Acta Oceanologica Sinica最新文献

Oxygen facilitates the breakdown of the organic material to provide energy for life. The concentration of dissolved oxygen (DO) in the water must exceed a certain threshold to support the normal metabolism of marine organisms. Located in the northern Beibu Gulf, Qinzhou Bay receives abundant freshwater and nutrients from several rivers which significantly influence the level of the dissolved oxygen. However, the spatial-temporal variations of DO as well as the associated driving mechanisms have been rarely studied through field observations. In this study, a three-dimensional coupled physical-biogeochemical model is used to investigate the spatial and seasonal variations of the DO and the associated driving mechanisms in Qinzhou Bay. The validation against observations indicates that the model can capture the seasonal and inter-annual variability of the DO concentration with the range of 5–10 mg/L. Sensitivity experiments show that the river discharges, winds and tides play crucial roles in the seasonal variability of the DO by changing the vertical mixing and stratification of the water column and the circulation pattern. In winter, the tide and wind forces have strong effects on the DO distribution by enhancing the vertical mixing, especially near the bay mouth. In summer, the river discharges play a dominant role in the DO distribution by inhibiting the vertical water exchange and delivering more nutrients to the Bay, which increases the DO depletion and results in lower DO on the bottom of the estuary salt wedge. These findings can contribute to the preservation and management of the coastal environment in the northern Beibu Gulf.

Marine sediments collected from the Zhujiang (Pearl) River Estuary (ZRE) and South China Sea (SCS) were utilized to study the occurrence and spatial distribution of tetrabromobisphenol A (TBBPA) and hexabromocyclododecane (HBCDD). The levels of TBBPA and HBCDD in sediments ranged from not detected (nd) to 6.14 ng/g dry weight (dw) and nd to 0.42 ng/g dw. TBBPA concentrations in marine sediments were substantially higher than HBCDD. The concentrations of TBBPA and HBCDD in the ZRE sediments were significantly greater than those in the SCS. α-HBCDD (48.7%) and γ-HBCDD (46.2%) were the two main diastereoisomers of HBCDD in sediments from the ZRE, with minor contribution of β-HBCDD (5.1%). HBCDD were only found in one sample from the northern SCS. The enantiomeric fraction of α-HBCDD in sediments from the ZRE was obviously greater than 0.5, indicating an accumulation of (+)-α-HBCDD. The enantiomers of HBCDD were not measured in sediments from the SCS. This work highlighted the environmental behaviors of TBBPA and HBCDD in marine sediments.

The sinking of phytoplankton is critical to organic matter transportation in the ocean and it is an essential process for the formation of coastal hypoxic zones. This study was based on a field investigation conducted during the summer of 2022 in the Changjiang River (Yangtze River) Estuary (CJE) and its adjacent waters. The settling column method was employed to measure the sinking velocity (SV) of different size fractions of phytoplankton at the surface of the sea and to analyze their environmental control mechanisms. The findings reveal significant spatial variation in phytoplankton SV (−0.55–2.41 m/d) within the CJE. High-speed sinking was predominantly observed in phosphate-depleted regions beyond the CJE front. At the same time, an upward trend was more commonly observed in the phosphate-rich regions near the CJE mouth. The SV ranges for different size-fractionated phytoplankton, including micro- (>20 µm), nano- (2–20 µm), and picophytoplankton (0.7–2 µm), were −0.50–4.74 m/d, −1.04–1.59 m/d, and −1.24–1.65 m/d, respectively. Correlation analysis revealed a significant negative correlation between SV and dissolved inorganic phosphorus (DIP), implying that the influence of DIP contributes to SV. The variations in phytoplankton alkaline phosphatase activity suggested a significant increase in SV across all size fractions in the event of phosphorus limitation. Phytoplankton communities with limited photosynthetic capacity (maximum photochemical efficience, Fv/Fm < 0.3) were found to have higher SV than that of communities with strong capacity, suggesting a link between sinking and alterations in physiological conditions due to phosphate depletion. The findings from the in situ phosphate enrichment experiments confirmed a marked decrease in SV following phosphate supplementation. These findings suggest that phosphorus limitation is the primary driver of elevated SV in the CJE. This study enhances the comprehension of the potential mechanisms underlying hypoxic zone formation in the CJE, providing novel insights into how nearshore eutrophication influences organic carbon migration.

Eutrophication caused by inputs of excess nitrogen (N) has become a serious environmental problem in Hangzhou Bay (China), but the sources of this nitrogen are not well understood. In this study, the August 2019 distributions of salinity, nutrients [nitrate (NO⁻₃), nitrite, ammonium, and phosphate], and the stable isotopic composition of NO⁻₃ (δ¹⁵N and δ¹⁸O) were used to investigate sources of dissolved inorganic nitrogen (DIN) to Hangzhou Bay. Spatial distributions of nitrate, salinity, and nitrate δ¹⁸O indicate that the Qiantang River, the Changjiang River, and nearshore coastal waters may all contribute nitrate to the bay. Based on the isotopic compositions of nitrate in these potential source waters and conservative mixing of nitrate in our study area, we suggest that the NO⁻₃ in Hangzhou Bay was likely derived mainly from soils, synthetic N fertilizer, and manure and sewage. End-member modeling indicates that in the upper half of the bay, the Qiantang River was a very important DIN source, possibly contributing more than 50% of DIN in the bay head area. In the lower half of the bay, DIN was sourced mainly from strongly intruding coastal water. DIN coming directly from the Changjiang River made a relatively small contribution to Hangzhou Bay DIN in August 2019.

The nutrients from the East China Sea (ECS) through the Tsushima/Korea Strait (TS) strongly impact the ecosystem of the Japan Sea (JS). The complex origins of the Tsushima Warm Current and the various nutrient sources in the ECS result in complex spatial-temporal variations in nutrients in the TS. Using a physical-biological model with a tracking technique, we studied the effects of nutrient sources from the ECS on the TS. Among all the nutrient sources, the Kuroshio has the highest nutrient concentrations in the TS. Its maximum concentration occurs at the bottom, while those of rivers and atmospheric deposition occur at the surface, and that of the Taiwan Strait occurs in the middle layer. The nutrient transport through the TS exhibits similar seasonal variations, as does the volume transport. The transport of nutrients from the Kuroshio accounts for more than 85% of the total. The transport of nutrients from the Taiwan Strait is greater during autumn and winter. The transport of dissolved inorganic nitrogen (DIN) from both rivers and atmospheric deposition through the TS peak in August. Nutrient transport cannot be equated with volume transport. The DIN in the less saline zone originates not only from rivers but also from atmospheric deposition and the Kuroshio. The transport of nutrients from the Taiwan Strait is not as significant as its volume transport in the TS.

Ocean fronts play important roles in nutrient transport and in the shaping ecological patterns. Frontal zones in small bays are typically small in scale, have a complex structure, and they are spatially and temporally variable, but there are limited data on how biological communities respond to this variation. Hangzhou Bay, a medium-sized estuary in China, is an ideal place in which to study the response of plankton to small-scale ocean fronts, because three water masses (Qiantang River Diluted Water, Changjiang River Diluted Water, and the East China Sea current) converge here and form dynamic salinity fronts throughout the year. We investigate zooplankton communities, and temperature, salinity and chlorophyll a (Chl a) in Hangzhou Bay in June (wet period) and December (dry period) of 2022 and examine the dominant environmental factors that affect zooplankton community spatial variability. We then match the spatial distributions of zooplankton communities with those of salinity fronts. Salinity is the most important explanatory variable to affect zooplankton community spatial variability during both wet and dry periods, in that it contributes >60% of the variability in community structure. Furthermore, the spatial distributions of zooplankton match well with salinity fronts. During December, with weaker Qiantang River Diluted Water and a stronger secondary Changjiang River Plume, zooplankton communities occur in moderate salinity (MS, salinity range 15.6 ± 2.2) and high salinity (HS, 22.4 ± 1.7) regions, and their ecological boundaries closely match the Qiantang River Diluted Water front. In June, different zooplankton communities occur in low salinity (LS, 3.9 ± 1.0), MS (11.7 ± 3.6) and HS (21.3 ± 1.9) regions. Although the LS region occurs abnormally in the central bay rather than its apex because of the anomalous influence of rising and falling tides during the sampling period, the ecological boundaries still match salinity interfaces. Low-salinity or brackish-water zooplankter taxa are relatively more abundant in LS or MS regions, and the biomass and abundance of zooplankton is higher in the MS region.

Massive bodies of low-oxygen bottom waters are found in coastal areas worldwide, which are detrimental to coastal ecosystems. In summer 2020, the response of coastal hypoxia to extreme weather events, including a catastrophic flooding, an extreme marine heatwave, and Typhoon Bavi, is investigated based on multiple satellite, four cruises, and mooring observations. The extensive fan-shaped hypoxia zone presents significant northward extension during July–September 2020, and is estimated as large as 13 000 km² with rather low oxygen minimum (0.42 mg/L) during its peak in 28–30 August. This severe hypoxia is attributed to the persistent strong stratification, which is indicated by flood-induced larger amount of riverine freshwater input and subsequent marine heatwave off the Changjiang River Estuary. Moreover, the Typhoon Bavi has limited effect on the marine heatwave and coastal hypoxia in summer 2020.

The Agulhas system is the strongest western boundary current system in the Southern Hemisphere and plays an important role in modulating the Indian-to-Atlantic Ocean water exchange by the Agulhas leakage. It is difficult to measure in situ transport of the Agulhas leakage as well as the Agulhas retroflection position due to their intermittent nature. In this study, an innovative kinematic algorithm was designed and applied to the gridded altimeter observational data, to ascertain the longitudinal position of Agulhas retroflection, the stability of Agulhas jet stream, as well as its strength. The results show that the east-west shift of retroflection is related neither to the strength of Agulhas current nor to its stability. Further analysis uncovers the connection between the westward extension of Agulhas jet stream and an anomalous cyclonic circulation at its northern side, which is likely attributed to the local wind stress curl anomaly. To confirm the effect of local wind forcing on the east-west shift of retroflection, numerical sensitivity experiments were conducted. The results show that the local wind stress can induce a similar longitudinal shift of the retroflection as altimetry observations. Further statistical and case study indicates that whether an Agulhas ring can continuously migrate westward to the Atlantic Ocean or re-merge into the main flow depends on the retroflection position. Therefore, the westward retroflection may contribute to a stronger Agulhas leakage than the eastward retroflection.

Sea ice surface roughness (SIR) affects the energy transfer between the atmosphere and the ocean, and it is also an important indicator for sea ice characteristics. To obtain a small-scale SIR with high spatial resolution, a novel method is proposed to retrieve SIR from Sentinel-1 synthetic aperture radar (SAR) images, utilizing an ensemble learning method. Firstly, the two-dimensional continuous wavelet transform is applied to obtain the spatial information of sea ice, including the scale and direction of ice patterns. Secondly, a model is developed using the Adaboost Regression model to establish a relationship among SIR, radar backscatter and the spatial information of sea ice. The proposed method is validated by using the SIR retrieved from SAR images and comparing it to the measurements obtained by the Airborne Topographic Mapper (ATM) in the summer Beaufort Sea. The determination of coefficient, mean absolute error, root-mean-square error and mean absolute percentage error of the testing data are 0.91, 1.71 cm, 2.82 cm, and 36.37%, respectively, which are reasonable. Moreover, K-fold cross-validation and learning curves are analyzed, which also demonstrate the method’s applicability in retrieving SIR from SAR images.