The coast of Hainan Island is dominated by embayed beaches, which is an important coastal tourist destination in China and also has a high incidence of drowning accidents caused by rip currents. To investigate the distribution characteristics of rip currents on embayed beaches, the occurrence and distribution characteristics of rip currents are interpreted from the satellite images of 54 embayed beaches around Hainan Island over the past 20 years, and compared with the field survey results. The results show that, in terms of regional distribution, the occurrence probability of rip currents on the eastern and southern coasts is significantly higher than that on the western and northern coasts of Hainan Island. In terms of location distribution, the number of rip currents in the middle of the headland bays is higher than that on the two sides. In terms of geometric characteristics, the average rip length was positively correlated with the average rip width, and the average rip spacing. The distribution density of rip currents is negatively correlated with significant wave height, average wave period, incident wave power, and wave energy. The scale of the headland has a significant effect on the number of rips, which is positively correlated with the bay width, the maximum bay indentation, and length of the headland, but the curvature of the shoreline has no significant correlation with the average number of rips. The results of satellite image interpretation for the occurrence of rips are consistent with the results of the field survey and the evaluation of the Ω-RTR model. These conclusions can provide useful references for the beach safety management and rip current warning work on Hainan Island.

In order to investigate the characteristics and spatial-temporal variations of the phytoplankton community in artificial reef areas, as well as to elucidate the relationship between phytoplankton abundance and environmental factors associated with artificial reef construction, four surveys were conducted in 2021 (May, August, November) and 2022 (January) at two artificial reef areas and a control area in Xiangyun Bay. A total of 70 phytoplankton taxa belonging to 39 genera and 3 classes were identified in this study. The annual average abundance of phytoplankton in the artificial reef areas was recorded as 313.5 × 10⁴ cells/m³, which were 1.4 times higher than that observed in the control area. Except in spring, the richness index, diversity index and evenness index of phytoplankton in the artificial reef areas were higher than those in the control area. The succession rate of dominant species from spring to summer and from summer to autumu in the reef areas were lower than that in the control area, suggesting greater stability of community structure within artificial reef areas compared to the control area. The biological increment index for each phytoplankton taxon ranged from 0.9 to 3.6; notably, Bacillariophyta displayed an average biological increment index value of 1.8. Pearson correlation analysis revealed that phytoplankton abundance was primarily influenced by TP, TN, NH₄-N, NO₃-N and DIP; significant seasonal differences were observed among these variables. These findings demonstrate that artificial reef construction has a positive conservation effect on phytoplankton communities closely related to temporal and spatial changes in nutrient availability.

Phosphonates in the ocean are a kind of potential phosphorus (P) source which could be utilized by phytoplankton. Although dinoflagellates cannot directly utilize phosphonates themselves, their symbiotic bacteria have the capability to degrade phosphonates into phosphate, thereby promoting the growth of algal cells. However, no studies focusing on a specific bacteria strain have been conducted thus far. In this study, Prorocentrum donghaiense was cultured under conditions with 2-Aminoethylphosphonic acid (2-AEP) as the sole P source. Isolation and purification of the symbiotic bacteria from the culture was conducted and five kinds of bacteria were obtained. Genome sequencing results revealed the presence of two types of C-P lyase pathways in the bacterial strain designated as Yoonia sp. PD-AEP-1. The function of the bacteira strain was verified through the co-culture of bacteria and algal cells. The results demonstrated that after the algal cells were treated to phosphorus-starved condition, when 2-AEP and the bacteria suspension were added together, as compared to conditions which only 2-AEP or the bacterial suspension of PD-AEP-1 was introduced, both the growth rate of algal cells and the phosphate concentration in the cultures showed a significant increase. Meanwhile, alkaline phosphatase activity and non-photochemical quenching of the algal cells decreased significantly, indicating that PD-AEP-1 has the ability to degrade 2-AEP into phosphate, thereby alleviating phosphorus limitation for P. donghaiense cells and effectively promoting the growth of algal cells. The study suggests that symbiotic bacteria of P. donghaiense might play a part in providing P sources to the algal cells through the degradation of phosphonates. This process could probably contribute to the outbreak of P. donghaiense bloom, highlighting the importance of algae-bacteira interactions in marine ecosystems.

Benthic harpacticoid copepods are widely distributed in marine ecosystem, which is particularly abundant in the epilithic algal matrix(EAM) of coral reefs. Due to its unique habitat, there is currently limited research on its developmental process and cultivation methods. In this study, we collected epilithic algal matrix from the coral reef of Luhuitou, Sanya, Hainan in the summer of 2023, from which we isolated one species of Harpacticoida, which was belonging to the genus Robertgurneya based on morphological characteristics. The effects of mono-and mixed-algal cultures on the developmental dynamics of Robertgurneya sp. were observed and recorded. The results showed that the adult body length of the Robertgurneya sp. was 0.5−0.7 mm, with an average life cycle of about 61−68 days. There were no significant differences in egg number per female each time, brood number, reproductive cycle, and life cycle between different feed cultivation techniques. However, under mixed algae culture, the average egg diameter, larval survival rate, and maximum body length of nauplii stage VI in the embryonic development stage of the Robertgurneya sp. were significantly higher than those in the mono-algae culture group (p < 0.05). Moreover, the embryonic development time and cumulative copepodid development time were significantly shorter under mixed-algal culture (p < 0.05). The results indicate that Robertgurneya sp. in EAM have a short life cycle and strong reproductive ability. Considering the abundant benthic microalgae and organic debris in the mats, harpacticoida may be able to provide huge potential food resources for predators such as small fish in coral reefs. Therefore, it may play an important role in the material cycle and energy flow of coral reef ecosystems.

In order to investigate the impact of climate change on the catch of bigeye tuna, we utilized the annual Pacific bigeye tuna catch data from 1960 to 2021, which was statistically compiled by the Western and Central Pacific Fisheries Commission. We also employed corresponding monthly climate indices, including Niño1+2, Niño3, Niño4, Niño3.4, Southern Oscillation Index (SOI), North Atlantic Oscillation (NAO), Pacific Decadal Oscillation (PDO), North Pacific Index (NPI), and global sea-air temperature anomaly (dT). By using a BP neural network and variable sensitivity analysis, we examined the relationship between these low-frequency climate factors and bigeye tuna catch. Our findings revealed that Niño1+2, SOI, NAO, PDO, NPI, and dT are relatively independent climate factors that have an impact on bigeye tuna catch. The optimal lag orders for these climate factors were determined to be 8 years for Niño1+2, 2 years for SOI, 9 years for NAO, 0 years for PDO, 9 years for NPI, and 3 years for dT. Among these factors, Niño1+2, SOI, and NAO were identified as the key climate factors influencing bigeye tuna catch. We constructed an optimal BP neural network model with a structure of 6-8-1, and the ratio of the difference between the predicted and actual bigeye tuna catch to the actual catch has been maintained within 15% since 1971. Additionally, the trend of the predicted and actual catch was found to be basically consistent, indicating a satisfactory level of model fit.

Sea ice is an indicator of global climate change, and the change of Arctic sea ice is related to global warming and sea level rise. Aiming at the problems such as inaccuracy and slow speed of extracting details from sea ice by traditional semantic segmentation model, an improved DeepLabV3+ sea ice extraction method was constructed. Firstly, we replaced the Xception backbone network with MobileNetV2, which significantly reduces the network’s parameter count and save time while maintaining the accuracy of sea ice extraction. Secondly, we enhanced the ASPP module to DenseASPP, further expanding the receptive field during multi-scale feature extraction for sea ice, resulting in denser features. Lastly, we introduced a coordinate attention mechanism to strengthen the focus on both channel and spatial features, enhancing the extraction of fine edge details in sea ice. The Greenland Sea in the Arctic is selected as the experimental area, and 10 Sentinel-1A dual-polarization SAR images from the winter of 2020 to 2022 in the sea area are processed and labeled to form a data set for the experiment, we compared our method with classic models such as U-Net, PSPNet and DeepLabV3+. The results showed that our method achieved anmIoU of 88.46% and an mPA of 94.16%. Compared to the traditional DeepLabV3+, mIoU increased by 2.35%, mPA increased by 2.90%, and the parameter count and GFLOPs decreased 45.08 M and 106.01 G, respectively. Meanwhile, the training time and sea ice extraction time decreased by 68% and 30%, respectively. Compared to U-Net、PSPNet and other models, the optimal results are also obtained. Compared with other models, the new model constructed in this paper has a stronger learning ability about sea ice characteristics, can obtain more detailed information of sea ice and greatly saves time, and can provide technical support for the study of sea ice degradation monitoring under global warming environment.

Coastal Acoustic Tomography (CAT) is an effective tool to observe the flow field in the large offshore range using high-frequency acoustic signals, of which direct observation range is still limited. The numerical ocean model provides a large-scale ocean background field with simulation errors, and the resolution and accuracy of the flow field results can be improved by assimilating the CAT data with the ocean background results. In this paper, we applied a method to obtain a larger range of two-dimensional ocean flow field results by fitting ocean-mode flow field results using Stream Function and assimilating CAT data using the Ensemble Kalman Filtering algorithm. The assimilation study used the unstructured grid Finite-Volume Community Ocean Model (FVCOM) as the background field, and the four CAT stations experiment conducted in Bali Strait, Indonesia, from 1st to 3rd June 2016 as the observational data. After fitting background field by Stream Function and assimilating CAT data, the two-dimensional flow field in Bali Strait is obtained. The assimilation results were compared with those of the same period of observation and tide level data, which is found that the flow function fitted and assimilated flow field can more accurately describe the high and low tides and flow conditions in the Bali Strait. By introducing the functional relationship between the CAT data and the flow field it can effectively reduce the error of the ocean model and the sparsity of the original observation data.

Tides act an important role in the transfer of ocean energy and mixing, and provide the main energy to maintain the global thermohaline circulation and influence the global ocean circulation. Previous work has explored the sensitivity of ocean circulation states to tidal forcing within an individual ocean model at a low resolution. To further investigate the influence of tidal forcing on ocean circulation and climate states, it is imperative to incorporate the tidal forcing into a coupled climate model. In this paper, the eight major equilibrium constituents are included into the coupled climate model FGOALS-g3 explicitly, and we evaluate its ability to simulate global ocean tides, which lays the basic for the further research on the influence of tidal forcing on large-scale circulation and climate states.We apply tidal harmonic analysis on the sea surface height data to obtain the harmonic constants of each constituent, and compare the model results with the global tidal models TPXO9 and FES2014, and the open ocean tide dataset from st102. The results show that the coupled model FGOALS-g3 can effectively simulate the barotropic tides in the global ocean, with relatively small errors compared to the global tidal models and the observation dataset. Compared with these two global tidal models, the mean square error is relatively small, and the errors are mostly distributed in the region of larger amplitudes. And compared with st102 dataset, the average amplitude relative errors of the eight major equilibrium constituents simulated by FGOALS-g3 are all less than 10%, and the total mean square errors are all less than 10 cm.

When black carbon deposits on snow/ice surface, it can reduce the albedo and increase the absorption of shortwave radiation. The changes in black carbon and their impact on the sea ice melting process are worth investigating. Study of the influence of black carbon in the Arctic Ocean was conducted using the CICE sea ice model. The results indicates that under the impact of black carbon deposition from different sources, from 1980 to 2014, the simulated summer albedo of the Arctic Ocean decreased by 0.82% to 1.71%, ultimately causing a decrease in sea ice extent by 0.97%−1.93%. In the Barents Sea, Kara Sea, and Laptev Sea, the summer sea ice area reduction caused by black carbon is approximately 2–3 times greater than the overall reduction in the Arctic Ocean. The simulation results under different black carbon deposition all show that from 1980 to 1995, the impact of black carbon on albedo in the Arctic exhibited a decreasing trend. However, from 1996 to 2014, the black carbon effect shifted to an increasing trend. In low-latitude regions, due to the retreat of sea ice, the effect of black carbon showed a decreasing trend, while in high-latitude regions, due to the cumulative effect of black carbon in multi-year ice, the radiative impact of black carbon showed an enhancing effect.

Based on the Acoustic Doppler Current Profiler carried by deep-sea moorings, we investigated the diel vertical migration of the acoustic scattering layer and the impact of internal solitary waves on it in the northern South China Sea. The observational results reveal that the acoustic scattering layer, influenced by zooplankton, ascends to shallower depth within approximately an hour after sunset, remains there throughout the night, then migrates to the deeper depth within about an hour before sunrise. The average migration velocities are 4.7 cm/s (upward) and 5.8 cm/s (downward). Additionally, internal solitary waves observed in the region induce a pair of downward and upward currents, with maximum vertical velocities exceeding 50 cm/s, leading to fluctuations of tens to hundreds of meters in the acoustic scattering layer. The acoustic backscattering strength of the ocean’s upper layer reaches its maximum value at the troughs of internal solitary waves. Further research indicates that the daytime internal solitary waves exhibit a stronger correlation between the vertical velocity and the depth-averaged backscattering strength variation compared to the nighttime internal solitary waves. When the vertical velocities induced by both types of waves are equal, the depth-averaged backscattering strength variations during the day are typically greater than those at night.