each span 3 decades of dynamic range. A unique data set was collected in oceanic, coastal, and inland waters (as shallow as 0.6 m) from the North Pacific Ocean, the Arctic Ocean, Hawaii, Japan, Puerto Rico, and the western coast of the United States. The data were partitioned using subjective categorizations to define a validation quality subset of conservative water masses (i.e., the inflow and outflow of properties constrain the range in the gradient of a constituent) plus 15 subcategories of more complex water masses that were not necessarily evolving conservatively. The dependence on optical complexity was confirmed with an objective methodology based on a cluster analysis technique. The latter defined five distinct classes with validation quality data present in all classes, but which also decreased in percent composition as a function of increasing class number and optical complexity. Four algorithms based on different validation quality end-members were validated with accuracies of 1.2 %–6.2 %, wherein pairs with the largest spectral span were most accurate. Although algorithm accuracy decreased with the inclusion of more subcategories containing nonconservative water masses, changes to the algorithm fit were small when a preponderance of subcategories were included. The high accuracy for all end-member algorithms was the result of data acquisition and data processing improvements, e.g., increased vertical sampling resolution to less than 1 mm (with pressure transducer precision of 0.03–0.08 mm) and a boundary constraint to mitigate wave-focusing effects, respectively. An independent evaluation with a historical database confirmed the consistency of the algorithmic approach and its application to quality assurance, e.g., to flag data outside expected ranges, identify suspect spectra, and objectively determine the in-water extrapolation interval by converging agreement for all applicable end-member algorithms. The legacy data exhibit degraded performance (as 44 % uncertainty) due to a lack of high-quality near-surface observations, especially for clear waters wherein wave-focusing effects are problematic. The novel optical approach allows the in situ estimation of an in-water constituent in keeping with the accuracy obtained in the laboratory. [ABSTRACT FROM AUTHOR]

Pacific water is an important nutrient source for sustaining biological production in the Chukchi Sea, western Arctic Ocean, which is one of the productive regions in the world. Therefore, to understand the impacts of future environmental changes on biological production in the sea, it is crucial to understand the origins, modification processes, and spatiotemporal variations of the water masses from the Bering Sea with changes in nutrient concentrations. To improve water mass analysis in the shelf regions of the Bering and Chukchi Seas, we observed levels of humic‐like fluorescent organic matter (FOMH) by using an in situ fluorometer directly connected to a temperature‐salinity sensor during a cruise in the early summer of 2013 and evaluated the potential of FOMH as a third parameter of water mass analysis. The levels of FOMH were different among specific water masses in the region, and FOMH seemed to behave semiconservatively in the shelf regions of the Bering and Chukchi Seas during the early summer of 2013. The distributional pattern of FOMH implies that FOMH estimated by the in situ fluorometer has the potential to (1) separate warm water into riverine‐affected Alaskan Coastal Water and historically photobleached summer Bering Basin Water; (2) distinguish the Anadyr Water, which has low FOMH levels and high nutrient concentrations, from the Bering Shelf Water; and (3) determine different formation/modification processes of dense shelf water that contains high nutrient concentrations. Plain Language Summary: The continental shelves of the northern Bering Sea and the Chukchi Sea are known to be the area characterized by a high abundance of organisms from phytoplankton to marine mammals and seabirds. Such a high density of marine organisms is sustained by intensive primary production induced by inputs of enough nutrients. The inputs of nutrients to the region are possibly sensitive to climate change. To project future changes in nutrient supply to the region, the present mechanism of nutrient supply should be quantitatively understood. To date, however, the mechanism is poorly determined because the origin and modification of water masses that convey nutrients have not been well understood. This study measured humic‐like fluorescent organic matter, products during the decay and transformation of biogenic remains, in addition to temperature and salinity by in situ sensors to characterize the water masses in the shelf region of the Bering and Chukchi Seas during the early summer of 2013. The levels were clearly different among water masses defined by temperature and salinity and characterized by different concentrations of nutrients, indicating that humic‐like fluorescent organic matter is useful to refine the water masses in the region for quantitative understanding of the mechanism of nutrient supply. Key Points: Distributions of fluorescent organic matter in the Bering and Chukchi Seas are measured by an in situ fluorometer to refine water massesFluorescent organic matter is useful for identifying Alaskan Coastal Water, summer Bering Basin Water, and nutrient‐rich Anadyr WaterFormation/modification processes of dense shelf water that is rich in nutrients can be evaluated by fluorescent organic matter [ABSTRACT FROM AUTHOR]

This study establishes an optical inversion scheme for deriving the absorption coefficient of colored (or chromophoric, depending on the literature) dissolved organic material (CDOM) at the 440 nm wavelength, which can be applied to global water masses with near-equal efficacy. The approach uses a ratio of diffuse attenuation coefficient spectral end members, i.e., a short and long wavelength pair. The global perspective is established by sampling "extremely" clear water plus a generalized extent in turbidity and optical properties that each span three decades of dynamic range. A unique data set was collected in oceanic, coastal, and inland waters (as shallow as 0.6 m) from the North Pacific Ocean, the Arctic Ocean, Hawaii, Japan, Puerto Rico, and the east and west coasts of the United States. The data were partitioned using subjective categorizations to define a validation quality subset of conservative water masses, i.e., the inflow and outflow of properties constrain the range in the gradient of a constituent, plus 15 subcategories of water masses that were not evolving conservatively. The dependence on subcategories was confirmed with an objective methodology based on cluster analysis techniques. The latter defined five distinct classes with validation quality data present in all classes, but which also decreased in percent composition as a function of increasing class number and optical complexity. Four different algorithms based on different validation quality end members were validated with accuracies of 1.-6.2 %, wherein pairs with the largest spectral span were most accurate. Although algorithm accuracy decreased with the inclusion of more subcategories containing non-conservative water masses, changes to the algorithm fit were small when a preponderance of subcategories were included. The high accuracy for all end-member algorithms was the result of data acquisition and data processing improvements, e.g., increased vertical sampling resolution to less than 1mm and a boundary constraint to mitigate wave focusing effects, respectively. An independent evaluation with a historical database confirmed the consistency of the algorithmic approach and its application to quality assurance, e.g., to flag data outside expected ranges, identify suspect spectra, and objectively determine the in-water extrapolation interval by converging agreement for all applicable end-member algorithms. The legacy data exhibit degraded performance (as 44 % uncertainty) due to a lack of high-quality near-surface observations, especially for clear waters wherein wave-focusing effects are problematic. The novel optical approach allows the in situ estimation of an in-water constituent in keeping with the accuracy obtained in the laboratory. [ABSTRACT FROM AUTHOR]