Southern Yellow Sea Research Articles

An integrated assessment of coastal habitat quality in Nantong, Jiangsu, China

Assessment of coastal habitat quality assessment is of great significance for sustainable coastal development. However, few attempts have been made to conduct an integrated assessment of coastal habitat quality. Consequently, we proposed a novel framework, in which cross-system threats from terrestrial and marine human activities are taken into account. The integrated assessment of the coastal habitat quality was realized by using the intertidal zone as a bridge after evaluating the two subsystems of the terrestrial and marine independently. Nantong, a typical development coast in Southern Yellow Sea, was used as a case study. The results showed that in 2020, the trend of the coastal habitat quality in Nantong steadily improved from the terrestrial to the marine. The distribution of the coastal habitat quality in Nantong was significantly polarized. The direction of the habitat quality distribution in Nantong’s coastal zone was generally northwest-southeast. Human activities and benthic biodiversity are the main factors affecting habitat quality in Nantong’s coastal zone. This study is a useful attempt of cross-system habitat quality assessment, which can support integrated land-sea conservation planning and integrated coastal management.

Combined Effects of Fishing and Environment on the Growth of Larimichthys polyactis in Coastal Regions of China

In fisheries’ stock assessments, the concept of “growth plasticity”—the ability of organisms to modulate their growth rates in response to environmental conditions—has gained attention in recent years. Historically, the impacts of fishing activities and environmental fluctuations were considered separately, while their combined effects have recently come into focus. This study collected 834 adult small yellow croakers (Larimichthys polyactis) from the northern Yellow Sea, the central Yellow Sea, the southern Yellow Sea, and the northern East Sea by trawling during 2020–2021. Using otolith increments as a proxy for annual somatic growth, the study reconstructed otolith chronologies during 2015–2020 for these four stocks. The results of the mixed-effects modeling suggested that temperature during spawning and previous overwintering seasons had comparable importance for the annual growth of small yellow croakers, with higher temperature promoting growth. The growth of small yellow croakers was also found to be correlated with ENSO events, with a lag of 1 to 2 years. A further investigation into combined effects revealed that higher fishing pressure might inhibit the small yellow croaker’s response to favorable environmental conditions. Furthermore, considering the potential differences in growth plasticity among stocks, an analysis was conducted on the spatial variations in growth response to these factors. The analysis revealed that, compared to the stocks in the Yellow Sea, the stock from the East China Sea could exhibit higher growth, superior adaptability to temperature, and a distinctive response to fishing pressure. In conclusion, the present study, while primarily focusing on temperature, preliminarily analyzed the combined effects of fishing and environment and underscored the differences in growth plasticity between stocks in the Yellow Sea and the East China Sea. Despite the limited factors analyzed in this study, it suggests a direction for future studies, highlighting the necessity to include more environmental factors, and even population factors (e.g., the biomass of preys), for a more comprehensive understanding of the combined effects. Based on the observed differences between the two potential subpopulations, this study also provides new insights for the management of the small yellow croaker based on metapopulation dynamics.

Seasonal dynamics of amino acids in the Southern Yellow Sea: Feedback on the mechanism of green tides caused by Ulva prolifera

The biogeochemical processes of amino acids in the Southern Yellow Sea (SYS) have become more dynamic under the influence of the world's largest-scale green tide. The potential relationship between amino acids and green tides has not been effectively assessed, despite its critical importance for exploring dissolved organic matter (DOM) cycling processes in marginal seas. In this study, three cruises were conducted to analyze the concentrations and compositions of total hydrolyzed amino acids (THAAs) in the SYS during the spring, summer, and autumn of 2019. The bioavailability potential of DOM was evaluated using the degradation index (DI) and THAA nitrogen normalized yield (THAA (%DON)) (DON as dissolved organic nitrogen). The variation dynamics of amino acid indicators during different stages of green tide were further explored. The results showed that the THAA concentrations and DOM bioavailability in the SYS were considerably influenced by biological processes. The THAA concentrations (0.96 ± 0.34 μmol L−1) exhibited the lowest mean values in the summer, while the DI values (0.106 ± 0.461) and mean THAA (%DON) values (18.20 ± 6.58 %) were the highest during this season. The distribution of amino acid indicators in the summer (the late-tide stage) was regulated by the green tide mechanism, and kept pace with the green tide floating region. In comparison with the waters in south of 35° N, the THAA concentrations and DI values experienced significant seasonal variations (p < 0.05) in north of 35° N, with the highest DI values (1.217) observed in the green tide aggregation area. This indicates the transformation of nutrient sources for Ulva prolifera in the late-tide stage and its impact on DOM bioavailability. Thus, as a potential feedback indicator of green tides, the study of amino acids is meaningful for understanding the occurrence of green tides and the source-sink pattern of organic nitrogen.

Spatiotemporal Distribution and Dispersal Pattern of Early Life Stages of the Small Yellow Croaker (Larimichthys Polyactis) in the Southern Yellow Sea

Nursery habitats play a significant role in completing fish life cycles, and they are now recognized as essential habitats. Monthly variations in nursery ground distributions of Larimichthys polyactis were investigated in the southern Yellow Sea in 2019. Bayesian hierarchical models with integrated nested Laplace approximation were utilized to model the preferential nursery habitats of L. polyactis larvae. The study analyzed the spatial and temporal distributions of the larvae and juveniles based on three environmental variables: sea surface temperature, sea surface salinity, and depth. Additionally, this study examined the utilization of habitats by different fish life stages and ontogenetic shifts. A total of 3240 individuals were collected from April to June, with the peak occurring in May (0.05 ind./m3), and the distribution areas varied between different months. The prediction of the model reveals the ecological adaptability of L. polyactis to temperature variations. The optimal temperature for L. polyactis density ranges from 12.5 °C to 16.5 °C in April and 16.5 °C to 17.5 °C in May, demonstrating a broad temperature tolerance for L. polyactis survival. In addition, there are variations in distribution patterns among different developmental stages. Larimichthys polyactis spawn in the inshore and nearshore waters, and after hatching, larvae in the pre-flexion stage tend to remain aggregated near the spawning beds. However, larvae in the advanced development stage (post-flexion) and juveniles move towards the sandy ridge habitats along the coast and start to migrate offshore in June. This study provides valuable insights for the effective management of fishery resources in the area and can be utilized to identify marine areas with specific habitat features that require conservation.

Why did the world's largest green tides occur exclusively in the southern Yellow Sea?

The world's largest green tide, caused by the nuisance green algae Ulva prolifera, has occurred in the southern Yellow Sea for 16 consecutive years. It is puzzling why the extensive floating green tide occurs exclusively in the southern Yellow Sea, rather than other waters. We speculate that the transition of U. prolifera from a sessile state to a surface-floating one is the underlying cause of the floating green tide. Here we founded that the floating of U. prolifera was attributed to detachment from substrata and appropriate desiccation. The convergence of unreasonable green algae disposal, geographical features and farming patterns of Porphyra (economic red algae) in Subei Shoal contributed to mass production of floating U. prolifera, resulting in the exclusive occurrence of the floating green tides. Inducing the natural inactivation of green algae to prevent the floating of U. prolifera may effectively mitigate the extensive Ulva bloom at zero cost.

Hazard assessment of dynamic marine processes during two extreme weather systems near the Shandong Peninsula

Typhoons and extratropical cyclones both occurred frequently on the Shandong Peninsula and have induced severe dynamic marine disasters. In this study, the typhoons and extratropical cyclones that occurred from 1979 to 2018 were counted and classified according to their tracks. We simulated the storm surge and disastrous waves using a coupled ocean-wave model and assessed the hazard distribution under different types of typhoons and extratropical cyclones. Through comparison, it was found that the southern Yellow Sea steering type typhoons and Yangtze-Huai cyclones occurred most frequently and were most likely to cause disasters. Regarding the hazard posed by the storm surge, there was a clear demarcation between the northern and southern coasts of the peninsula, as well as between the eastern and western coasts. The western coast of the peninsula was more exposed to storm surges induced by typhoons, while the northern coast of the peninsula was more exposed to storm surges induced by extratropical cyclones. The hazard posed by waves exhibited a localized characteristic. The wave hazard was the greatest near the peninsula cape and the lowest surrounding the Laizhou Bay, which was contrary to the distribution of the storm surge hazard. The identification of key management areas for disasters provides basic data and a technical reference for coastal development planning and can play a positive role in disaster prevention.

Distribution, abundance, and realized niches of meroplankton by two different mesh size nets during spring 2017 in the Southern Yellow Sea, China

Distribution, abundance, and realized niches of meroplankton by two different mesh size nets during spring 2017 in the Southern Yellow Sea, China

Riverine Influences and Seasonal Dynamics: Exploring Carbonate System Variations and Air‐Sea CO2 Fluxes in the Southern Yellow Sea and East China Sea

AbstractIn this work, the seasonal variations of the carbonate system and air‐sea CO2 fluxes were investigated by two cruises in the southern Yellow Sea (SYS) and the East China Sea (ECS), which were significantly influenced by the Changjiang riverine inputs across seasons. Biological‐mediated change of dissolved inorganic carbon (DIC) was first quantitated through a three end‐member mixing model from winter to spring. Our modeling results suggested that DIC addition through biological remineralization persisted in the SYS during winter and spring, while DIC removal was evident in the Changjiang River Plume and the offshore ECS triggered by river inputs in spring. Horizontal and vertical mixing together constituted the largest contribution to the interseasonal variability of partial pressure of CO2 (pCO2) in the SYS (−69.5%), the river plume (−59.3%), and the ECS offshore (−43.8%), followed by temperature effects in the plume area (29.4%), and biological processes in the ECS offshore (24.1%) and the SYS (−7.7%), with air‐sea CO2 exchange contributing the least in both three subregions. The SYS, the river plume, and the ECS offshore all acted as atmospheric CO2 sinks in both winter and spring. Furthermore, their ability to absorb atmospheric CO2 increased from winter to spring, as reflected in a ∼1.8‐fold increase in the overall spring air‐sea CO2 flux compared to winter estimation.

Interannual variability of air-sea CO2 exchange in the Northern Yellow Sea and its underlying mechanisms

A three-dimensional (3-D) physical-biogeochemical-carbon cycle coupled model is used to investigate the interannual variability of the air-sea carbon dioxide (CO2) flux (FCO2) in the Northern Yellow Sea (NYS) from 2009 to 2018. The verification of the model indicate that the simulation results for multiple variables exhibit consistency and fit well with the observed data. The study show that although the multi-year average FCO2 in the NYS is close to the source-sink balance, there are obvious interannual differences between different years. In particular, a relatively strong source of atmospheric CO2 (1.0 mmol m–2 d–1) is exhibited in 2014, while a relatively strong sink of atmospheric CO2 (–0.7 mmol m–2 d–1) emerges in 2016. Mechanism analysis indicates that the abnormally high temperature is the main controlling factor for the relatively high CO2 efflux rate in the NYS in 2014, while the abnormally low dissolved inorganic carbon (DIC) concentration is the main factor contributing to the relatively high CO2 influx rate in 2016. Further analysis reveals that the primary reason for the low DIC concentration since the onset of winter in 2016 is the high net decrease rate of DIC in the NYS in 2015, influenced by net community production in the summer and advection processes during the autumn. The abnormally high primary production during the summer of 2015 results in the excessive reduction of DIC concentration through biological processes. In addition, due to the strong northeasterly wind event in November 2015, low-concentration-DIC water from the Yellow Sea (YS) extends into the Bohai Sea (BS). This further leads to higher DIC flux from the NYS into the BS in the upper mixed layer and increases the inflow of low-concentration-DIC water from the Southern Yellow Sea (SYS) into the NYS. These ultimately result in the abnormal reduction of DIC concentration in the upper mixed layer of the NYS during the autumn of 2015. This study enriches our understanding of interannual variability of FCO2 in the NYS, which will not only help to further reveal the variations of FCO2 under human activities and climate change, but also provide useful information for guiding the comprehensive assessment of the carbon budget.

Deciphering Multi‐Scale Submarine Groundwater Discharge in a Typical Eutrophic Bay

AbstractAs a significant source of nutrients and other components into the coastal ocean, submarine groundwater discharge (SGD) is a combination of multiple spatial‐temporal scale processes, including fresh terrestrial groundwater (FSGD), recirculating seawater (RSGD), and porewater exchange (PEX). Quantifying the different types of SGD is extremely important for understanding the coastal biogeochemical cycles of various materials. The green tide bloom in the southern Yellow Sea (China) has attracted increasing attention during the past decade. Haizhou Bay is thought to be an important green tide proliferating area, but the source of an apparent high supply of nutrients has not been identified yet. We report here on our investigations of the distribution patterns of Ra and Rn isotopes in groundwater and seawater in Haizhou Bay. By solving a combined mass balance for 224Ra, 223Ra, 226Ra, and 222Rn, we estimated that the bay's water residence time is 28.8 days, FSGD is highest at 3.6 cm d−1, RSGD is 2.7 cm d−1, and PEX is lowest at 0.6 cm d−1. The total SGD into Haizhou Bay is estimated at 9.40 × 107 m3 d−1 (6.9 cm d−1), about 12 times that of the local river discharge into the bay. SGD‐derived nutrients are shown to play an important role when considered among all known nutrient sources. Our results suggest that dissolved inorganic nitrogen (DIN) and silicate (DSi) are transported by SGD mainly via FSGD. We also note that the phosphorus (DIP) budget is heavily influenced by RSGD.

Dynamics of green macroalgal micro-propagules and the influencing factors in the southern Yellow Sea, China

Micro-propagules (banks of microscopic forms) play important roles in the expansion of green tides, which are spreading on eutrophic coasts worldwide. In particular, large-scale green tides (Yellow Sea Green Tide, YSGTs) have persisted in the Yellow Sea for over 15 years, but the dynamics and functions of micro-propagules in their development remain unclear. In the present study, year-round field surveys were conducted to identify the reservoirs and investigate the persistence mechanisms and associated biotic and abiotic factors driving the temporal and spatial variations of micro-propagules. Micro-propagules in the southern Yellow Sea (SYS) showed evident spatial heterogeneity in terms of seasonal patterns and major influencing factors. Offshore of the SYS, the micro-propagule population underwent ephemeral expansion along with a large-scale bloom of floating Ulva algae in late spring and early summer. The Subei Shoal, particularly the sediments in the central raft region, had the highest micro-propagule abundance (MA) and was a major reservoir. The pronounced seasonal variation of MA in the Subei Shoal was primarily associated with the attached Ulva algae on Neopyropia aquaculture rafts. Vast aquaculture rafts provided essential substrates for micro-propagules to complete their life cycle and replenish the seed bank, thereby sustaining persistent YSGTs. It implied that habitat modification has pronounced ecological impacts on this intertidal muddy flat. The unique environmental conditions (enriched nutrients, esp. nitrate, favourable seawater temperatures in spring, and strong tidal mixing) facilitated the abundance, seasonal variation and recruitment of micro-propagules in the Subei Shoal. Given the current mitigation measures implemented in the raft region, further research is required to monitor and investigate the physiological and ecological responses of micro-propagule populations to the complex hydrobiological, geochemical, and physical matrices.

Population connectivity of small yellow croaker (Larmichthys polyactis) in the southern Yellow Sea: Implications from multiple otolith-based approaches

Connectivity is a fundamental aspect governing the viability of marine species populations, particularly during their early life stage. However, a dearth of comprehensive studies exists regarding connectivity patterns in Larmichthys polyactis within the southern Yellow Sea region. To address this gap, we conducted a thorough investigation into the population connectivity of L. polyactis collected from Haizhou Bay (HZB) and Lvsi (LS) fishing grounds, employing a suite of otolith-based methodologies. Initially, our analysis of otolith microstructure revealed a notable incubation peak occurring between early and mid-April for juvenile L. polyactis sampled in HZB, despite the sea surface temperature (SST) during that period not aligning with the known spawning requirements of L. polyactis. By juxtaposing the spawning period and associated SST conditions of L. polyactis across different fishing grounds, we deduced a high likelihood that the HZB samples originated from the LS. Subsequently, we investigated the stable isotope ratios (δ13C and δ18O) of L. polyactis collected in HZB, LS, and offshore locations, focusing on the core zone indicative of the early life stage. Statistical analysis using Dunn’s multiple comparison test revealed that no significant disparity in otolith δ13C and δ18O values between HZB and LS samples (P > 0.01). However, these values were notably lower than those observed I n offshore samples (P < 0.01). Furthermore, through otolith morphological analysis, we identified two distinct aggregations within the HZB samples, one aligning with the entirety of the LS samples and another representing the remaining HZB sample subset. This finding provides additional support for the population connectivity between HZB and LS stocks. Drawing upon these findings, we posit the existence of a a mixed population of L. polyactis within HZB, likely resulting from the transportation of eggs or larvae originating from LS to HZB via prevailing northward currents. An enhanced understanding of connectivity dynamics among L. polyactis holds significant implications for the formulation of effective management strategies aimed at bolstering stock recovery efforts.

Distribution of Ulva macroalgae before and after the outbreak of the Yellow Sea green tide in aquaculture ponds and rivers nearshore Jiangsu, China

Green tide caused by the extensive proliferation of Ulva macroalgae in the Yellow Sea has been continuously occurring for 17 years. Previous studies indicate the Neopyropia aquaculture area in Jiangsu to be the main source of green tides in the Southern Yellow Sea. However, whether there are additional supplementary sources for the green tide in the Yellow Sea remains to be studied. This study conducted sampling of Ulva macroalgae in 14 aquaculture ponds and rivers nearshore Jiangsu in February and November in 2022. The molecular biology identification of samples through ITS and 5S indicates that the Ulva macroalgae in aquaculture ponds and rivers are composed of Ulva prolifera, Ulva linza, Ulva simplex, Ulva torta, Ulva flexuosa, and Ulva meridionalis. The results of micropropagules indicate that the abundance of micropropagules in aquaculture ponds is significantly higher than in rivers. In both February and November, the points with the highest abundance of micropropagules were located in aquaculture ponds, reaching 2090 inds L−1 and891 inds L−1, respectively. This study found that before and after the outbreak of green tide, the dominant species of green tide outbreak,5S-II U. prolifera,existed in aquaculture ponds and rivers nearshore Jiangsu. 5S-II U. prolifera in aquaculture ponds and rivers may have a potential impact on the outbreak of green tide in the Yellow Sea.

Distribution, Seasonality, and Water‐Mass Transformation of Temperature and Salinity Inversions in the Southern Yellow Sea

AbstractIn the southern Yellow Sea (SYS) there are temperature and salinity inversions associated with the presence of middepth low temperature and salinity water. Many questions regarding with the inversions, such as the seasonal variation and transformation mechanism, still remain unexplained. In this study, based on hydrographic measurements obtain from 12 cruises within 6‐year (2017–2022), we explored the distribution, seasonality, generation, and transformation mechanism of temperature and salinity inversions in the SYS. Observations reveal widespread temperature and salinity inversions in the Yellow Sea (YS) during both spring and summer. More temperature and salinity inversions are found in spring and summer, respectively. The relative motion between the fresh, cold, high‐dissolved‐oxygen (DO) water at the local shelf/slope and the salty, warm, low‐DO water at the central trough is the key to answering the formation of temperature/salinity inversions. The salinity at the low‐salinity water (LSW) layer increases towards the central YS suggesting the origin of LSW at the local shelf/slope region. Assuming the gain of salinity is caused primarily by vertical diffusion at the upper and lower boundary of the LSW, the propagation speed of the LSW lense is estimated which compares favorably with the magnitude of observed residual currents at the LSW layer. We further note that the depth of subsurface dissolved oxygen maximum agrees well with the LSW layer depth, especially for summer. The influence of the lateral transport of nutrients, and dissolved oxygen on the local biogeochemical environment in the YS is worth further examination.

Distribution of Permian and Triassic strata in the Laoshan Uplift, Southern Yellow Sea, using seismic attributes

The Laoshan Uplift (LSU) in the South Yellow Sea Basin of the Lower Yangtze Plate is a potential site for hydrocarbon exploration; thus, determining the strata attributes and distribution is imperative. Tracking a wide range of seismic reflection horizons for lateral comparisons is challenging because the first well (CSDP-2) encountered in the Paleozoic strata of the LSU is located in a structurally complex deformation zone and has yielded poor seismic data quality. In this study, we present a novel approach to seismic horizon tracking in areas with few or no wells and adopt a cross-regional analogy and inverse analysis method to track the seismic horizon. Seismic attribute characteristics of the marine strata of the LSU were compared with the seismic horizons calibrated in the Wx1 well in the stable Qingdao Depression (QDD), and horizontal tracking was performed. An inverse analysis was then performed on the CSDP-2 well area to predict the distribution of marine strata in the LSU. The results revealed that the Permian and Triassic strata of the LSU and QDD have similar seismic attributes. Finally, the seismic horizon of the LSU was tracked horizontally based on a comparison of the seismic attributes of the LSU and QDD. Previous studies showed that seismic reflection horizons are difficult to calibrate, and the marine strata attributes in the LSU remain controversial. This study adopted a cross-regional comparison and inversion analysis method to perform a seismic multi-attribute comparison based on the calibrated results of the QDD for the Wx1 well. The remaining Permian and Triassic strata lie almost entirely within the LSU. Moreover, the maximum reported thicknesses of the Upper Permian and Lower-Middle Triassic strata in the LSU were approximately 500 and 2750 m, respectively. Our findings will aid in the exploration and evaluation of early hydrocarbon resources in the LSU.

Long-term trends and extreme events of marine heatwaves in the Eastern China Marginal Seas during summer

Marine heatwaves (MHWs) are a type of widespread, persistent, and extreme marine warming event that can cause serious harm to the global marine ecology and economy. This study provides a systematic analysis of the long-term trends of MHWs in the Eastern China Marginal Seas (ECMS) during summer spanning from 1982 to 2022, and occurrence mechanisms of extreme MHW events. The findings show that in the context of global warming, the frequency of summer MHWs in the ECMS has increased across most regions, with a higher rate along the coast of China. Areas exhibiting a rapid surge in duration predominantly reside in the southern Yellow Sea (SYS) and southern East China Sea (ECS, south of 28°N). In contrast, the long-term trends of mean and maximum intensities exhibit both increases and decreases: Rising trends primarily occur in the Bohai Sea (BS) and Yellow Sea (YS), whereas descending trends are detected in the northern ECS (north of 28°N). Influenced jointly by duration and mean intensity, cumulative intensity (CumInt) exhibits a notable positive growth off the Yangtze River Estuary, in the SYS and southern ECS. By employing the empirical orthogonal function, the spatio-temporal features of the first two modes of CumInt and their correlation with summer mean sea surface temperature (SST) and SST variance are further examined. The first mode of CumInt displays a positive anomalous pattern throughout the ECMS, with notable upward trend in the corresponding time series, and the rising trend is primarily influenced by summer mean SST warming. Moreover, both of the first two modes show notable interannual variability. Extreme MHW events in the SYS in 2016 and 2018 are examined using the mixed layer temperature equation. The results suggest that these extreme MHW events originate primarily from anomalous atmospheric forcing and oceanic vertical mixing. These processes involve an anomalous high-pressure system over the SYS splitting from the western Pacific subtropical high, augmented atmospheric stability, diminished wind speeds, intensified solar radiation, and reduced oceanic mixing, thereby leading to the accumulation of more heat near the sea surface and forming extreme MHW events.

Overwintering and summer survival of Ulva prolifera in sediments: Indoor simulation of temperature impacts

Green tides, a globally prevalent marine ecological anomaly observed in coastal regions, have received substantial attention. However, there is limited research on the burial of Ulva prolifera in sediments during the late stages of green tide outbreaks. This study investigates the effect of temperature on U. prolifera buried in sediment over 30 days. The measurements included the length, biomass, relative growth rate, chlorophyll composition and maximum quantum yield (Fv/Fm) of PS II at different stages. The results indicate that at −20 °C, numerous seedlings emerged after 14 days of recovery culture, suggesting the release of spores or gametes; survival was possible from −2 °C to 15 °C; but at 20 °C and 30 °C, all U. prolifera died. The U. prolifera buried in sediment during the late stage of green tide outbreaks may serve as one of the sources for the subsequent year's green tide eruption. This research provides insights into the origins of green tide outbreaks in the southern Yellow Sea.

Unveiling the evolution of phytoplankton communities: Decades-long insights into the southern Yellow Sea, China (1959–2023)

We obtained historical and observational data on phytoplankton communities from 1959 to 2023 to explore the responses of the phytoplankton community structure to long-term environmental changes in the southern Yellow Sea (SYS), China. The results revealed a decrease in the proportions of diatom cell abundance within the phytoplankton community by 8 %, accompanied by a corresponding increase in that of dinoflagellates. Dominant phytoplankton species were mainly chain-forming diatoms before 2000, and large dinoflagellate species from the genera Tripos and Noctiluca increased their dominance after 2000. Warm-water phytoplankton species have increased in dominance over the study period. Correlation analysis revealed that the ocean warming and alterations in nutrient structure (N/P and Si/N ratios) were mostly responsible for the long-term evolution trend, and these changes may result in an increase in dinoflagellate harmful algal blooms, reduced efficiency of the biological carbon pump, and heightened hypoxia in the future, which should draw our attention.

Marine Heatwaves in the East Asian Marginal Seas Facilitated by Boreal Summer Intraseasonal Oscillations

AbstractDuring the summer of 2016, the northern East China Sea and the southern Yellow Sea (NECS‐SYS) experienced one of the most severe and devastating marine heatwaves (MHWs) on record, with a temperature anomaly exceeding 4°C. This shallow semi‐enclosed continental shelf region is widely recognized as a significant hotspot for MHWs with associated incidences of harmful algae blooms. Previous studies have highlighted the importance of mixed layer shoaling as a crucial factor in the genesis of MHWs in the global ocean. The current study employed the Hybrid Coordinate Ocean Model reanalysis data set during 1994–2015 to delve into the mechanisms driving mixed layer shoaling during NECS‐SYS MHW genesis. Our findings reveal the significant role of the northward propagating boreal summer intraseasonal oscillation in promoting MHW genesis and intensification. Specifically, boreal summer intraseasonal oscillation phases 5, 6, and 7 contribute to the favorable conditions that facilitate MHW formation by inducing mixed layer shoaling and increasing solar influx, with mixed layer shoaling playing a more dominant role. The current study provides insights into the relative influences of wind, salinity, and temperature on mixed layer shoaling. We observe that wind plays the most significant role in mixed layer shoaling, followed by temperature and salinity. The boreal summer intraseasonal oscillation induced wind relaxation, increased shortwave radiation, and freshwater influx lead sea surface temperature by 7, 5, and 4 days, respectively. Importantly, mixed layer shoaling leads SST anomalies by 1–2 days. Therefore, the current study also suggests an intraseasonal predictability source for NECS‐SYS MHWs.

Spatial variation in the optical and molecular properties of dissolved organic matter in the Yellow Sea and East China Sea

Marine dissolved organic matter (DOM) plays a crucial role in the global carbon cycle. Carbon cycling in marginal seas is highly complex because it is regulated by numerous source-sink processes. Here, we investigated the spatial variability of the bulk, optical properties, and molecular compositions of DOM in the Yellow Sea (YS) and East China Sea (ECS) and its influencing factors. The DOM in the study area was dominated by protein-like component, highly unsaturated and phenolic compounds, and CHO formulas. The dissolved organic carbon (DOC), chromophoric DOM, and fluorescent DOM decreased with increasing salinity and depth, and within the three regions they were ordered: northern YS > southern YS > ECS. Due to the greater influence of anthropogenic inputs and phytoplankton production in the YS, it contained significantly more heteroatomic compounds and labile compounds (aliphatic, peptide-like, and sugar-like compounds) than the ECS. Moreover, unlike in the open ocean, none of the compound categories exhibited significant variability with depth in these shelf seas. Redundancy analysis suggested that dilution was a main factor determining the spatial distributions of DOM. The polycyclic aromatics and polyphenolics decreased in the seaward direction, but they were still present offshore, indicating they contributed to the open-ocean carbon pool. Anthropogenic inputs obscured potential correlations among the labile compounds and salinity and phytoplankton production. In addition, compounds with high molecular weights and low unsaturated CC functionalities were preserved during the carbon cycle, due to their recalcitrant nature. By providing optical and molecular data, this study helps to better understand the biogeochemical cycling of DOM in marginal seas.