Nutrients structure changes impact the competition and succession between diatom and dinoflagellate in the East China Sea

Temperature and nutrients drive distinct successions between diatoms and dinoflagellates over the past 40 years: Implications for climate warming and eutrophication

Diatoms and dinoflagellates are two typical functional groups of phytoplankton, playing important roles in ecosystem processes and biogeochemical cycles. Changes in diatoms and dinoflagellates are thought to be one of the possible mechanisms for the increase in harmful algal blooms (HABs), due to changing hydrological conditions associated with climate change and human activities. However, little is known about their ability to adapt to changing ocean environments, thus making it difficult to know whether and how they are adapting. By analyzing a 44-year monitoring dataset in the central Bohai Sea during 1978-2021, we found that the abundance ratio of diatoms to dinoflagellates showed a decreasing trend seasonally and ecologically, indicating that the phytoplankton community underwent distinct successional processes from diatom dominance to diatom-dinoflagellate co-dominance. These processes exhibited varying responses to temperature, nutrient concentrations and ratios, and their interactions, of which temperature primarily drove the seasonal succession whereas nutrients were responsible for the ecological succession. Specifically, diatoms showed a preference for lower temperatures and higher DIP concentrations, and were able to tolerate lower DIN at lower temperatures. In contrast, dinoflagellates tended to prevail at conditions of warming and high N/P ratios. These different traits of diatoms and dinoflagellates reflected the fact that warming as a result of rising temperature and eutrophication as a consequence of nutrient input would favor dinoflagellates over diatoms. Moreover, the increasing dominance of dinoflagellates indicated that dinoflagellate blooms were likely to become more frequent and intense in the central Bohai Sea.

Environmental controls on the seasonal variations of diatoms and dinoflagellates in the Qingdao coastal region, the Yellow Sea

Diatoms and dinoflagellates are two typical functional groups of phytoplankton assemblages, which play a crucial role in the structure and functioning of most marine ecosystems. To date, a novel challenge in ecology and biogeochemistry is to address the influences of environmental changes associated with climate change and human activities on the dynamics of diatoms and dinoflagellates. However, the knowledge of the key environmental factors controlling the diatom-dinoflagellate dynamics remains to be improved, particularly in the coastal ecosystems. Therefore, we conducted four cruises along the Qingdao coastline in spring, summer, autumn, and winter 2022 to explore how diatoms and dinoflagellates varied in response to regional environmental changes. The results showed that the phytoplankton communities were dominated by diatoms and dinoflagellates in terms of abundance and species diversity throughout the year in the study region. Yet, there were significant seasonal variability of diatoms and dinoflagellates across the four seasons. For example, diatom species was the most diverse during autumn, and the higher average abundance was observed in the fall and winter. In contrast, the average abundance of dinoflagellates was maximum during the summer and minimum in the autumn season. Moreover, the abundance and species ratios of diatoms/dinoflagellates (dia/dino) also showed significant seasonal variations in the region. The dia/dino abundance ratio was lowest in summer, while the dia/dino species ratio showed an increasing trend from spring to fall and a slight descending trend during winter. Based on the redundancy analysis, we revealed that diatoms and dinoflagellates responded differently to various environmental variables in different seasons, of which temperature and nutrients (especially dissolved inorganic nitrogen, DIN) had highly significant correlations with both the dia/dino abundance and species ratios. Thus, we suggested that temperature and DIN were the key factors controlling the seasonal dynamics of diatoms and dinoflagellates in the Qingdao coastal area.

Nutrient characteristics driven by multiple factors in large estuaries during summer: A case study of the Yangtze River Estuary

Nutrients directly control the level of primary productivity and are crucial for the stability of marine ecosystems. Focusing on the survey results in August 2020 of the Yangtze River Estuary, this study elucidated the distribution characteristics and controlling factors of three nutrients: NO3-N, PO4-P, SiO3-Si. The results showed that the concentrations of NO3-N, PO4-P, SiO3-Si in the study area were generally higher near the shore than far shore, with average concentrations of 11.40, 0.70, and 23.73 μmol/L, respectively. The ocean currents drove the distribution of nutrients, and the transport of CDW and YSCC increased the nutrient levels. The resuspension of sediment caused by factors such as terrain and weather may lead to an abnormal increase in nutrients in the bottom waters. The main controlling factors of the three nutrients were different. NO3-N was significantly affected by human activities, PO4-P and SiO3-Si were mainly affected by natural factors.

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.

Latitudinal pronounced variations in tintinnid (Ciliophora) community at surface waters from the South China Sea to the Yellow Sea: Oceanic-to-neritic species shift, biotic-abiotic interaction and future prediction

The oceanic-to-neritic species shift of microzooplanktonic tintinnids and their interaction with relevant abiotic variables are two crucial processes in the marine ecosystem. However, these processes remain poorly documented in China's marginal seas. In the summer of 2022, we investigated the community structure of pelagic tintinnids in surface waters from the South China Sea (SCS) to the Yellow Sea (YS), passing through the East China Sea (ECS). A number of 58 species from 23 genera were identified, with 36 and 22 species belonging to oceanic and neritic genera, respectively. The abundance proportion of oceanic and neritic genera exhibited a decreasing and increasing trend, respectively, from the SCS to YS. Furthermore, four distinctive tintinnid community groups were classified based on cluster analysis using tintinnid species and abundance data, and the position of southern Taiwan Strait was identified as the "Shift Point" for oceanic-to-neritic species dominance. The top two tintinnid species in each group showed distinct variations in body size. Additionally, multivariate biotic-abiotic statistical analyses revealed that temperature determined tintinnid species richness, while temperature, salinity, Si(OH)4, and Chl a determined tintinnid abundance. Our study provides a substantial foundation for recognizing the oceanic-to-neritic species shift of tintinnids in the China's marginal seas, and highlights the role of biotic-abiotic factors in driving biogeochemical fluxes and the potential response of microzooplankton to future climate change.

Membrane lipid remodeling and autophagy to cope with phosphorus deficiency in the dinoflagellate Prorocentrum shikokuense

Dinoflagellates, which are responsible for more than 80% of harmful algal blooms in coastal waters, are competitive in low-phosphate environments. However, the specific acclimated phosphorus strategies to adapt to phosphorus deficiency in dinoflagellates, particularly through intracellular phosphorus metabolism, remain largely unknown. Comprehensive physiological, biochemical, and transcriptomic analyses were conducted to investigate intracellular phosphorus modulation in a model dinoflagellate, Prorocentrum shikokuense, with a specific focus on membrane lipid remodeling and autophagy in response to phosphorus deficiency. Under phosphorus deficiency, P. shikokuense exhibited a preference to spare phospholipids with nonphospholipids. The major phospholipid classes of phosphatidylcholine and phosphatidylethanolamine decreased in content, whereas the betaine lipid class of diacylglyceryl carboxyhydroxymethylcholine increased in content. Furthermore, under phosphorus deficiency, P. shikokuense induced autophagy as a mechanism to conserve and recycle cellular phosphorus resources. The present study highlights the effective modulation of intracellular phosphorus in P. shikokuense through membrane phospholipid remodeling and autophagy and contributes to a comprehensive understanding of the acclimation strategies to low-phosphorus conditions in dinoflagellates.

Variation in intracellular polyphosphate and associated gene expression in response to different phosphorus conditions in the dinoflagellate Karenia mikimotoi

Polyphosphate (polyP) has long been recognized as a crucial intracellular reservoir for phosphorus in microorganisms. However, the dynamics of polyP and its regulatory mechanism in eukaryotic phytoplankton in response to variations in external phosphorus conditions remain poorly understood. A comprehensive investigation was conducted to examine the intracellular polyP-associated metabolic response of the dinoflagellate Karenia mikimotoi, a harmful algal bloom species, through integrated physiological, biochemical, and transcriptional analyses under varying external phosphorus conditions. Comparable growth curves and Fv/Fm between phosphorus-replete conditions and phosphorus-depleted conditions suggested that K. mikimotoi has a strong capability to mobilize the intracellular phosphorus pool for growth under phosphorus deficiency. Intracellular phosphate (IPi) and polyP contributed approximately 6-23 % and 1-3 %, respectively, to the overall particulate phosphorus (PP) content under different phosphorus conditions. The significant decrease in PP and increase in polyP:PP suggested that cellular phosphorus components other than polyP are preferred for utilization under phosphorus deficiency. Genes involved in polyP synthesis and hydrolysis were upregulated to maintain phosphorus homeostasis in K. mikimotoi. These findings provide novel insights into the specific cellular strategies for phosphorus storage and the transcriptional response in intracellular polyP metabolism in K. mikimotoi. Additionally, these results also indicate that polyP may not play a crucial role in cellular phosphorus storage in phytoplankton, at least in dinoflagellates.

Interspecific competition between the bloom-causing dinoflagellates Hetrocapsa bohaiensis and the local species Chlorella pyrenoidosa

Harmful algal blooms caused by Heterocapsa bohaiensis have broken out in aquaculture areas near Liaodong Bay, China, since 2012, resulting in mass mortality of Eriocheir sinensis larvae and substantial economic loss. Chlorella pyrenoidosa is a local phytoplankton species that is found in aquaculture ponds. However, the reason why H. bohaiensis dominated and proliferated in the phytoplankton community remains unknown. Previous studies have revealed the toxicity and hemolytic activity of H. bohaiensis. It is suspected that the out-competition of H. bohaiensis to C. pyrenoidosa was associated with toxicity. Filtrate and bi-algal cultures were investigated to determine the interspecific competition between H. bohaiensis and C. pyrenoidosa in this study. Filtrate experiments revealed that H. bohaiensis showed no toxin allelopathy in C. pyrenoidosa. However, the C. pyrenoidosa filtrates had significant allelopathic effects on the growth of H. bohaiensis. The bi-algal culture experiments and the simulation showed that the dominant species were dependent on the initial cell density ratios of the species and nutrient ratios. Therefore, H. bohaiensis achieved competitive advantage through exploitation competition but not allelopathy. The results contribute to the reasons for the occurrence of H. bohaiensis blooms in a further study.

Biodiversity and Interannual Variation of Harmful Algal Bloom Species in the Coastal Sea of Qinhuangdao, China

For the frequent occurrence of harmful algal blooms (HABs) in the Qinhuangdao coastal sea (QCS) of the Bohai Sea in summer, we tested the hypothesis that high-biodiversity HAB species exist in the area, and a series of censuses of HAB species were conducted in the QCS in the summers of 2014-2019. Through morphological identification, we found 100 algae species representing 42 genera in 3 phytoplankton phyla in this study, among which Bacillariophyta was the most dominant phylum. We also found that the population density of Dinoflagellata increased from 2016 to 2019. In total, 59 HAB species were annotated in this study, including 39 of Bacillariophyta, 18 of Dinoflagellata and 2 of Ochrophyta, of which 13 HAB species were reported in the Bohai Sea for the first time, and most HAB species were widely distributed in the QCS in summer. Notably, four dominant HAB species displayed unique temporal and spatial distribution characteristics, while their distribution ranges and population densities increased from 2014 to 2019. The distributions of five environmental factors were different in the QCS, while the temperature, salinity, and dissolved inorganic nitrogen might be the key environmental factors influencing the distribution of dominant HAB species in the summer. In conclusion, this study provides a detailed evaluation of phytoplankton diversity and interannual variation in the QCS. The existence of a high level of biodiversity of algal bloom species suggests the need for long-term monitoring in order to further study and prevent potential HABs.

Eukaryotic community succession on discarded face masks in the marine environment

Wearing facemasks remains an essential strategy for combating the COVID-19 pandemic. However, used masks are becoming plastic wastes that are widespread in the oceans, which is raising concerns about the potential impacts of these novel plastic niches on marine organisms. To delve into this issue, we exposed surgical masks to coastal waters for 30 days. Valuable information was recorded weekly in regard to the succession of the eukaryotic community inhabiting the masks via high-throughput 18S rRNA gene sequencing. Generally, the community on masks was significantly distinct from that in the surrounding seawater. With 1150 different eukaryotic taxa identified, the diversity of the vigorous colonizers of masks peaked at the beginning and decreased over time. A hallmark of initial colonization was the aggregation of diatoms, which formed biofilms on masks, followed by dinoflagellates that acted as a turning point for subsequent development of calcified species and other predators. This study provides insight into the eukaryotic community dynamics on discarded masks in the marine environment and highlights that the potential mask-mediated harmful species clustering may threaten the marine ecosystem.

Why do red tides occur frequently in some oligotrophic waters? Analysis of red tide evolution history in Mirs Bay, China and its implications

The process and management of red tide in oligotrophic waters are poorly understood as most studies on red tide were focused on eutrophic areas. In this study, 404 red tide events together with the historical water quality dynamics during 1991-2020 were investigated in an anthropogenically influenced bay in China - Mirs Bay, whose most region is oligotrophic except small inshore areas. Red tides of oligotrophic offshore accounted for 20 % of all. With the effective governmental management on inshore areas, concentration of PO₄ and DIN has been decreased to a low level (PO₄ <0.01 mg/L while DIN <0.1 mg/L) in the bay since about 2000. However, the reduction of nutrients was still accompanied by the frequent outbreaks of red tides, as well as a shift of dominant algae from diatoms to dinoflagellates, which might be due to the unbalanced nutrient reduction, such as N:P ratio fluctuation and organic nutrient increase. This shift might trigger more red tide events and even some super ones (long-duration or large-scale) in oligotrophic areas. Detailed analysis on red tide events combined with model simulation proved that the outbreak of red tide in Mirs Bay was caused by the joint contribution of nutrients and hydrodynamics. Nutrients of inshore area supported the red tides there, and with the help of physical conditions, red tides inshore could be transferred to offshore areas and then were likely to bloom again or be preyed to support blooms of other organisms. This study acknowledged that the reduction of both N and P either inorganic or organic nutrients was essential to control red tides, even in oligotrophic waters, but a balanced strategy considering the dual reduction of both nitrogen and phosphorus was of pivotal role to restore the health of coastal water systems disturbed by human.

Establishment and preliminary study of electrophysiological techniques in a typical red tide species

Electrophysiology studies the electrical properties of cells and tissues including bioelectrical signals and membrane ion channel activities. As an important means to reveal ion channel related physiological functions and the underlying mechanisms, electrophysiological techniques have been widely used in studies of animals, higher plants and algae that are closely related to higher plants. However, few electrophysiological studies have been carried out in red tide organisms, especially in dinoflagellates, which is mainly due to the complex surface structure of dinoflagellate amphiesma. In this study, the surface amphiesma of Alexandrium pacificum, a typical red tide species, was removed by centrifugation, low-temperature treatment and enzymatic treatment. In all three treatments, low-temperature treatment with 4 °C for 2 h had high ecdysis rate and high fixation rate, and the treated cells were easy to puncture, so low-temperature treatment was used as a preprocessing treatment for subsequent current recording. Acquired protoplasts of A. pacificum were identified by calcofluor fluorescence and immobilized by poly-lysine. A modified "puncture" single-electrode voltage-clamp recording was first applied to dinoflagellates, and voltage-gated currents, which had the characteristics of outward K⁺ current and inward Cl^- current, were recorded and confirmed by ion replacement, indicating the voltage-gated currents were mixed. This method can be used as a technical basis for the electrophysiological study of dinoflagellates and provides a new perspective for the study of stress tolerance, red tide succession, and the regulation of physiological function of dinoflagellates.

Review of Effects of Dam Construction on the Ecosystems of River Estuary and Nearby Marine Areas

Dams have made great contributions to human society, facilitating flood control, power generation, shipping, agriculture, and industry. However, the construction of dams greatly impacts downstream ecological environments and nearby marine areas. The present manuscript presents a comprehensive review of the influence of human activities on the environment, especially the effect of dam construction on the ecosystems of river estuaries and nearby marine areas, so as to provide a scientific basis for ecological environment protection. To summarize these impacts, this review used recent studies to comprehensively analyze how dam construction has affected river hydrology, geomorphology, and downstream ecosystems globally. Effects of dams on ecosystems occur through reduced river flow, reduced sediment flux, altered water temperature, changed estuary delta, altered composition and distribution of nutrients, altered structure and distribution of phytoplankton populations, habitat fragmentation, and blocked migration routes in river sections and adjacent seas. Effects of dam construction (especially the Three Gorges Dam) on the Yangtze River were also reviewed. Performing community and mitigation planning before dam construction, exploring new reservoir management strategies (including targeted control of dam storage and flushing sediment operations), banning fishing activities, and removing unnecessary dams (obsolete or small dams) are becoming crucial tools for ecosystem restoration.

An overview of Prorocentrum donghaiense blooms in China: Species identification, occurrences, ecological consequences, and factors regulating prevalence

Prorocentrum donghaiense Lu (also identified as Prorocentrum shikokuense Hada and Prorocentrum obtusidens Schiller) is a bloom-forming dinoflagellate species distributed worldwide. Blooms of P. donghaiense occur annually in adjacent waters of the East China Sea (ECS), especially in the waters near the Changjiang River Estuary. Blooms of this species have also been reported in nearby Japanese and Korean waters. There has been an apparent bloom-forming species succession pattern in the ECS since 2000, with diatom blooms in the early spring, shifting to long-lasting and large-scale dinoflagellate blooms dominated by P. donghaiense during the spring, and finally ended by diatom and/or Noctiluca scintillans blooms in summer. These bloom succession patterns were closely correlated with changes in environmental factors, such as temperature increase and anthropogenic eutrophication. Decreasing silicate by the construction of the Three Gorges Dam and increasing dissolved inorganic nitrogen flux were mainly influenced by high intensity human activities in the Changjiang River watershed, resulting in low Si/N ratio and high N/P ratios, possibly accelerating outbreak of P. donghaiense blooms. Phosphorous deficiency might be the most critical factor controlling the succession of microalgal blooms from diatoms to dinoflagellates. Prorocentrum donghaiense is a nontoxic species, but it can disrupt marine ecosystem by decreasing phytoplankton biodiversity and changing the structure of the food chain. Prorocentrum donghaiense blooms in the ECS have been intensively studied during the last two decades. Several possible mechanisms that contribute or trigger the annual blooms of this species have been proposed, but further research is required particularly on the aspect of nutrient budget, ecosystem impacts, as well as social-economic impact assessment.

Metagenome-Assembled Genomes From Pyropia haitanensis Microbiome Provide Insights Into the Potential Metabolic Functions to the Seaweed

Pyropia is an economically important edible red alga worldwide. The aquaculture industry and Pyropia production have grown considerably in recent decades. Microbial communities inhabit the algal surface and produce a variety of compounds that can influence host adaptation. Previous studies on the Pyropia microbiome were focused on the microbial components or the function of specific microbial lineages, which frequently exclude metabolic information and contained only a small fraction of the overall community. Here, we performed a genome-centric analysis to study the metabolic potential of the Pyropia haitanensis phycosphere bacteria. We reconstructed 202 unique metagenome-assembled genomes (MAGs) comprising all major taxa present within the P. haitanensis microbiome. The addition of MAGs to the genome tree containing all publicly available Pyropia-associated microorganisms increased the phylogenetic diversity by 50% within the bacteria. Metabolic reconstruction of the MAGs showed functional redundancy across taxa for pathways including nitrate reduction, taurine metabolism, organophosphorus, and 1-aminocyclopropane-1-carboxylate degradation, auxin, and vitamin B12 synthesis. Some microbial functions, such as auxin and vitamin B12 synthesis, that were previously assigned to a few Pyropia-associated microorganisms were distributed across the diverse epiphytic taxa. Other metabolic pathways, such as ammonia oxidation, denitrification, and sulfide oxidation, were confined to specific keystone taxa.

Parasitic infections by Group II Syndiniales target selected dinoflagellate host populations within diverse protist assemblages in a model coastal pond

Protists are integral to marine food webs and biogeochemical cycles; however, there is a paucity of data describing specific ecological niches for some of the most abundant taxa in marker gene libraries. Syndiniales are one such group, often representing the majority of sequence reads recovered from picoplankton samples across the global ocean. However, the prevalence and impacts of syndinian parasitism in marine environments remains unclear. We began to address these critical knowledge gaps by generating a high-resolution time series (March-October 2018) in a productive coastal pond. Seasonal shifts in protist populations, including parasitic Syndiniales, were documented during periods of higher primary productivity and increased summer temperature-driven stratification. Elevated concentrations of infected hosts and free-living parasite spores occurred at nearly monthly intervals in July, August, and September. We suggest intensifying stratification during this period correlated with the increased prevalence of dinoflagellates that were parasitized by Group II Syndiniales. Infections in some protist populations were comparable to previously reported large single-taxon dinoflagellate blooms. Infection dynamics in Salt Pond demonstrated the propagation of syndinian parasites through mixed protist assemblages and highlighted patterns of host/parasite interactions that better reflect many other marine environments where single taxon blooms are uncommon.

Trophic states regulate assembly processes and network structures of small chromophytic phytoplankton communities in estuarine and coastal ecosystem

Small chromophytic phytoplankton (SCP) are anticipated to be more important for a significant proportion of primary production in estuarine-coastal ecosystems. However, responses of SCP community to coastal eutrophication are still unclear. In this study, we investigated diversity, co-occurrence and assembly features of SCP communities, as well as relationship with environmental factors in subtropical Beibu Gulf. The results exhibited that the alpha diversity and beta diversity of SCP communities were significantly different among eutrophic states. Co-occurrence network revealed a complex interaction that most amplicon sequence variants (ASVs) in modules of the network were specific to trophic states. Further, phylogenetic based β-nearest taxon distance analyses revealed that stochastic processes mainly provided 69.26% contribution to SCP community assembly, whereas deterministic processes dominated community assembly in heavy eutrophic state. Overall, our findings elucidate the mechanism of diversity and assembly in SCP community and promote the understanding of SCP ecology related to subtropical coastal eutrophication.

Monitoring, modeling and projection of harmful algal blooms in China

This review assesses monitoring, modeling and projection of harmful algal blooms (HABs) in China, with a focus on near-term warning and long-term trend projection. We reviewed phytoplankton monitoring as early warning, remote sensing for offshore environments, building models to describe ecological processes, modeling and forecasting near-term or seasonal HAB events, and projection of long-term HAB trend in China. Over the past 40 years, great progresses were made in traditional observation capability of HABs, and some reliable remote sensing algorithms were developed for HABs in optically complex coastal waters in Chinese seas. Numerical models have been applied to simulating real-world algal bloom events successfully; and these models, to some degree, are capable of predicting the time and place of HAB occurrence. In terms of long-term trend, HABs appeared to have shown diversified forms, being more miniaturized and more harmful. The development of an integrated monitoring and early-warning system of algal blooms and HABs should be a necessary first step to provide an effective management tool for mitigating damages associated with the occurrence of HABs in China.

Metagenomic insights into co-proliferation of Vibrio spp. and dinoflagellates Prorocentrum during a spring algal bloom in the coastal East China Sea

Coastal harmful algal blooms (HABs), commonly termed 'red tides', have severe undesirable consequences to the marine ecosystems and local fishery and tourism industries. Increase in nitrogen and/or phosphorus loading is often regarded as the major culprits of increasing frequency and intensity of the coastal HAB; however, fundamental understanding is lacking as to the causes and mechanism of bloom formation despite decades of intensive investigation. In this study, we interrogated the prokaryotic microbiomes of surface water samples collected at two neighboring segments of East China Sea that contrast greatly in terms of the intensity and frequency of Prorocentrum-dominated HAB. Mantel tests identified significant correlations between the structural and functional composition of the microbiomes and the physicochemical state and the algal biomass density of the surface seawater, implying the possibility that prokaryotic microbiota may play key roles in the coastal HAB. A conspicuous feature of the microbiomes at the sites characterized with high trophic state index and eukaryotic algal cell counts was disproportionate proliferation of Vibrio spp., and their complete domination of the functional genes attributable to the dissimilatory nitrate reduction to ammonia (DNRA) pathway substantially enriched at these sites. The genes attributed to phosphorus uptake function were significantly enriched at these sites, presumably due to the P_i-deficiency induced by algal growth; however, the profiles of the phosphorus mineralization genes lacked consistency, barring any conclusive evidence with regard to contribution of prokaryotic microbiota to phosphorus bioavailability. The results of the co-occurrence network analysis performed with the core prokaryotic microbiome supported that the observed proliferation of Vibrio and HAB may be causally associated. The findings of this study suggest a previously unidentified association between Vibrio proliferation and the Prorocentrum-dominated HAB in the subtropical East China Sea, and opens a discussion regarding a theoretically unlikely, but still possible, involvement of Vibrio-mediated DNRA in Vibrio-Prorocentrum symbiosis. Further experimental substantiation of this supposed symbiotic mechanism may prove crucial in understanding the dynamics of explosive local algal growth in the region during spring algal blooms.

Coupling high frequency monitoring and bioassay experiments to investigate a harmful algal bloom in the Bay of Seine (French-English Channel)

Coastal ecosystems are increasingly threatened by eutrophication and dystrophy. In this context, the full pattern of a bloom dominated by the dinoflagellate, Lepidodinium chlorophorum, was investigated by a high frequency monitoring buoy equipped with sensors allowing nutrients and photosynthesis measurements. An increase of the N/P ratio affected phytoplankton physiology leading to bloom collapse with a slight oxygen depletion. In parallel, enrichment experiments were performed on the natural bloom population. After 5 days of incubation the community structure, using flow cytometry and several physiological parameters were analysed. The data reveal a potential N and P co-limitation and a decoupling between primary production and productivity in fully enriched conditions. Under unbalanced N/P inputs, high level of alkaline phosphatase activity and transparent exopolymeric particle production, which favour phytoplankton sedimentation, were observed. Nutrient inputs and their stoichiometry control phytoplankton growth, the community structure, physiological regulations, the fate of the bloom and consequences.

Long-term changes of nutrients and biocenoses indicating the anthropogenic influences on ecosystem in Jiaozhou Bay and Daya Bay, China

Long-term changes of nutrients, plankton and macrobenthos were studied to research the transformation of ecosystem in Jiaozhou Bay and Daya Bay in the past 30 years. Concentrations of dissolved inorganic nitrogen and phosphate increased with significant changes in nutrient compositions and ratios. Concentrations of Chl a slightly decreased in Jiaozhou Bay but increased in Daya Bay. Phytoplankton abundances increased and diatoms were dominant, however, dinoflagellate gradually had the competitive advantage under high N/P and N/Si in the two bays. Zooplankton biomass significantly increased in Jiaozhou Bay, but only increased slightly in Daya Bay over the past years. Polychaetes were dominant in macrobenthos in the bays, indicating their adaptation to the changing benthic environments. The long-time variations of biocenoses and nutrients reflected that the ecological environments have changed under the influence of anthropogenic activities in the two bays.

Photosynthesis acclimation under severely fluctuating light conditions allows faster growth of diatoms compared with dinoflagellates

BACKGROUND

Diatoms contribute 20% of the global primary production and are adaptable in dynamic environments. Diatoms always bloom earlier in the annual phytoplankton succession instead of dinoflagellates. However, how diatoms acclimate to a dynamic environment, especially under changing light conditions, remains unclear.

RESULTS

We compared the growth and photosynthesis under fluctuating light conditions of red tide diatom Skeletonema costatum, red tide dinoflagellate Amphidinium carterae, Prorocentrum donghaiense, Karenia mikimotoi, model diatom Phaeodactylum tricornutum, Thalassiosira pseudonana and model dinoflagellate Dinophycae Symbiodinium. Diatoms grew faster and maintained a consistently higher level of photosynthesis. Diatoms were sensitive to the specific inhibitor of Proton Gradient Regulation 5 (PGR5) depending photosynthetic electron flow, which is a crucial mechanism to protect their photosynthetic apparatus under fluctuating light. In contrast, the dinoflagellates were not sensitive to this inhibitor. Therefore, we investigate how PGR5 functions under light fluctuations in the model diatom P. tricornutum by knocking down and overexpressing PGR5. Overexpression of PGR5 reduced the photosystem I acceptor side limitation (Y (NA)) and increased growth rate under severely fluctuating light in contrast to the knockdown of PGR5.

CONCLUSION

Diatoms acclimatize to fluctuating light conditions better than dinoflagellates. PGR5 in diatoms can regulate their photosynthetic electron flow and accelerate their growth under severe light fluctuation, supporting fast biomass accumulation under dynamic environments in pioneer blooms.

Alkaline phosphatase activity during a phosphate replete dinoflagellate bloom caused by Prorocentrum obtusidens

Prorocentrum obtusidens Schiller (formerly P. donghaiense Lu), a harmful algal species common in the East China Sea (ECS), often thrives with the depletion of phosphate. Three cruises in the spring of 2013 sampled an entire P. obtusidens bloom process to investigate the dynamics of alkaline phosphatase activity (APA) and phosphorus (P) status of the bloom species using both bulk and cell-specific assays. Unlike previous studies, the bloom of P. obtusidens occurred in a phosphate replete environment. Very high APA, with an average of 76.62 ± 90.24 nmol L^-1 h^-1, was observed during the early-bloom phase, a value comparable to that in low phosphate environments. The alkaline phosphatase (AP) hydrolytic kinetics also suggested a more efficient AP system with a lower half-saturation constant (K_s), but higher maximum potential hydrolytic velocity (V_max) in this period. The APA decreased significantly with an average of 24.98 ± 30.98 nmol L^-1 h^-1 when the bloom reached its peak. The lack of a correlation between dissolved inorganic phosphate (DIP) or dissolved organic phosphate (DOP) concentration and APA suggested that the APA was regulated by the internal P growth demand, rather than the external P availability during the phosphate replete P. obtusidens bloom. These findings facilitate an understanding of the P. obtusidens acclimation strategy with respect to P variations in terms of AP expression during blooms in the ECS.

Responses of Primary Productivity and Phytoplankton Community to the Atmospheric Nutrient Deposition in the East China Sea

Atmospheric deposition of nutrients to the surface seawater may significantly affect marine phytoplankton growth. Two in situ bioassay experiments were performed in the East China Sea (ECS) by adding nutrients (N, P, and Si) and atmospheric aerosols into the surface seawater. Chlorophyll a (Chl a) concentrations were largely enhanced by simultaneous input of N and P with the maximal increase of 0.68–0.78 μg Chl a per μmol N addition. This Chl a increment was significantly lower (0.19–0.47 μg) in aerosol treatments as a result of initial N-replete condition (N/P ratio ~50) and extremely high N/P ratio in aerosols (>300). Among the multiple influencing factors, atmospheric dry flux of NH4+ + NO3− (AN) was found to be an effective predictor for springtime Chl a in the ECS with a time lag of three days and were strongly correlated with Chl a concentrations on day 3 (r = 0.81, p < 0.001), which might be partly explained by the asynchronous supplies of N (atmospheric deposition) and P (subsurface water). Although dinoflagellates dominated the phytoplankton community in both initial seawaters, additions of P and N + P + Si profoundly enhanced the cell densities and dominance of diatom species Thalassiosira sp. and Nitzschia closterium in the 2012 and 2014 bioassay experiments, respectively. Moreover, the percentage of dinoflagellates were promoted by adding higher NH4+/NO3− ratio (6/4 vs. 1/9) when silicate was at a low concentration (~2 μmol L−1). Atmospheric deposition is likely to be an important N source supporting the high primary production in the ECS and its supply of excess N relative to P may influence dominant phytoplankton groups.

Spatiotemporal Dissolved Silicate Variation, Sources, and Behavior in the Eutrophic Zhanjiang Bay, China

Dissolved silicate (DSi) is an important nutrient in coastal water, which is used by planktonic diatoms for cell division and growth. In this study, surface water samples were collected in the eutrophic Zhanjiang Bay (ZJB) in 2019, covering a seasonal variation of coastal water and land-based source water discharge. The spatiotemporal DSi distribution, land-based sources flux input and behaviors in ZJB were studied and discussed. The results show that the DSi concentration had significant differences in spatiotemporal scale. The average concentration of DSi in ZJB was 38.00 ± 9.48 μmol·L−1 in spring, 20.23 ± 11.27 μmol·L−1 in summer, 12.48 ± 1.42 μmol·L−1 in autumn and 11.96 ± 4.85 μmol·L−1 in winter. The spatiotemporal DSi distribution showed a decreasing gradient from the top to the mouth of ZJB, which was affected by land source inputs and hydrodynamics. The land-based sources’ input concentration of DSi in ZJB ranged from 0.021 to 0.46 mol·L−1, with an average of 0.14 mol·L−1, and the total annual flux of DSi was 1.06 × 109 mol, comprising up to 8.28%, 41.55% and 50.17% in dry, normal, and wet seasons, respectively. The Suixi River contributed the highest DSi flux proportion in all seasons. The DSi in land-based source water was mainly affected by water flow discharge, diatom uptake and impacts from anthropogenic activities. Compared with dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP), the DSi was the limitation nutrient in ZJB. Additionally, the DSi concentration in the coastal water was negatively correlated with salinity. The seasonal DSi/DIN and DSi/DIP ratios in land-based sources discharge water was significantly higher than that in coastal water (p < 0.05). Land-based sources of DSi input played an important role in nutrients composition that sustained diatoms as the dominant species in ZJB.

Dissolution kinetics of biogenic silica and the recalculated silicon balance of the East China Sea

We conducted field observations in the East China Sea (ECS) in 2010 and 2011 to determine the content and dissolution dynamics of bSiO₂ in sediments of the ECS. The influencing factors on bSiO₂ dissolution were investigated, and the regional silicon budget was recalculated. The sediment bSiO₂ content in the ECS varied from 143 to 583 μmol g^-1. The burial flux of bSiO₂ ranged from 0.11 to 19 mol m^-2 yr^-1 and gradually decreased eastward offshore from within and north of the Changjiang River. Continuous flow experiments showed that the solubility of bSiO₂ in surface sediments varied from 213 to 357 μM-Si, and the dissolution rate constant of bSiO₂ was 14.9-56.6 nmol g^-1 h^-1; both ranges are lower than those of other marginal seas, such as the Arabian and Scotia Seas. The release of soluble aluminum from lithogenic minerals was suggested to influence the pore water build-up of dissolved silica in the ECS. The silicon budget of the ECS was recalculated based on up-to-date research. Due to the low dissolution rate constant and high sediment accumulation rate, sediment bSiO₂ in the ECS is well preserved, with a burial efficiency of 81%; this accounts for 9.9% of the global burial rate and is significantly higher than that of the Yellow Sea and the global ocean average.

Dynamics of phytoplankton and nutrient uptake following dust additions in the northwest Pacific

Dust deposition can supply nutrients that affect marine phytoplankton, but changing trophic statuses of the surface ocean increase the complexity of interpreting the process. In this study, four onboard incubation experiments amended with various nutrients and dust were performed in the Kuroshio Extension (KE) and Kuroshio-Oyashio transition (TR) of the northwest Pacific (NWP), which are characterised by lower and higher trophic statuses, respectively. According to the nutrient-addition experiments, phytoplankton were limited by nitrogen (N) in the KE, and limited by iron (Fe) or co-limited by Fe and phosphorus (P) in the TR. Dust additions supplied a considerable amount of N and Fe but negligible amount of P to stimulate phytoplankton growth, as indicated by chlorophyll a (Chl a) concentration. In the KE incubations, dust additions enhanced the shift of phytoplankton size structure towards larger cells from dominantly pico-sized (0.2-2 μm) Chl a to comparable contributions from each size class (i.e. pico-, nano-: 2-20 μm, micro-: >20 μm). On the basis of the large shift of size structure towards nano- or micro-phytoplankton in the unamended control treatments in the TR, dust additions furtherly promoted the shift towards micro-phytoplankton becoming the dominant contributor to the total Chl a. The collective analysis of the data from experiments in both regions revealed that, the extent of phytoplankton growth stimulation and the shift towards larger cells were enhanced gradually with increasing amounts of nutrient uptake (including N, P, and silicon). The nutrient uptake ratios of phytoplankton converged towards the Redfield ratio in comparison to the wider range of nutrient ratios in the dust-amended seawater. This study suggested consistencies in the dynamic of phytoplankton growth, shift of size structure, and nutrient uptake following dust additions in the KE and TR, although the trophic status and limiting nutrient varied between these two regions.

Water conservancy project on the Yellow River modifies the seasonal variation of Chlorophyll-a in the Bohai Sea

The Water Sediment Regulation Scheme (WSRS) is a unique engineering measure that has been regularly performed to reduce reservoir sedimentation and increase the flood capacity of the Yellow River in China since 2002. As a side effect, the WSRS greatly increases the monthly input flux of nutrients to the Bohai Sea (BHS) in summer, potentially exacerbating eutrophication levels therein and subsequently affecting the growth of phytoplankton. However, its influence on the Chlorophyll-a (Chl-a) dynamics over the BHS is still poorly understood. In this study, two approaches were adopted to investigate it: 1) long-term in-situ observations and satellite-derived data of surface Chl-a were used to study its seasonal variations before and since 2002, and 2) one 1D physical-biological coupled model was developed to evaluate the impact of WSRS on seasonal Chl-a. The results showed that the surface Chl-a exhibited two peaks in spring and autumn until 2002, but has exhibited only one peak in spring-summer since 2002. Satellite-derived Chl-a concentrations in spring-summer since 2002 have increased by 56% compared to those until 2002. The simulated results showed that the change in Yellow River discharge induced by the WSRS has resulted in the appearance of high concentrations of Chl-a in summer over the Central Bohai Sea since 2002. The WSRS increased the ratio of added Chl-a owing to the riverine nutrients to total Chl-a by 19% compared to that until 2002. Overall, WSRS greatly affects the seasonal cycling of Chl-a in the Bohai Sea, and the side effect needs to be considered.

Seawater silicate fertilizer facilitated nitrogen removal via diatom proliferation

Dissolved inorganic nitrogen (DIN) enrichment accompanied by silicate deficiency in seawater can promote dinoflagellate growth over diatom growth and induce further negative ecological consequences. Here, we propose an easily exercisable method for silicate fertilization as a remedy of eutrophication. In the laboratory, rice husk ash (RHA) released silicate and phosphate in an atomic ratio range of 38-113 without a significant influence on DIN. During incubations of silicate-limited waters, low-dose fertilization increased the diatom/dinoflagellate ratio by 1-5 times. With the high-dose fertilizer addition, DIN, with an initial concentration of 7.63 ± 0.95 μmol l^-1, was exhausted in three days, and the diatom abundance increased by 19 times on the 5th day. The silicate fertilization method presented here can be applied independently in eutrophicated waters for dinoflagellate suppression and dissolved nitrogen removal; this method could also work as a supplementary measure to existing nutrient (N, P) reduction and biomanipulation efforts.

Comparative uptake and assimilation of nitrate, ammonium, and urea by dinoflagellate Karenia mikimotoi and diatom Skeletonema costatum s.l. in the coastal waters of the East China Sea

The nitrogen uptake kinetics and physiological growth of Karenia mikimotoi and Skeletonema costatum sensu lato grown on different N substrates and concentrations were compared in the laboratory. In the presence of three N substrates, both species preferred to take up NH₄⁺. K. mikimotoi and S. costatum s.l. showed the highest substrate affinities for urea and NO₃^-, respectively. Both species grew well on three N substrates, and the growth parameters were comparable among the different N substrates. However, K. mikimotoi assimilated urea more efficiently than it assimilated either NO₃^- or NH₄⁺. Different with S. costatum s.l., K. mikimotoi grew slowly and steady and the physiological and growth activities in N-depleted conditions were higher than those in N-replete conditions. Our results suggested that K. mikimotoi shows a greater readiness for uptake and assimilation of urea, and that this species is more competitive in an N-depleted environment when compared with S. costatum s.l.

Variability in water chemistry of the Three Gorges Reservoir, China

The environmental influence of the Three Gorges Reservoir (TGR) on the Changjiang River has been widely studied since the Three Gorges Dam (TGD) began operation in 2003. However, the changes in water chemistry in the reservoir in response to damming effect variations are poorly documented in the area of this large reservoir. The results suggest that in comparison to the water chemistry before the TGR operation, the inflow concentrations of Mg²⁺, K⁺, Na⁺ and Cl⁻ increased in the TGR, and the abundance of Ca²⁺ and HCO₃^- decreased in the inflow in the period after the TGR filling as a result of climate change and human activities in the Changjiang River basin. The ionic composition in the TGR is primarily controlled by contributions from the upstream region of the Changjiang River but was modified by the interaction between water and rocks within the TGR. The concentrations of most major ions as well as the equivalent ratios of the major ions increased in the TGR after the operation of TGD. This change yielded a 6% increase in the major ion loading downstream of the TGD. The Three Gorges area strongly contributes to the increase in ion loading in the TGR due to enhanced water and rock interactions in comparison with the period before TGD operation.

Alkaline phosphatase activities and regulation in three harmful Prorocentrum species from the coastal waters of the East China Sea

Harmful blooms of Prorocentrum donghaiense occur annually in the phosphorus-scarce coastal waters of the East China Sea (ECS). The enzymatic activities of alkaline phosphatase (AP) and its regulation by external phosphorus were studied during a P. donghaiense bloom in this area. The AP characteristics of P. donghaiense was further compared with Prorocentrum minimum and Prorocentrum micans in monocultures with both bulk and single-cell enzyme-labeled fluorescence AP assays. Concentrations of dissolved inorganic phosphorus (DIP) varied between 0.04 and 0.73 μmol l^-1, with more than half recording stations registering concentrations below 0.10 μmol l^-1. Concentrations of dissolved organic phosphorus (DOP) were comparable or even higher than those of DIP. P. donghaiense suffered phosphorus stress and expressed abundant AP, especially when DIP was lower than 0.10 μmol l^-1. The AP activities showed a negative correlation with DIP but a positive correlation with DOP. The AP activities were also regulated by internal phosphorus pool. The sharp increase in AP activities was observed until cellular phosphorus was exhausted. Most AP of P. donghaiense was located on the cell surface and some were released into the water with time. Compared with P. minimum and P. micans, P. donghaiense showed a higher AP affinity for organic phosphorus substrates, a more efficient and energy-saving AP expression quantity as a response to phosphorus deficiency. The unique AP characteristic of P. donghaiense suggests that it benefits from the efficient utilization of DOP, and outcompete other species in the phosphorus-scarce ECS.

Variations in the phytoplankton community due to dust additions in eutrophication, LNLC and HNLC oceanic zones

Dust deposition can bring nutrients and trace elements to the upper ocean and affect phytoplankton growth and community structure. We conducted a comparative study using on-board microcosm experiments amended with varying amounts of dust (0.2, 1, and 2 mg L^-1) in the East China Sea (eutrophic zone), the subtropical gyre (low-nutrient and low-chlorophyll zone, LNLC), and the Kuroshio-Oyashio transition region (high-nutrient and low-chlorophyll zone, HNLC) of the Northwest Pacific Ocean. The additions of dust supplied a considerable amount of nitrogen (N) and negligible phosphorus (P) relative to the seawater collected for incubation experiments (baseline), contributing to increases in Chlorophyll a with increasing dust additions. Significant linear correlations were observed between the net growth rates of larger cells (i.e., micro-size: >20 μm and nano-size: 2-20 μm) and available N (sum of baseline and added N) at each zone, demonstrating that phytoplankton size structure shifts towards larger cells with the increasing dust additions. In the experiments conducted in LNLC and HNLC zones, micro-sized phytoplankton (primarily consisting of diatoms) benefited most from dust additions. In the experiments conducted in eutrophic zone, however, the primary beneficiary was the nano-sized phytoplankton (primarily consisting of dinoflagellates). When a time lag of one day in relative abundance of diatoms (RAD, the abundance of diatoms divided by the sum of diatoms and dinoflagellates) relative to the N:P ratio was considered, we found the RAD increased substantially with increases in the N:P ratio until the ratio approached the Redfield ratio (N:P = 16:1), and then the RAD decreased gradually as the N:P ratio increased. This was ascribed to the lower sensitivity of dinoflagellates to nutrient shortage, relative to diatoms. Overall, our results suggested that the overwhelming input of N relative to P by dust deposition might cause significant ecological impacts by altering the N:P ratio of varying trophic seawaters.

Nutrient-limitation induced diatom-dinoflagellate shift of spring phytoplankton community in an offshore shellfish farming area

As mariculture expands offshore in response to the increasing demand for seafood, a new set of ecological concerns arises. We report on presented phosphate and silicate deficiencies in spring in the Zhangzi Island area, northern Yellow Sea, used for farming scallops. Silicon limitation was observed at up to 77.3% of stations, with an average silicate concentration as low as 1.7 μM in March 2014. Average phosphate concentration decreased from 0.12 to 0.05 μM from March to May. Stoichiometric ratios and absolute concentrations indicate that 78%-90% of stations showed phosphate limitation. Correspondingly, the phytoplankton community shifted from predominately diatoms to dinoflagellates. The higher frequency of nutrient limitation in farmed areas, compared with unseeded areas and northern Yellow Sea in general, imply intensified bottom-up controls on scallop production. The "bottle-neck" effect of limited food availability in spring suggests that carrying capacity was originally overestimated, when calculated from annual primary production.

The stresses of allelochemicals isolated from culture solution of diatom Phaeodactylum tricornutum Bohlin on growth and physiology of two marine algae

The allelopathic effects of extracts isolated from the culture filtrate of diatom Phaeodactylum triconutum Bohlin on typical marine microalgae Prorocentrum donghaiense Lu and Dunaliella salina Teodoresco were investigated by determining different physiological and biochemical parameters, such as growth rate, membrane systems and esterase activity under controlled laboratory conditions. The growth of P. donghaiense was significantly inhibited immediately after exposure to the allelochemicals, while the algae density of D. salina was less sensitive. Chlorophyll-a content, membrane systems, as well as esterase activity were simultaneously investigated by flow cytometry with particular fluorescent markers and exhibited changeable sensitivities. The results demonstrated that the membrane systems of P. donghaiense were suppressed by the allelochemicals directly, causing loss of integrity and membrane penetration. Esterase activity was the most sensitive indicator as that of P. donghaiense cells significantly increased in short time and was inhibited subsequently. However, the membrane of D. salina remained intact still after exposure to the extracts and the esterase activity was only inhibited on last day during experiment period. Membrane potential and chlorophyll-a content of the two marine algae also showed somewhat different changes, as that of P. donghaiense cells were impaired after 5 day exposure to all volume conditions while these two characteristics of D. salina was only suppressed by exposure to high volume of the allelochemicals on day 6. The present results indicated that the inhibition of culture filtrate of P. triconutum on P. donghaiense was algicidal whereas the effect on D. salina appeared to be algistatic.

Effects of increasing nutrient disturbances on phytoplankton community structure and biodiversity in two tropical seas

Statistical analysis of rainfall data from 2005 to 2015 showed that atmospheric deposition supplied large amount of dissolved inorganic nitrogen (38-155 mg·m^-2·month^-1) in N-deficient South China Sea and Eastern Indian Ocean. To understand marine ecosystem responses to increasing nutrient disturbances, we implemented field mesocosm experiments to study phytoplankton community structure and biodiversity responses to nutrient treatments with nitrate, phosphate and iron across tropical seas. Our results showed that DIN supply would change phytoplankton community structure and stimulated the regime shift from cyanobacteria to diatoms (relative dominance R > 0). Phytoplankton communities were dominated by diatoms (relative abundance >50%) accompanied by high chlorophyll a content with 1.58-39.27 μg·L^-1 in DIN-added cultures, whereas cyanobacteria dominated communities (relative abundance >60%) with low biomass of 0.12-0.18 μg·L^-1 in undisturbed cultures. Simultaneously increased DIN loading from atmospheric deposition would decrease ecological diversity of tropical seas owing to species competition and succession (Shannon diversity H' decreased to <1).

Influence of humic substances on iron distribution in the East China Sea

The influence of humic substances (specifically humic and fulvic acids, referred to as HS-HA and HS-FA) as well as other factors, such as major nutrient concentrations of total dissolved nitrogen (TDN), total dissolved phosphate (TDP) and hydrologic factors, on the distribution of total dissolved iron (DFe) and the chemical speciation of DFe was studied in the East China Sea (ECS) during a summer cruise in 2013. As the wide rage fraction of nature organic matter, the HS-HA, HS-FA in ESC contains most part of the organic ligand (Lt) of DFe. The concentrations of HS-HA, DFe and Lt in coastal water masses were higher than those in the water masses affected by the Kuroshio Current. The highest concentrations of HS-HA and DFe were observed in surface water at stations MT1 and MC4, with the value of 336.5 μg SRHA/L and 20.3 nmol/L, respectively, whereas, the lowest concentrations of HS-HA and DFe were observed in surface waters with the value of 149.6 μg SRHA/L and 0.4 nmol/L, respectively. HS-HA concentrations were more conservative than that of DFe. The DFe which were combined by unit weight HS-HA (mg^-1, IB) in the surface and bottom waters quickly decreased with increasing salinities from the Yangtze River estuary to the southeast of the ECS. Average IB values in bottom waters were higher than those in surface waters. This study indicated that Yangtze River dilution water and cold water from the Yellow Sea were the main source of HS-HA and DFe in ECS.