Northern Beibu Gulf Research Articles

Fugacity- and biotransformation-based mechanistic insights into the trophic transfer of organophosphate flame retardants in a subtropical coastal food web from the Northern Beibu Gulf of China

The bioaccumulation and trophic transfer of organophosphate flame retardants (OPFRs) in marine ecosystems have attracted great attention in recent research, but our understanding of the trophic transfer mechanisms involved is limited. In this study, we investigated the trophodynamics of OPFRs and their metabolites in a subtropical coastal food web collected from the northern Beibu Gulf, China, and characterized their trophodynamics using fugacity- and biotransformation-based approaches. Eleven OPFRs and all seven metabolites were simultaneously quantified in the shellfish, crustacean, pelagic fish, and benthic fish samples, with total concentrations ranging from 164 to 4.11 × 104 and 4.56–4.28 × 103 ng/g lipid weight, respectively. Significant biomagnification was observed only for tris (phenyl) phosphate (TPHP) and tris (2-ethylhexyl) phosphate (TEHP), while other compounds except for tris(2-chloroethyl) phosphate (TCEP) displayed biomagnification trends based on Monte Carlo simulations. Using a fugacity-based approach to normalize the accumulation of OPFRs in biota to their relative biological phase composition, storage lipid is the predominant biological phase for the mass distribution of 2-ethylhexyl diphenyl phosphate (EHDPHP) and TPHP. The water content and structure protein are equally important for TCEP, whereas lipid and structure protein are the two most important phases for other OPFRs. The mass distribution of these OPFRs along with TLs can explain their trophodynamics in the food web. The organophosphate diesters (as OPFR metabolites) also displayed biomagnification trends based on bootstrapped estimation. The correlation analysis and Korganism-water results jointly suggested the metabolites accumulation in high-TL organisms was related to biotransformation processes. The metabolite-backtracked trophic magnification factors for tri-n‑butyl phosphate (TNBP) and TPHP were both greater than the values that accounted for only the parent compounds. This study highlights the incorporation of fugacity and biotransformation analysis to characterize the trophodynamic processes of OPFRs and other emerging pollutants in food webs.

Spring circulation characteristics and formation mechanism in the Beibu Gulf

Spring circulation in the Beibu Gulf remains an underinvestigated research topic. Model results validated by observations reveal that the northern and southern parts of the Beibu Gulf are controlled by an enclosed cyclonic gyre and a cyclonic gyre, respectively, while in the vertical, the circulation at the surface differs from those in the middle and deep layers. Contrary to that of the classical view, the modeled Qiongzhou Strait (QS) current flows eastward during some days in spring. This eastward current has an occurrence proportion of 29.3–55.4%, and its longest duration period is 7~18 d. The average current in the QS in spring is approximately westward except in some typical years. In spring, the northern Beibu Gulf is affected by the cold water mass (CWM), which is depicted by the 12°C and 10°C isolines in strong and weak years, respectively. Sensitivity experiments show that the heat flux is dominant in the northern Beibu Gulf via the effect of the CWM, and the monsoon wind favors the formation of the cyclonic gyre. Conversely, the cyclonic gyre in the southern Beibu Gulf is jointly driven by the monsoon wind and heat flux, with the effect of the monsoon wind being more significant. The current off the northern coast of Vietnam (VNC) can be either northeastward or southwestward in spring in different years. The VNC is mainly controlled by the elevation gradient along the northern coast of Vietnam, and the Coriolis force, horizontal advection, and vertical diffusion also play significant roles. Additionally, the CWM is mainly controlled by the heat flux whereas the monsoon wind plays a secondary role. And the VNC, current in the QS, CWM, and cyclonic gyre jointly affect nutrient distributions in the northern Beibu Gulf.

Metabarcoding reveals a high diversity and complex eukaryotic microalgal community in coastal waters of the northern Beibu Gulf, China.

Beibu Gulf is an important semi-enclosed bay located in the northwestern South China Sea, and is famous for its high bio-productivity and rich bio-diversity. The fast development along the Beibu Gulf Economical Rim has brought pressure to the environment, and algal blooms occurred frequently in the gulf. In this study, surface water samples and micro-plankton samples (20-200 μm) were collected in the northern Beibu Gulf coast. Diversity and distribution of eukaryotic planktonic microalgae were analyzed by both metabarcoding and microscopic analyses. Metabarcoding revealed much higher diversity and species richness of microalgae than morphological observation, especially for dinoflagellates. Metabarcoding detected 144 microalgal genera in 8 phyla, while microscopy only detected 40 genera in 2 phyla. The two methods revealed different microalgal community structures. Dinoflagellates dominated in microalgal community based on metabarcoding due to their high copies of 18 s rRNA gene, and diatoms dominated under microscopy. Altogether 48 algal bloom and/or toxic species were detected in this study, 34 species by metabarcoding and 19 species by microscopy. Our result suggested a high potential risk of HABs in the Beibu Gulf. Microalgal community in the surface water samples demonstrated significantly higher OTU/species richness, alpha diversity, and abundance than those in the micro-plankton samples, although more HAB taxa were detected by microscopic observations in the micro-plankton samples. Furthermore, nano-sized taxa, such as those in chlorophytes, haptophytes, and chrysophyceans, occurred more abundantly in the surface water samples. This study provided a comprehensive morphological and molecular description of microalgal community in the northern Beibu Gulf.

A numerical model study on the spatial and temporal variabilities of dissolved oxygen in Qinzhou Bay of the northern Beibu Gulf

A numerical model study on the spatial and temporal variabilities of dissolved oxygen in Qinzhou Bay of the northern Beibu Gulf

Nutrient budgets and biogeochemical dynamics in the coastal regions of northern Beibu Gulf, South China Sea: Implication for the severe impact of human disturbance

This study examined the nutrient budgets and biogeochemical dynamics in the coastal regions of northern Beibu Gulf (CNBG). Nutrient concentrations varied spatially and seasonally among the different bays. High nutrient levels were found in the regions with high riverine inputs and intensive mariculture. Using a three end-member mixing model, nutrient biogeochemistry within the ecosystem was estimated separately from complex physical mixing effects. Nutrient consumption dominated in most bays in summer, whereas nutrient regeneration dominated in winter, likely due to phytoplankton decomposition, vertical mixing and desorption. Through the Land-Ocean Interaction Coastal Zone (LOICZ) model, the robust nutrient budgets were constructed, indicating that the CNBG behaved as a sink of dissolved inorganic nitrogen, phosphorus and silicon. River-borne nutrient inputs were the dominant nutrient source, while residual flows and water exchange flows transported nutrient off the estuaries. This study could help us better understand nutrient cycles and nutrient sources/sinks in the CNBG.

Effects of intensive oyster farming on nitrogen speciation in surface sediments from a typical subtropical mariculture bay

The spatial-temporal distributions of various nitrogen (N) species in surface sediments were examined in a typical subtropical mariculture bay (Maowei Sea) in the northern Beibu Gulf to assess the impact of intensive oyster culture activities on sedimentary N speciation. The results indicated that the mean contents of total nitrogen (TN), extractable (labile) nitrogen (LN) and residual nitrogen (RN) in the surface sediments were 33.3 ± 15.5 μmol g−1, 13.8 ± 1.3 μmol g−1 and 19.5 ± 15.0 μmol g−1, respectively, which lacked significant seasonal variability (P > 0.05). Four forms of LN, namely ion extractable form (IEF-N), weak acid extractable form (WAEF-N), strong alkali extractable form (SAEF-N) and strong oxidant extractable form (SOEF-N) were identified based on sequential extraction. SOEF-N was the dominant form of LN, accounting for 67.8 ± 2.5 % and 63.7 ± 5.9 % in summer and winter, respectively. Spatially, the contents of sedimentary TN, LN, RN, WAEF-N and SOEF-N in intensive mariculture areas (IMA) were significantly higher than those in non-intensive mariculture areas (NIMA) during summer (P < 0.05). Stable nitrogen isotope (δ15N) mixing model revealed that shellfish biodeposition was the predominant source of sedimentary TN in IMA with a contribution of 67.8 ± 23.0 %, approximately 5.4 times that of NIMA (12.6 ± 3.3 %). Significant positive correlations (P < 0.05) were observed between most forms of N species (WAEF-N, SOEF-N, LN and RN) and shellfish-biodeposited N in the surface sediments during summer, indicating that intensive oyster farming greatly enhanced sedimentary TN accumulation.

Effects of seasonal variations and environmental factors on phytoplankton community structure and abundance in Beibu Gulf, China

The seasonal variation in phyla within the phytoplankton community is evident in the Beibu Gulf region, with Bacillariophyta being the most diverse group, accounting for 55% of genera in spring and slightly increasing to 57.9% in autumn. In contrast, Pyrrophyta exhibits a higher number of genera and taxa in spring at 43.3%, but this percentage declines to 40% in autumn. During both seasons, the phylum Cyanophyta had a limited occurrence, making up only 1.7% and 2.1% of the total species counts, but in the spring season, it emerged as the dominant phylum with a high cell density of 95.4%, surpassing other phyla such as Aphanocapsa elachista, Guinardia flaccida, Chaetoceros laciniosus, Trichodesmium thiebautii, and Thalassiosira subtilis. Conversely, in the autumn season, cyanophyta constituted merely 4% of the overall cell density. The relative abundance of Bacillariophyta species (Bacteriastrum sp. >Chaetoceros affinis > Chaetoceros lorenzianus > Thalassionema nitzschioides > Chaetoceros curvisetus) displayed a distinct trend, reaching its peak cell density of 94% during autumn. The significance of this study lies in its findings that different types of phytoplankton, such as dinoflagellates and diatoms, have distinct growth patterns in the northern Beibu Gulf. It also highlights the importance of nutrient levels, particularly nitrate, nitrite, silicate, and soluble reactive phosphorus, in influencing the composition of phytoplankton communities during the spring season. Additionally, the study reveals a negative relationship between ammonium concentration and water temperature during both spring and autumn. These findings suggest that changes in nutrient availability during seasonal shifts play a critical role in shaping phytoplankton dynamics in marine ecosystems. The observed negative relationship between ammonium concentration and water temperature raises questions about the potential impacts of climate change on phytoplankton dynamics in the Beibu Gulf. As temperatures continue to rise, it is possible that the nutrient-rich conditions in the spring season may be altered, leading to changes in the composition and abundance of phytoplankton populations. This could potentially have cascading effects on the overall ecosystem. Therefore, studying the interplay between temperature and nutrient availability is essential for accurately assessing the future health and productivity of the Beibu Gulf's phytoplankton communities.

General Features of MCSs with the Organization of Multiple Parallel Rainbands in China

Abstract Multiple parallel rainbands (MPRBs) involve the organization of mesoscale convective systems (MCSs) characterized by multiple parallel convective rainbands, which may produce high rainfall accumulation. A total of 178 MPRBs were identified from 2016 to 2020 in China, which were classified into the initiation type (∼40%), where rainbands initiate individually, and differentiation type (∼60%), where rainbands form through the splitting of large rainbands or merging of smaller cells. Results showed that the occurrence frequency of MPRBs peaks in July with a midnight major peak and a morning minor peak. The highest occurrence frequency is observed in the northern Beibu Gulf and its coastal areas, with minor high frequencies in Guangdong, northern Jiangxi, and southern Shandong provinces, typically in a southwesterly low-level jet to the west of the subtropical high. MPRBs mainly contain 3–4 rainbands with a spacing distance of 30–50 km and an orientation generally consistent with the direction of 850-hPa winds and 0–1-km vertical wind shear. MPRBs generally move slower than that of squall lines in East China ranging from 4 to 8 m s−1 with 16% being quasi-stationary, which is mainly due to the occurrence of band back building mainly associated with cold pool. Most MPRBs have training effects with band training as the dominant mode. Because of the band training effect and slower movement of MPRBs mainly due to band back building, 71% of MPRBs are associated with enhanced maximum hourly rainfall. Rainfall severity may be alleviated somewhat by the generally short duration of MPRBs with 78% being shorter than 2 h. Significance Statement The purpose of this study is to document the general features of mesoscale convective systems (MCSs) with a specific organization of multiple parallel rainbands (MPRBs). MCSs with this unique organization tend to produce extremely heavy rainfall partly due to the training of multiple rainbands as well as their slow movement because of back building. The organization pattern of MPRBs was previously found in a case study. The possible formation mechanism was also previously examined based on case studies. As a complement to these studies, this work aims to reveal the temporal and spatial distributions, movement and duration, morphology, precipitation patterns, and environmental features of MPRBs in China based on statistics using 5-yr radar reflectivity data.

Dramatic changes in the horizontal structure of mangrove forests in the largest delta of the northern Beibu Gulf, China

Dramatic changes in the horizontal structure of mangrove forests in the largest delta of the northern Beibu Gulf, China

Atmospheric radioactive nuclide deposition on the coast of the Maowei Sea, northern Beibu Gulf, China

The natural radioisotopes 7Be, 210Pb, and 210Po, with different half-lives, are all particle-reactive and serve as natural tracers to study sources and transportation of sediments, sedimentation rates, and sediment chronology. Atmospheric deposition of these radioisotopes is the premise and foundation of their tracing application. The Maowei Sea is a semi-closed bay along the Beibu Gulf, which is an important gulf in the northwest of the South China Sea, but the atmospheric deposition of the abovementioned radioisotopes has not been systematically reported along the coast. In this research paper, the atmospheric depositional fluxes of 7Be, 210Pb, and 210Po were observed over a period from June 2018 to December 2021 on the coast of the Maowei Sea. The annual atmospheric depositional fluxes (Bq m-2 yr-1) of 7Be, 210Pb, and 210Po on the coast of the Maowei Sea were 496.80, 201.72, and 58.08, respectively. The distributions for 7Be and 210Pb depositional fluxes during a whole year (years 2019 and 2021) followed a bimodal pattern, with one peak from February to April and another peak from August to October, while the distribution for 210Po depositional flux showed only one peak during the year of 2019 and another during 2021. The deposition flux and activity of 7Be showed a strong positive correlation with the deposition flux and activity of 210Pb, respectively; the deposition flux and activity of 210Po also showed positive correlations with the deposition fluxes and activities of 210Pb and 7Be, respectively, indicating a similarly scavenging behavior from the atmosphere. A Pearson correlation matrix was used to illustrate the factors influencing the atmospheric depositions and found that precipitation, air quality index (AQI), and PM (both PM2.5 and PM10) were the major factors that influenced the deposition of these three radionuclides. Precipitation had significant positive correlations with the deposition fluxes of all three radionuclides, indicating that, for these radionuclides, rainfall was the main scavenging way from the atmosphere. The observations for specific single rainfall events and their air mass backward trajectory analyses showed that the air masses movement during the rainfall may be another important factor that impacted the depositional fluxes for 7Be, 210Pb, and 210Po.

High-frequency mangrove degradation events during the Holocene climatic optimum in the Maowei Sea of tropical China

Mangrove forests are ecologically and economically significant ecosystems in the (sub)tropics, and it is important to reconstruct their historical changes to understand the future of mangrove forests in the context of global warming. Although the Holocene climatic optimum (HCO) is regarded as an analogue of future climatic change, high-resolution studies of mangrove ecosystem changes during this period are rare. This study presents a high-resolution sedimentary record for the HCO (7800–6900 cal. yr BP) in the Maowei Sea, northern Beibu Gulf, China. Contributions of mangrove-derived organic matter in the sediments were quantified using an end-member mixing model and a Monte Carlo simulation. The model reveals there were high-frequency degradation events (11 times) of the mangrove forests during 7800–6900 cal. yr BP. Palaeo-environmental records (e.g., sea-level, stalagmite δ18O values, and sediment Ti contents) from near the study area demonstrate that an intensified Asian winter monsoon (AWM), rather than sea-level changes, was the key factor responsible for the high-frequency mangrove degradation events during the HCO in this region. The results indicate that the effects of the intensified winter monsoon on tropical ecosystems may be greater than previously thought, particularly in the context of anthropogenic warming scenarios.

Source, distribution, and transformation of dissolved and particulate organic matters in Qinzhou Bay, Northern Beibu Gulf

Both dissolved and particulate organic matters (DOM and POM) provide a reduced carbon pool of considerable size in coastal ecosystems, and the two are closely linked. Currently, however, the integrated study of DOM and POM remains limited, precluding a more in-depth understanding of their interaction in coastal regions. In April 2021, 13 surface water samples were collected from Qinzhou Bay, in the northern Beibu Gulf. The DOM samples were characterized using dissolved organic carbon (DOC) analysis and UV-visible and fluorescence spectroscopy techniques. We determined the POM for the particulate organic carbon and nitrogen (POC and PN) and also isotopic composition (δ13C and δ15N). The weak to negligible relationships found between the DOC, colored and fluorescent DOM, salinity, and chlorophyll a together suggested that DOM’s distribution in Qinzhou Bay is concurrently shaped by various processes, namely, hydrological and in situ biological processes. A high C/N ratio of ~17, high POC/chlorophyll a ratio (253 ± 112), and depleted δ13C (−25.7 ± 1.6‰) confirmed that POM is highly degraded and originates mainly from allochthonous input, to which the terrigenous organic matter and freshwater phytoplankton each contributes 35%. The total organic carbon (TOC = DOC + POC) was positively correlated with the humic-like peak M, revealing the transformation of labile DOM and POM into recalcitrant DOM components. The in situ production efficiency of peak M in surface waters of Qinzhou Bay is one order of magnitude greater than that in inland waters or open oceans, indicating that not only temperature but also the activity of substrate is a key factor controlling the in situ production of recalcitrant DOM in Qinzhou Bay. High levels of TOC and humic-like fluorescent DOM suggest the mass coexistence of organic matter differing in its reactivity, highlighting the large potential for photochemical as well as microbial degradation in the future.

Distribution Characteristics of Dissolved Oxygen in Spring in the Northern Coastal Beibu Gulf, a Typical Subtropical Bay

Dissolved oxygen (DO) is an important parameter for evaluating the seawater quality of the oce-anic environment, but the distribution of DO and its possible mechanisms of marine environ-mental regulation are less studied. Here, the spatial distribution of DO and the influence of main environmental factors on DO were explored using in-situ observation data collected in the northern coastal waters of the Beibu Gulf in April 2021, based on cluster analysis, partial correla-tion analysis, and multiple linear regression. The results showed that DO in surface water of the study area gradually increased from nearshore to offshore, with the nearshore’s DO being nearly saturated and the offshore supersaturated. Tieshan Port and the Maowei Sea had the lowest degree of DO saturation. The partial correlation analysis between DO concentration and environmental factors revealed that the distribution of DO concentration was mainly influenced by temperature and salinity as well as other biochemical factors (i.e., chlorophyll-a, turbidity, and pH). Significant differences existed between the dominant factors in various parts of the northern Beibu Gulf. The western river (Beilun River, Fangcheng River, Maoling River, Qinjiang River) estuaries and offshore areas were dominated by photosynthesis, while the eastern river (Fengfeng River, Nanliu River) estuaries were dominated by oxidation-decomposition. In addition, DO was also affected by the mangrove ecosystem, aquaculture, and coastal industry.

Seasonal variability of nektonic community structure and phylogenetic diversity in Weizhou Island, the Beibu Gulf

The favorable natural conditions and variety of habitats in the Beibu Gulf provide a basis for harboring a high diversity of marine organisms. Sustainable coastal ecosystem management can be benefited from a comprehensive assessment of species diversity. In this study, we analyzed the seasonal changes in nektonic phylogenetic and community structures in the waters of Weizhou Island in the northern Beibu Gulf. The results showed that both the nektonic phylogenetic diversity and community structure in the northern Beibu Gulf exhibited strong seasonal differences between spring and autumn. The catch density was 291.9 kg per km2 in spring and 1081.1 kg per km2 in autumn. Phylogenetic diversity of nektonic communities obviously increased from spring to autumn, while phylogenetic patterns changed from clustering to overdispersion. The seasonal patterns of nektonic communities were mainly related to the different fishing intensities in spring and autumn. Summer fishing intensity in the Beibu Gulf was effectively controlled by a mid-summer fishing moratorium, during which nektonic diversity and fish stocks rapidly recovered from the larval pool. Our study revealed that fishing intensity had a greater impact on nektonic communities at smaller spatial scales, and even exceeded the effects of environmental factors.

Ranging pattern development of a declining delphinid population: A potential cascade effect of vessel activities

It is generally accepted that vessel activity causes various behavioral responses of cetaceans and undermines individual fitness. Whether or how it can lead to a demographic response of populations remains rarely examined. In the northern Beibu Gulf, China, vessel activities have sharply increased in the past two decades, while abnormal demographic dynamics was recently noted for the resident Indo-Pacific humpback dolphins. The present study first examined the humpback dolphins' utilization distribution (UD) from 2003 to 2019. Habitat suitability was then modeled with the sighting data collected before the most recent population reduction. Finally, we tried to disentangle the anthropogenic driver of dolphin demography by cross-referring the spatiotemporal development of dolphins' UD, vessel activities, and habitat suitability. Our results showed that the dolphins’ UD shrank substantially during the port expansion in the early 2010s, and we suggest that the consequential increase in vessel activities might impose extra marine stressors on the resident humpback dolphins. To reduce the boat interaction, the dolphins steadily shifted their core area to a less suitable area in the east during 2015–2017, when unnaturally low survivals were recorded. Afterward, the dolphin core area partially shifted back to the more suitable area in the west, which corresponded to the improving dolphin survival in 2018. Our finding suggested that the vessel activity may be responsible for the dolphin displacement, while staying in the less suitable area may further lead to a more severe and acute demographic consequence on the population. The underlying and indirect impact of vessel activities as disclosed by the present study is particularly important for port management, marine planning, and conservation practice regarding coastal cetaceans, especially for those resident and endangered populations inhabiting the urbanized coastal areas.

Chemically mediated rheotaxis of endangered tri-spine horseshoe crab: potential dispersing mechanism to vegetated nursery habitats along the coast.

An enhanced understanding of larval ecology is fundamental to improve the management of locally depleted horseshoe crab populations in Asia. Recent studies in the northern Beibu Gulf, China demonstrated that nesting sites of Asian horseshoe crabs are typically close to their nursery beaches with high-density juveniles distributed around mangrove, seagrass and other structured habitats. A laboratory Y-maze chamber was used to test whether the dispersal of early-stage juvenile tri-spine horseshoe crab Tachypleus tridentatus is facilitated by chemical cues to approach suitable nursery habitats. The juvenile orientation to either side of the chamber containing controlled seawater or another with various vegetation cues, as well as their movement time, the largest distance and displacement were recorded. The juveniles preferred to orient toward seagrass Halophila beccarii cues when the concentration reached 0.5 g l-1, but ceased at 2 g l-1. The results can be interpreted as a shelter-seeking process to get closer to the preferred settlement habitats. However, the juveniles exhibited avoidance behaviors in the presence of mangrove Avicennia marina and invasive saltmarsh cordgrass Spartina alterniflora at 2 g l-1. The juveniles also spent less time moving in the presence of the A. marina cue, as well as reduced displacement in water containing the S. alterniflora cue at 1 and 2 g l-1. These results may explain the absence of juvenile T. tridentatus within densely vegetated areas, which have generally higher organic matter and hydrogen sulfide. Early-stage juvenile T. tridentatus are capable of detecting and responding to habitat chemical cues, which can help guide them to high-quality settlement habitats. Preserving and restoring seagrass beds in the intertidal areas should be prioritized when formulating habitat conservation and management initiatives for the declining horseshoe crab populations.

Assessing the sources and dynamics of organic matter in a high human impact bay in the northern Beibu Gulf: Insights from stable isotopes and optical properties

Severe human activities in coastal areas have greatly impacted the sources and biogeochemical behaviors of organic matter (OM), including particulate OM (POM) and dissolved OM (DOM). However, few studies have incorporated the indices of POM and DOM to address this issue. Here, a dataset of the combination of stable isotopes of carbon and nitrogen in POM and the optical properties of DOM was presented in Xi Bay, a semi-enclosed bay with a highly developing industrial port in Beibu Gulf, South China, to reveal the origin, distribution, and fate of OM during the rainy season. In the upper bay, depleted δ13C suggested that particulate organic carbon (POC) mainly originated from terrestrial sources. However, the negative relationship between chromophoric DOM (CDOM) and particulate nitrogen (PN) suggested that bacterial-mediated decomposition of POM may be the primary source of CDOM. The negative correlation between humic-like fluorescent components (C1 and C2) and salinity suggested that those two components were mainly affected by terrestrial input. The significant correlation between the protein-like component (C3) and Chl a suggested that C3 was mainly derived from phytoplankton production in the upper bay. In the lower bay, the increase of δ13C values indicated an increased contribution of marine POC. The high levels of CDOM may be due to the decomposition of marine (fresh) POM. However, the low levels of C1 and C2 might be affected by dilution with seawater, and the increased levels of the protein-like C3 were due to enhanced primary production. In addition, the enhancement of δ15N values in both the upper and lower bays indicated serious nitrogen pollution in the bay. This study highlights that biological production fueled by excess nutrients is the dominant OM dynamic process in the bay with high human impact in Beibu Gulf.

Linkage between mangrove wetland dynamics and wave attenuation during a storm–a case study of the Nanliu Delta, China

Frequently, large-scale typhoon-induced casualties and financial losses in global mega-delta cities can be buffered to a large extent by the presence of mangrove wetlands occurring in tropical coastal zones. However, there is little information about how mangrove wetland dynamics induce wave energy damping. Here, a series of biomorphodynamic field data was acquired during a storm period induced by Typhoon Sinlaku over a mangrove wetland in the Nanliu Delta, the largest delta of the northern Beibu Gulf in China; these data revealed the wave attenuation process of native Aegiceras corniculatum (AC). The results indicated that the wave damping coefficient of AC was higher during the storm period than during normal weather conditions. Sapling and adult AC were densely distributed in the field and had a greater effect on reducing wave height than seedlings, which were sparsely distributed, even though seedlings might have a stronger damping ability. Moreover, wave height was linearly attenuated with landward wave propagation distance along a transect of AC plots of different ages. Our work further indicated that the slopes and intercepts of the linear fits between wave height and landward wave propagation distance under storm and normal conditions were closely related to incident wave height, the water level and submerged vegetation volume. These results highlight the role of AC trees of different ages and densities under distinct weather conditions in the wave attenuation process.

Tetrodotoxin in Asian horseshoe crabs Carcinoscorpius rotundicauda and Tachypleus tridentatus across different life stages from northern Beibu Gulf, China.

Horseshoe crabs (HSCs) are a group of ancient chelicerates with great ecological and biomedical importance. Food poisonings caused by the consumption of Asian HSCs have significant impacts on public health and safety. This study measured tetrodotoxin (TTX) concentrations in two HSC species across various life stages in May 2020 from the northern Beibu Gulf, their most important spawning and nursery habitats in China. The average TTX contents in both Carcinoscorpius rotundicauda and Tachypleus tridentatus ranged 6.2-8.0μg/kg and 3.8-8.4μg/kg, respectively. While sampling location, growth and molt stages have little influence on TTX distribution in both species, significantly higher levels of TTX were detected in hemolymph, but lower in pooled tissues of early-instar juvenile T. tridentatus. These results provide a regional view of TTX occurrence and distribution in HSCs during their spawning season, which are critical for future studies to enhance understanding of TTX dynamics and formation in HSCs.

Distribution, spectral characteristics, and seasonal variation of dissolved organic matter in the northern Beibu Gulf, South China Sea

Dissolved organic matter (DOM) in the land–ocean interface plays a critical role in the global carbon cycle, yet its dynamic is not well understood. Dissolved organic carbon (DOC) in the northern Beibu Gulf was measured seasonally from April 2020 to January 2021. Chromophoric and fluorescent DOM (CDOM and FDOM) were synchronously characterized by absorption and fluorescence spectroscopy. Three fluorescent components were identified from 597 samples using parallel factor analysis, including two humic-like and one protein-like component. DOC displays a significant seasonality with the average level being highest in summer (177 μmol/L) and lowest in winter (107 μmol/L). CDOM and FDOM levels in summer are also higher than those in winter. Although the variation ranges of DOC, CDOM, and FDOM in surface water are generally greater than in bottom water, the difference between the two layers is statistically insignificant. River discharge and anthropogenic input are important external sources of DOM in the low-salinity nearshore waters (i.e., estuaries and coast), whereas DOM in offshore waters is mainly produced by in situ biological activity. The mixing behavior of DOM in the northern Beibu Gulf varies spatially but minimally on a seasonal scale. Two distinct conservative mixing behaviors of DOC, CDOM, and humic-like FDOM occur in the nearshore waters from Tieshangang Bay and excluding Tieshangang Bay, respectively. The removal of protein-like FDOM along the salinity gradient and the negative correlation between protein-like FDOM intensity and apparent oxygen utilization are pronounced in the nearshore waters excluding Tieshangang Bay, jointly indicating that oxygen is consumed by microbial communities. In contrast, a net addition of DOC occurs in the high-salinity offshore waters, whereas the CDOM and humic-like FDOM undergoes quasi-conservative mixing. Overall, this study reveals notable spatial and seasonal variations in the concentration, source, and mixing of DOM at the land–ocean interface and highlights the importance of sources and processes in shaping the amount and composition of DOM exported to the ocean margin.