Search

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 FOMvisdistribution 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 FOMviswere 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 FOMviswere observed, accompanied by negative N*, an indicator of denitrification, and phosphate and silicic acid inputs from anoxic sediments. These observational results imply that FOMvisobservation via in situ sensors can be useful for tracing the input of nutrient‐rich sediment‐affected water masses from coastal areas to offshore oceans. 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. 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 A portion of the fluorescent organic matter in the Kuroshio‐Oyashio transition zone could be laterally transported from coastal areas High levels of fluorescent organic matter in subsurface layers in coastal regions are linked with sediment‐derived nutrients Sensor‐based observation of fluorescent organic matter can be used to effectively trace the export of coastal sediment‐derived nutrients

The impact of mesoscale eddies on phytoplankton communities attracts considerable research attention because phytoplankton play numerous roles in marine ecosystems. Using remote sensing techniques, this study considered a synoptic relationship between phytoplankton size structure, which is a determinant of energy transfer efficiency in marine ecosystems, and mesoscale eddies, which are ubiquitous ocean features. We found clear spatial variation in the impacts of mesoscale eddies on phytoplankton size structure; phytoplankton communities with larger cell sizes were supported by anticyclonic eddies in oligotrophic regions and by cyclonic eddies in eutrophic regions. Differences in the phytoplankton size structure within these two types of mesoscale eddies became greater, accompanied by interannual trends in the phytoplankton size structure for 2003–2014. Our findings are the first to demonstrate the important linkage between mesoscale eddies and phytoplankton size structure on a global scale; this new knowledge might help better predict variability in marine ecosystems. Mesoscale eddies are ubiquitous ocean features and they play a significant role in phytoplankton dynamics. This study used satellite remote sensing techniques to examine the impact of mesoscale eddies on phytoplankton size structure, an important factor of marine ecosystems. We found clear regional and temporal variations in the impacts of two types of mesoscale eddies: communities of larger‐sized phytoplankton were observed in anticyclonic eddies in oligotrophic regions and in cyclonic eddies in eutrophic regions, and differences in the phytoplankton size structure inside these two eddy types increased during the period 2003–2014. Our findings are the first to demonstrate the important linkage between the cell‐size composition of phytoplankton communities and mesoscale eddies on a global scale; this new knowledge might help us to better predict variability in marine ecosystems both now and in the future. This study presents the first findings on the linkage between phytoplankton size and mesoscale eddies on a global scalePhytoplankton communities with larger cell sizes were supported by anticyclonic (cyclonic) eddies in oligotrophic (eutrophic) regionsThese differences in phytoplankton size structure between two types of mesoscale eddies became greater over the 2003–2014 time period

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 FOMHas a third parameter of water mass analysis. The levels of FOMHwere different among specific water masses in the region, and FOMHseemed to behave semiconservatively in the shelf regions of the Bering and Chukchi Seas during the early summer of 2013. The distributional pattern of FOMHimplies that FOMHestimated 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 FOMHlevels 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. 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. 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

A fixed‐point observation station was set up in the northern Chukchi Sea during autumn 2013, and for about 2 weeks conductivity‐temperature‐depth (CTD)/water samplings (6 h) and microstructure turbulence measurements (2 to 3 times a day) were performed. This enabled us to estimate vertical nutrient fluxes and the impact of different types of turbulent mixing on biological activity. There have been no such fixed‐point observations in this region, where incoming low‐salinity water from the Pacific Ocean, river water, and sea‐ice meltwater promote a strong pycnocline (halocline) that stabilizes the water column. Previous studies have suggested that because of the strong pycnocline, wind‐induced ocean mixing could not change the stratification to impact biological activity. However, the present study indicates that a combined effect of an uplifted pycnocline accompanied by wind‐induced inertial motion and turbulent mixing caused by intense gale‐force winds (>10 m s−1) did result in increases in upward nutrient fluxes, primary productivity, and phytoplankton biomass, particularly large phytoplankton such as diatoms. Convective mixing associated with internal waves around the pycnocline also increased the upward nutrient fluxes and might have an impact on biological activity there. For diatom production at the fixed‐point observation station, it was essential that silicate was supplied from a subsurface silicate maximum, a new feature that we identified during autumn in the northern Chukchi Sea. Water mass distributions obtained from wide‐area observations suggest that the subsurface silicate maximum water was possibly derived from the ventilated halocline in the Canada Basin. Ocean and biological responses to strong‐wind events were observed in the ArcticWind‐induced ocean mixing and inertial motion impact primary productionSubsurface silicate maximum plays an important role in diatom production

Both historic and currently operational chlorophyll algorithms of the satellite-borne ocean color sensors, such as SeaWiFS, were evaluated for in situspectral radiation and chlorophyll data in some Case I waters, including the waters in the Indian Ocean sector of the Southern Ocean. Chlorophyll aconcentration of the data set (n= 73) ranged from 0.04 to 1.01 mg m−3. The algorithms had higher accuracy for the low- and mid-latitude waters (RMSE: 0.163–0.253), specifically the most recently developed algorithms of OCTS and Sea WiFS showed 0.163 and 0.170 of Root Mean Square Errors, respectively. However, these algorithms had large errors (0.422–0.621) for the Southern Ocean data set and underestimated the surface chlorophyll by more than a factor of 2.6. The absorption coefficients in the blue spectral region retrieved from remote sensing reflectance varied in a nonlinear manner with chlorophyll aconcentration, and the value in the Southern Ocean was significantly lower than that in the low- and mid-latitude waters for each chlorophyll aconcentration. The underestimation of chlorophyll aconcentration in the Southern Ocean with these algorithms was caused by the lower specific absorption coefficient in the region compared with the low- and mid-latitude waters under the same chlorophyll aconcentration.

Surface temperature, salinity, concentrations of silicate (Si) and nitrate + nitrite (N), and in vivofluorescence (Fluor) were investigated in the marginal ice zone (MIZ) and the seasonally open oceanic zone (SOOZ) (32–40°E, 64–69°S) from February 23 to 28 1992. In the MIZ the mean Si and N were 67.8 ± 2.2 μM and 32.5 ± 1.7 μM, respectively. There was a trend that low N values coincided with high Fluor values. Observation conducted at one point (64°S, 38°E) revealed a diel variation pattern in Fluor. Applying this pattern of deviation from noon value, all Fluor data were normalized to value at local noon. In the MIZ a significant negative correlation was observed between the normalized Fluor and N but not Si. On the other hand, Si decreased continuously from south to north in the SOOZ and was negatively correlated with the normalized Fluor. Difference in Si concentration was about 30 μM between the sea around 64°S and the MIZ, while the difference in N concentration was estimated as less than 10 μM. If diatoms take up silicate and nitrogen at an approximate ratio of 1:1, additional nitrogenous nutrients other than nitrate and nitrite (e.g. ammonia, urea etc.) would be required. In this case, an f-ratio of lower than 33% is obtained. It is suggested that in the MIZ abundance of phytoplankton community dominated by non-diatom increases utilizing nitrate while in the SOOZ abundance of phytoplankton community dominated by diatoms increases consuming Si and regenerated nitrogen.