Assessing responses of phytoplankton to consecutive typhoons by combining Argo, remote sensing and numerical simulation data

Global marine phytoplankton dynamics analysis with machine learning and reanalyzed remote sensing

Phytoplankton are the world's largest oxygen producers found in oceans, seas and large water bodies, which play crucial roles in the marine food chain. Unbalanced biogeochemical features like salinity, pH, minerals, etc., can retard their growth. With advancements in better hardware, the usage of Artificial Intelligence techniques is rapidly increasing for creating an intelligent decision-making system. Therefore, we attempt to overcome this gap by using supervised regressions on reanalysis data targeting global phytoplankton levels in global waters. The presented experiment proposes the applications of different supervised machine learning regression techniques such as random forest, extra trees, bagging and histogram-based gradient boosting regressor on reanalysis data obtained from the Copernicus Global Ocean Biogeochemistry Hindcast dataset. Results obtained from the experiment have predicted the phytoplankton levels with a coefficient of determination score (R2) of up to 0.96. After further validation with larger datasets, the model can be deployed in a production environment in an attempt to complement in-situ measurement efforts.

Tracks of typhoon movement (left and right sides) control marine dynamics and eco-environment in the coastal bays after typhoons: A case study in Zhanjiang Bay

Coastal oceans are highly responsive to typhoons, making them one of the most affected regions. However, our understanding of the impact of typhoon intensity and movement path on marine dynamic processes and eco-environmental factors remains limited because there are very few on-site investigations, especially continuous field observations in the bay during typhoon events. This study investigated dual water isotopes through a continuous survey (with a 5-day interval) during ten cruises in Zhanjiang Bay, associated with two typhoons of varying intensities and landing tracks (left and right sides). After typhoons, the water mass mixing intensified and lasted for several weeks, depending on the intensity of typhoons. During the typhoon periods, there was a considerable increase in contributions from freshwater to nutrient loads; however, this contribution was higher from the stronger typhoon than the weaker one. The weaker Typhoon Lionrock, which landed on the left side of the bay, enhanced the ocean front due to onshore winds induced by the typhoon, causing intrusion of high-salinity seawater into the bay and retaining pollutants in the bay. However, when stronger Typhoon Chaba landed on the right side, offshore winds induced by counterclockwise wind stress during the typhoon resulted in more seawater flowing toward the lower and outer bay. This prevented the forming of an ocean front and played a dilution role in pollutants through its hydrodynamic process. This was primarily due to the fact that the landing trajectory of typhoons directly influenced the direction of seawater flow in Zhanjiang Bay, while the intensity of typhoons further amplifies these flow patterns. This study suggests that tracks of typhoon movement, rather than their intensity and terrestrial runoff, play a crucial role in governing marine dynamics and nutrient supplies in coastal bays during typhoon events.

Thermal bleaching in the northern South China Sea: impact of abnormal environment and climate on high-latitude coral reefs

Extensive coral bleaching events can result in catastrophic degradation of coral reefs and reorganization of coral communities. In the present study, we analyzed the spatial differences in coral bleaching and possible reasons of large-scale coral bleaching, based on the results of a survey carried out in the northern South China Sea in 2020. In addition, we have continuously monitored the sea surface temperature (SST) of the northernmost Weizhou Island for more than six years. The living coral cover at Weizhou Island (W), Xuwen Nature Reserve (X), and Haihua Island (H) was relatively high at 24.6% ± 4.8%, 12.1% ± 3.8%, and 8.1% ± 2.6%, respectively, whereas their bleaching rates were 9.7% ± 2.6%, 9.7% ± 3.3%, and 6.9% ± 2.1%, respectively. Among them, the living coral cover of W was significantly different from those of X and H, whereas the bleaching rate was not significantly different among the three areas. In all three areas, the massive and encrusting corals predominate and exhibit relatively high bleaching rates, with Porites lutea and Bernardpora stutchburyi being the dominant species. In addition, the temperature monitoring results of Weizhou Island for six consecutive years showed that the critical SST of coral bleaching was 31.5 ℃. The monitoring results also showed that the average SST of Weizhou Island was 32.1 ℃, exceeding 32 ℃ in July 2020 for up to 533 h. The longest continuous time when the SST exceeded 32 ℃ was 97 h. These findings indicated that the coral bleaching event that occurred in the Beibu Gulf during 2020 was a large-scale and high-temperature transient event that presented a relatively homogeneous threat to the coral communities. We inferred that this sudden heat stress event was caused by the enclosed tidal current in the Beibu Gulf, which prevented the southern upwelling from reaching the north, as well as by the inability of the SST to decrease without rainfall caused by typhoon cyclones. Our findings suggested that abnormal heat waves can result in coral bleaching at high latitudes and even coral reef degradation. Furthermore, our study provides a new perspective for investigating the self-recovery and reorganization of coral communities following accumulated coral bleaching.

A review of the marine biogeochemical response to typhoons

Typhoons are extreme weather events that can not only affect marine dynamics, but also change marine biogeochemistry, considerably impacting the climate. Based on the satellite remote sensing data, the upwelling of abundant nutrients induced by typhoons from deeper eutrophic water to the upper oligotrophic layer triggers phytoplankton blooms in the upper oceans, thereby increasing new productivity (as a carbon sink). However, field observations have shown that organic matter decomposition (as a carbon source) is the dominant process regardless of whether phytoplankton blooms occur after typhoons, resulting in oxygen consumption in the water column. Therefore, it is particularly important to comprehensively study the coupling mechanisms of biogeochemistry and dynamics in the ocean after typhoons. Here, we present a systematic overview summarizing the effects of typhoons on marine dynamics and biogeochemistry and elaborating on the characteristics and mechanisms of organic matter decomposition induced by typhoons.

Marine environmental monitoring with unmanned vehicle platforms: Present applications and future prospects

Basic monitoring of the marine environment is crucial for the early warning and assessment of marine hydrometeorological conditions, climate change, and ecosystem disasters. In recent years, many marine environmental monitoring platforms have been established, such as offshore platforms, ships, or sensors placed on specially designed buoys or submerged marine structures. These platforms typically use a variety of sensors to provide high-quality observations, while they are limited by low spatial resolution and high cost during data acquisition. Satellite remote sensing allows monitoring over a larger ocean area; however, it is susceptible to cloud contamination and atmospheric effects that subject the results to large uncertainties. Unmanned vehicles have become more widely used as platforms in marine science and ocean engineering in recent years due to their ease of deployment, mobility, and the low cost involved in data acquisition. Researchers can acquire data according to their schedules and convenience, offering significant improvements over those obtained by traditional platforms. This study presents the state-of-the-art research on available unmanned vehicle observation platforms, including unmanned aerial vehicles (UAVs), underwater gliders (UGs), unmanned surface vehicles (USVs), and unmanned ships (USs), for marine environmental monitoring, and compares them with satellite remote sensing. The recent applications in marine environments have focused on marine biochemical and ecosystem features, marine physical features, marine pollution, and marine aerosols monitoring, and their integration with other products are also analysed. Additionally, the prospects of future ocean observation systems combining unmanned vehicle platforms (UVPs), global and regional autonomous platform networks, and remote sensing data are discussed.