The meridional distributions of fluorescent dissolved organic matter (FDOM) and various hydrologic properties were investigated along 67°E in the western Indian Ocean. Our results showed that the ...highest fluorescence of the humic FDOM (FDOMH) was discovered in the Indian Deep Water (IDW), and relatively lower values were observed in the intruding water masses from the upper layer (e.g., Circumpolar Deep Water (CDW), Antarctic Intermediate Water (AAIW), and South Indian Central Water (SICW)). The deep FDOMH was robustly correlated with apparent oxygen utilisation (AOU), as suggested by previous studies. In particular, the slopes of the regression line AOU on FDOMH varied for different water masses and the two humic components. In this study, to identify the factor inducing the variations of the slope, we estimated the relative water mass fraction of different water masses using a three-end-member mixing model with a salinity-FDOMH diagram. The distribution of water mass fractions was in good agreement with water mass distribution from the conventional method from temperature and salinity distribution and previous studies. The FDOMH components were positively correlated with the aged water mass fraction (i.e., IDW; r = 0.93) and negatively correlated with fresher ones originating from the upper water (r = −0.93, −0.51, and − 0.95 for CDW, AAIW, and SICW, respectively). The fluorescence ratio between the two FDOMH components was also observed to be linked to the water mass fractions. The results indicate that the distribution of FDOMH is attributed to the mixing of various deep-water masses during the global ocean circulation.
•Humic FDOM is enriched in IDW, but relatively depleted in AAIW, SICW, and CDW.•The slope of the humic FDOM-AOU relationship varies with depth and water masses.•Water mass fraction of Indian deep water was defined using salinity and humic FDOM.•The major factor controlling the deep FDOM distribution is water-mass mixing.
There is an increasing emergence of antibiotic-resistant Vibrio alginolyticus, a zoonotic pathogen that causes mass mortality in aquatic animals and infects humans; therefore, there is a demand for ...alternatives to antibiotics for the treatment and prevention of infections caused by this pathogen. One possibility is through the exploitation of bacteriophages. In the present study, the novel bacteriophage pVa-21 was classified as Myoviridae and characterised as a candidate biocontrol agent against V. alginolyticus. Its morphology, host range and infectivity, growth characteristics, planktonic or biofilm lytic activity, stability under various conditions, and genome were investigated. Its latent period and burst size were estimated to be approximately 70 min and 58 plaque-forming units/cell, respectively. In addition, phage pVa-21 can inhibit bacterial growth in both the planktonic and biofilm states. Furthermore, phylogenetic and genome analysis revealed that the phage is closely related to the giant phiKZ-like phages and can be classified as a new member of the phiKZ-like bacteriophages that infect bacteria belonging to the family Vibrionaceae.
The carbon monoxide (CO) in the marine boundary layer and in the surface waters and water column were measured along the western limb of the North Pacific from the Korean Peninsula to Alaska, USA, in ...summer 2012. The observation allows us to estimate the CO budgets in the surface mixed layer of the three distinct regimes: the East Sea (Sea of Japan) (ES), the Northwest Pacific (NP), and the Bering Sea (BS). CO photochemical production rates were 56(±15) µmol m−2 d−1, 27(±3) µmol m−2 d−1, and 26(±2) µmol m−2 d−1, while microbial consumption rates were 30(±8) µmol m−2 d−1, 24(±5) µmol m−2 d−1, and 63(±19) µmol m−2 d−1 in the ES, NP, and BS, respectively, both of which are the dominant components of the CO budget in the ocean. The other two known components, air–sea gas exchange and downward mixing, remained negligible (less than 3 µmol m−2 d−1) in all regimes. While the CO budget in the surface mixed layer of the NP was in balance, the CO production surpassed the consumption in the ES, and vice versa in the BS. The significant imbalances in the CO budget in the ES (25 ± 17 µmol m−2 d−1) and the BS (40 ± 19 µmol m−2 d−1) are suggested to be compensated by external physical transport such as lateral advection, subduction, or ventilation. Notably, the increase in the CO column burden correlated with the imbalance in the CO budget, highlighting the significant role of the physical transport in the marine CO cycles. Our observation, for the first time, underscores the potential importance of physical transport in driving CO dynamics in the marine environment.
Abstract
Meridional overturning circulation (MOC) is vital to distributing heat, freshwater, and dissolved matter in semienclosed deep marginal seas such as the East Sea (ES) (Sea of Japan). As our ...understanding of the ES MOC remains incomplete, we attempted to fill this research gap. We analyzed the ES MOC and its decadal change (1993–2012), employing Hybrid Coordinate Ocean Model (HYCOM) global reanalysis. We found that the ES MOC, consisting of two counterrotating overturning cells in the late 1990s, changed into a single full-depth cell in the 2000s and reverted to two cells in the 2010s. The decadal change relates to weakening of the southward western boundary current at the intermediate layer and northward eastern boundary currents at the deep abyssal layer. We propose that surface warming and salinification favored reduced intermediate water formation and enhanced bottom water formation in the northwestern ES in the 2000s and were, therefore, key to the decadal change. Conditions unfavorable to intermediate water formation and favorable to bottom water formation in the winters of the 2000s, compared with the late 1990s, enhanced northward (westward) Ekman transport in the southern (northeastern) ES, successive advection of surface warm, saline water into water formation areas, and air–sea heat and freshwater exchanges linked to the January Arctic Oscillation. Our results indicated that the ES MOC is sensitive to both external atmospheric forcing and internal ES processes, which have implications for significant changes in the response of other marginal seas and global oceans to future climate variability.
We investigated seasonal net community production (NCP) variations in the productive Amundsen Sea Polynya, integrating observational data and ecosystem modeling. NCP estimates (NCPO2/Ar) from in situ ...O2/Ar data during the austral summer (January‐March) from 2011 to 2018 were compared with those from a one‐dimensional ecosystem model. Early January saw the highest NCPO2/Ar values ranging from 115 to 139 mmol O2 m−2 d−1 among observations. Over the summer, NCPO2/Ar gradually decreased, reaching 40 mmol O2 m−2 d−1 by late February. Late summer values, though one‐third of early January, remained notably positive, indicating net autotrophy. This persisted despite sea surface temperature dropping from >−0.4°C in January to −1.33°C in late February. Refining NCPO2/Ar, we modified bacterial dynamics in our ecosystem model. Significantly improved model performance resulted from two key modifications. First, we introduced bacterial uptake dependency on Phaeocystis primary production. Second, we heightened temperature‐dependent bacterial respiration and production approximately fifteenfold. These changes revealed NCP's remarkable sensitivity to minor temperature fluctuations (<1°C). Furthermore, modified bacterial dynamics delayed the net primary production peak by 2 weeks, underlining the importance of phytoplankton‐bacteria interaction in the ocean carbon cycle. Model results estimated annual NCP in the Amundsen Sea Polynya at 4.04 mol C m−2, aligning with summer NCP estimates (0.2–5.9 mol C m−2) in observational study. Our study advances NCP understanding in polar regions, emphasizing comprehensive observations, including bacterial processes, for understanding intricate biotic interactions. These findings align with past observations on bacterial metabolism and Phaeocystis ecological properties in the Antarctic oceans.
Plain Language Summary
Our study delved into the seasonal changes in productivity in the Amundsen Sea Polynya, a bustling region near Antarctica. We combined real‐world observations with computer modeling to unravel these patterns. In early January, this area was super productive, generating lots of oxygen. As summer progressed, productivity slowly declined, but it remained positive even in late summer, despite a significant drop in ocean temperature. This suggested that the ecosystem continued to thrive. To refine our model, we adjusted how bacteria interacted in the ecosystem. The results were fascinating. When we made bacteria more reliant on a specific algae, Phaeocystis, and more sensitive to temperature, our model better matched real‐world conditions. This revealed that even minor water temperature changes, less than 1°C, could significantly impact this ecosystem. Furthermore, altering bacterial dynamics led to a 2‐week delay in the peak period of ocean plant growth, highlighting the crucial role of the interaction between tiny plants called phytoplankton and bacteria in the ocean's carbon cycle. Our study provides valuable insights into polar regions, emphasizing the need for comprehensive observations that include bacterial activities. Understanding these intricate interactions is vital for grasping the bigger picture of life in these remote and extreme environments.
Key Points
Net community production in the Amundsen Sea Polynya was investigated by integrating observational data with ecosystem modeling
Phaeocystis‐driven nutrient remineralization by bacteria stimulates phytoplankton growth
Bacterial respiration peaks within specific temperature range, affecting carbon cycling in Antarctic polynyas
The variation of the North Equatorial Current (NEC) bifurcation is investigated using results from a high‐resolution ocean general circulation model (OGCM). The bifurcation occurs at about 15.5°N for ...the annual average and is easily identifiable in the upper 500 m, but it varies with time and depth. In agreement with recent observations, during the summer season the NEC bifurcation moves equatorward with a weak poleward shift with depth, while a large poleward movement with a poleward shift with depth is found during the winter season. Vertical mode decomposition indicates that the seasonal variation of the NEC bifurcation is dominated by the first two baroclinic modes. On the interannual timescale, the meridional migration of the NEC bifurcation is strongly influenced by El Niño/Southern Oscillation (ENSO); its correlation with the Southern Oscillation Index exceeds 0.8 in magnitude at depths around the thermocline. The NEC bifurcation occurs at its northernmost position during El Niño years and at its southernmost position during La Niña years. This variation is mainly accounted for by westward propagation of upwelling (downwelling) Rossby waves generated by winds in the central equatorial Pacific and by an anomalous anticyclone (cyclone) located in the western North Pacific when a warm (cold) event matures. The interannual variability of the NEC transport is highly correlated with that of the Mindanao Current (MC) and the Kuroshio transports. It is also found that the interannual variability of the NEC bifurcation latitude is highly correlated with the variations of transports in the NEC and the Kuroshio, but is less correlated with transport variations in the MC.
Information about wind variations and future wind conditions is essential for a monsoon domain such as the Northwest Pacific (NWP) region. This study utilizes 10 Generalized Circulation Models (GCM) ...from CMIP6 to evaluate near-future wind changes in the NWP under various climate warming scenarios. Evaluation against the ERA5 reanalysis dataset for the historical period 1985–2014 reveals a relatively small error with an average of no more than 1 m/s, particularly in the East Asian Marginal Seas (EAMS). Future projections (2026–2050) indicate intensified winds, with a 5–8% increase in the summer season in the EAMS, such as the Yellow Sea, East Sea, and East China Sea, while slight decreases are observed in the winter period. Climate mode influences show that winter El Niño tends to decrease wind speeds in the southern study domain, while intensifying winds are observed in the northern part, particularly under SSP5-8.5. Conversely, summer El Niño induces higher positive anomalous wind speeds in the EAMS, observed in SSP2-4.5. These conditions are likely linked to El Niño-induced SST anomalies. For the application of CMIP6 surface winds, the findings are essential for further investigations focusing on the oceanic consequences of anticipated wind changes such as the ocean wave climate, which can be studied through model simulations.
In this study, a machine learning (ML)-based Tropical Cyclones (TCs) Rapid Intensification (RI) prediction model has been developed by using the Net Energy Gain Rate Index (
NGR
). This index ...realistically captures the energy exchanges between the ocean and the atmosphere during the intensification of TCs. It does so by incorporating the thermal conditions of the upper ocean and using an accurate parameterization for sea surface roughness. To evaluate the effectiveness of
NGR
in enhancing prediction accuracy, five distinct ML algorithms were utilized: Decision Tree, Logistic Regression, Support Vector Machine, K-Nearest Neighbors, and Feed-forward Neural Network. Two sets of experiments were performed for each algorithm. The first set used only traditional predictors, while the second set incorporated
NGR
. The outcomes revealed that models trained with the inclusion of
NGR
exhibited superior performance compared to those that only used traditional predictors. Additionally, an ensemble model was developed by utilizing a hard-voting method, combining the predictions of all five individual algorithms. This ensemble approach showed a noteworthy improvement of approximately 10% in the skill score of RI prediction when
NGR
was included. The findings of this study emphasize the potential of
NGR
in refining TC intensity prediction and underline the effectiveness of ensemble ML models in RI event detection.
The Amundsen Sea continental shelf (ACS) water ecosystem is expected to undergo changes since increasing melt rate of glacier and decreasing sea ice extent by global warming would lead to the ...mitigation of iron and light limitation. We investigated how diatoms and Phaeocystis, two dominant taxa, and primary production in the ACS water would respond to variations in iron and light availabilities by using a 1‐D pelagic ecosystem model. In the model, we added sea ice effects that reduce light penetration and optimized model parameters for diatoms and Phaeocystis. The results from our model showed good agreement with 20‐year observations of Chl‐a as well as the biomass proportion of diatoms and Phaeocystis and nutrient distributions during the growing season. Our model experimental results suggest that the current moderate iron and high light conditions favor the growth of Phaeocystis over diatoms. Moreover, as iron increases, the organic carbon exudation by phytoplankton increases more rapidly than net primary production (NPP), leading to a decline in phytoplankton biomass. On the other hand, irradiance plays a role in controlling NPP in terms of photoinhibition which is reduced by increasing iron. Increases in both iron and irradiance lead to an advance in the timing of the bloom peak (surface Chl‐a maximum) due to increases in phytoplankton carbon loss and photoinhibition. Our results imply that the dominance of Phaeocystis can continue and that the carbon uptake capacity of the ACS in the summer seasons might increase given that iron availability will increase with future climate change.
Plain Language Summary
We firmly believe that this research based on an optimum biogeochemical model developed for the Amundsen Sea continental shelf water is of sufficiently broad interest for the public as well as the scientific community, since it advances the study of phytoplankton ecology and carbon uptake in the Antarctic coastal area. A complex coupled ocean‐ecosystem model was developed to investigate dynamics of the two dominant species (diatoms and Phaeocystis) in the ACS water, one of the most rapidly warming regions on Earth with extremely high primary productivity. These two groups are particularly important as they could significantly contribute to the carbon export into the deep ocean. The model is able to accurately address the temporal evolution of diatoms and Phaeocystis responding to the variations of nutrients, iron, and irradiance. We show these two groups of phytoplankton are controlled by the rate of irradiance and iron availability in terms of the net primary production (NPP), biomass contribution, and bloom period. This research presents new evidence that the short‐term export production of Phaeocystis‐derived carbon can become stronger in the future with increasing basal melt in the coastal waters around the Antarctic continent.
Key Points
One‐dimensional physical‐biogeochemical model successfully reproduced key ecodynamics of diatoms and Phaeocystis in the Amundsen Sea
Current moderate iron availability and excessive light conditions in the Amundsen Sea favor Phaeocystis over diatoms, which would persist
Net primary production of the Amundsen Sea could increase with increasing ice sheets melt in the future
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