A bibliometric review of Green Tide research between 1995-2023.

In recent years, the frequent occurrence of green tides has attracted attention from academia and industry. Despite some literature reviews, systematic bibliometric and visualization analyses are still lacking. The study employs CiteSpace and VOSviewer tools to conduct a bibliometric and visualization analysis of green tide-related literature from the Web of Science (1995 to 2023). The study identifies key countries, institutions, journals, disciplines, and authors, and maps out their collaborative networks. Co-citation analysis provides an initial overview of various aspects within the green tide field. Keyword analysis has reveals six core themes: remote sensing applications, eutrophication and green tides, phylogenetic analysis, the impact of climate change, green tide management and applications, and studies focused on green tides in the China Sea. Additionally, keyword burst analysis has revealed two emerging trends. This study provides a strategic framework for future research, serving as a navigational guide in the field of green tide studies.

Temporal succession of micropropagules during accumulation and dissipation of green tide algae: A case study in Rudong coast, Jiangsu Province.

Over the past 18 years, green tides have persistently occurred in the Yellow Sea. Micropropagules of these algae are key to bloom formation, yet their species composition and succession during dissipation remain underexplored. During the dissipation process of accumulated green tide algae, a large number of micropropagules are released. This study monitored the dissipation of green tide algae at a coastal site, tracking micropropagules in water and sediment using an internal transcribed spacer (ITS) and 5S rDNA primers. Results showed that the dissipation lasted about one month, with significant micropropagule release. Initially, micropropagules matched 5S-II Ulva prolifera, but later species like Ulva torta, Ulva simplex, Ulva flexuosa, and Ulva meridionalis emerged. Ulva meridionalis dominated sediment in July and August, while U. torta was prevalent in water, and U. flexuosa was dominant in other months. Accumulated U. prolifera in the intertidal zone may not contribute to the seeding of the next year's bloom. This study sheds light on the dissipation process and succession patterns of micropropagules in coastal environments.

How did the floating Ulva prolifera develop into the world's largest green tide?

It has been 16 years since the world's largest Ulva bloom appeared in the Yellow Sea. However, it remains unclear how the floating Ulva prolifera developed into the immense green tide within two months especially considering that source control measures have been conducted since 2019. In this study, we investigated the growth mechanism of the floating population by examining the production and regeneration of U. prolifera explants and constructing a growth model. The results showed that U. prolifera explants exhibited rapid regeneration mainly through branches at 15, 20 and 25 °C. Algivore feeding and sporulation (at 20-25 °C) contributed to mass production of explants. The growth of floating population could be attributed to the coupling of mass production and rapid regeneration of explants through multiple cycles, which accounted for the outbreak of floating green tide. This finding presented a novel population growth model to elucidate macroalgal bloom occurrences.

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

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

Seasonal and interannual variations of nutrients in the Subei Shoal and their implication for the world's largest green tide.

The world's largest "green tide" (Ulva prolifera) has occurred every year since 2007 in the Yellow Sea. The Subei Shoal area is thought to be the origin of the green tide. Based on field data from 2016 to 2023, seasonal and interannual variations of dissolved nutrients and their ecological effects in the Subei Shoal were analyzed. Spatial distribution of dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP) and dissolved silicate (DSi) showed clear terrestrial sources, while ammonia (NH4-N) and dissolved organic nitrogen (DON) were not solely controlled by terrestrial sources. The seasonal variations of NH4-N, DIN, DON, DIP and DSi concentrations were significant, and the interannual variations of DIN, DON, DIP and DSi concentrations showed general decreasing trends from 2016 to 2023. The key factors affecting the seasonal and interannual variations of DIN and DIP concentrations were terrestrial input, aquaculture wastewater discharge, atmospheric deposition, submarine groundwater discharge and macroalgae absorption, while the dominant factor determining the variations of DSi concentrations was terrestrial input. NH4-N and DON concentrations were mainly influenced by aquaculture wastewater discharge and the absorption and release of macroalgae. The high nutrient concentrations in the Subei Shoal throughout the year provided sufficient material basis for the growth of Ulva prolifera in the source area of green tide outbreak.

Exploring the response and prediction of phytoplankton to environmental factors in eutrophic marine areas using interpretable machine learning methods.

Coastal marine areas are frequently affected by human activities and face ecological and environmental threats, such as algal blooms and climate change. The community structure of phytoplankton-primary producers in marine ecosystems-is highly sensitive to environmental factors, such as temperature, salinity, and nutrients. However, traditional methods for exploring the relationship between phytoplankton communities and environmental factors in eutrophic marine areas are limited by various factors. Therefore, this study employed interpretable machine learning models, integrating high-dimensional data analysis and complex system modeling, to quantitatively and thoroughly analyze the dynamic relationship between phytoplankton communities and environmental variables in high-frequency samples collected over 53 weeks from eutrophic marine areas. The cell abundance of phytoplankton exhibited a distinct "two-peak pattern" variation. Interpretable machine learning model analysis revealed the dynamic contributions of different environmental factors during changes in the phytoplankton community structure. The results showed that temperature was a key environmental factor that affected phytoplankton growth during peak periods. In addition, the contribution of salinity increased during the second peak in phytoplankton abundance, highlighting its central role in the ecological dynamics of this phase. During green tide outbreaks, particularly in Area 01, the contributions of factors such as temperature and salinity increased, whereas those of phosphates and silicates decreased, indicating that green tide outbreaks substantially altered the nutritional dynamics of the ecosystem. Furthermore, different phytoplankton species, such as Skeletonema costatum, Thalassiosira spp., and Nitzschia spp., exhibit varying responses to environmental factors. Hence, the predictions made using random forest and generalized additive models for phytoplankton cell abundance in two marine areas revealed complex nonlinear relationships between environmental factors, such as temperature, salinity, and phytoplankton abundance.

Differential effects of petroleum hydrocarbons on the growing development and physiological characteristics of Ulva species.

To compare the different effects of petroleum hydrocarbons on intertidal Ulva macroalgae, three dominant Ulva species (U. prolifera, U. linza, and U. lactuca) were exposed to two water-accommodated fractions (WAFs) of 0# diesel oil and crude oil at three concentration levels. The results indicated that two WAFs had significant concentration effects on the physiological characteristics of Ulva, the toxicity of 0# diesel oil was greater than crude oil, and crude oil had hormesis effect. Exposure of high WAFs concentrations, the growth, pigment, carbohydrate, and protein contents of Ulva were inhibited, while the antioxidant system was activated. In addition, the integrated biomarker response (IBR) indicated that U. prolifera had higher resistance to WAFs than U. linza and U. lactuca. Considering that U. prolifera is the main species of green tide in the Yellow Sea (YS) of China, the comparative effects of WAFs on different development stages of U. prolifera were also explored. The results showed that spore was the most sensitive to WAFs, while adult thalli was the most tolerant. The increased resistance of U. prolifera thalli and the hormesis effect triggered by crude oil may influence the outbreak scale of green tides. This study provides a new perspective for understanding the formation of green tides in the YS.

Network and evolutionary analysis of green tide management policies in the Yellow Sea, China.

Since 2007, persistent green tides in the Yellow Sea of China (YSC) have inflicted substantial economic and social losses. In response, the Chinese government has enacted various policies to mitigate these impacts. This study introduced an evolutionary-multiple streams framework and employed Social Network Analysis (SNA) and Text Analysis from 2007 to 2023, tracing three phases of policy development: growth (2007-2012), stability (2013-2017), and explosion (2018-2023). Findings indicated that during the growth phase, the most of the policy themes were "monitoring" and "emergency"; in the stability phase, policy issuance by provincial and municipal agencies began to increase; in the explosion phase, a basic consensus was reached on source control and intergovernmental cooperation in the management of green tides. Themes such as "ecology", "extreme weather", and "green tide exploitation" have been emphasized. This analysis provides insights for future policy formulation in green tide control and broader marine environmental governance.

Review of Allelopathy in Green Tides: The Case of Ulva prolifera in the South Yellow Sea.

The proliferation of large green macroalgae in marine environments has led to the occurrence of green tides, particularly in the South Yellow Sea region of China, where Ulva prolifera has been identified as the primary species responsible for the world's largest green tide events. Allelopathy among plants is a critical factor influencing the dynamics of green tides. This review synthesizes previous research on allelopathic interactions within green tides, categorizing four extensively studied allelochemicals: fatty acids, aldehydes, phenols, and terpenes. The mechanisms by which these compounds regulate the physiological processes of green tide algae are examined in depth. Additionally, recent advancements in the rapid detection of allelochemicals are summarized, and their potential applications in monitoring green tide events are discussed. The integration of advanced monitoring technologies, such as satellite observation and environmental DNA (eDNA) analysis, with allelopathic substance detection is also explored. This combined approach addresses gaps in understanding the dynamic processes of green tide formation and provides a more comprehensive insight into the mechanisms driving these phenomena. The findings and new perspectives presented in this review aim to offer valuable insights and inspiration for researchers and policymakers.

The green tide causative-species Ulva prolifera responding to exposure to oil and dispersant.

In order to study the role of oil spills in the occurrence of green tide in the Yellow Sea, the physiological characteristics and photosynthetic activities of green tide causative-species Ulva prolifera was monitored under different conditions including two oil water-accommodated fractions (WAFs) of diesel oil and crude oil, dispersed water-accommodated fractions (DWAFs) and dispersant GM-2. The results showed that, the physiological parameters of U. prolifera including the growth, pigment, carbohydrate and protein contents decreased with the increased diesel oil WAF (WAFDO) concentration, while crude oil WAF (WAFCO) showed low concentration induction and high concentration inhibition effect. In addition, with the increase of WAFs concentration, two antioxidant activities were activated. However, compared with WAFDO alone and WAFCO alone, the mixture of oil and dispersant enhanced the toxicity on the above physiological characteristics of U. prolifera. On the other hand, the photosynthetic efficiency of U. prolifera showed a similar trend. Two WAFs showed significant concentration effects on the chlorophyll-a fluorescence transients and JIP-test. The addition of dispersant further blocked the electron flow beyond QA and from plastoquinone (PQ) to PSI acceptor side, damaged the active OEC centers at the PSII donor side, suppressed the pool size and the reduction rate of PSI acceptor side, and reduced the energy transfer efficiency between PSII functional units. These results implied that the crude oil spills may induce the formation of U. prolifera green tide, and the oil dispersant GM-2 used after the oil spills is unlikely to further stimulate the scale of bloom, while the diesel oil spills is always not conducive to the outbreak of green tide of U. prolifera.

Spatio-temporal distribution of micropropagules of green algae along the Jiangsu coast.

We conducted continuous monitoring at 13 stations along the Jiangsu coast to study the spatiotemporal distribution, population succession of micropropagules of green algae, and their impact on the outbreak of Southern Yellow Sea green tide. The study discovered that: 1) Green algae micropropagules had obvious temporal and spatial distribution and population changes along the Jiangsu coast. The monthly average abundance of micropropagules of green algae at station BH1, which was the high-value area, was 1230 inds/L. Station XS2 had the second-highest value area. Green algae micropropagules had an average monthly abundance of 836 inds/L. Between stations XS2 and BH1, the amount of green algae micropropagules steadily declined in comparison to other stations. The abundance was greatest from spring to early summer, and Ulva prolifera micropropagules predominated; 2) Compared with salinity, temperature had a more obvious effect on the micropropagules of green algae along the Jiangsu coast; 3) Green algae micropropagules on the Jiangsu coast could be a potential additional source on the outbreak of Southern Yellow Sea green tide. More data are needed to corroborate this conclusion. For the purpose of preventing and managing green tide, it is crucial to investigate the Southern Yellow Sea's potential supplementary source. This study analyzes the spatiotemporal distribution and population changes of green algae micropropagules along the Jiangsu coast, as well as their impact on green tide outbreaks, providing scientific data support for the prevention and control of green tides in the Southern Yellow Sea.

The survival, gene expression, and DNA methylation of Paralichthys olivaceus impacted by the decay of green tide and bacterial infection in both laboratory and field simulation experiments.

The recurring appearance of Ulva prolifera green tides has become a pressing environmental issue, especially for marine transportation, tourism, and aquaculture in the stage of decomposition. An abundance of decaying U. prolifera leads to water acidification, hypoxia and pathogenic microorganism proliferation, threatening marine germplasm resources, particularly benthic organisms with weak escape ability. Epigenetic modification is considered to be one of the molecular mechanisms involved in the plastic adaptive response to environmental changes. However, few studies concerning the specific impact of decaying green tide on benthic animals at the epigenetic level. In this study, decomposing algal effluents of U. prolifera, sediments containing uncorrupted U. prolifera, pathogenic microorganism were considered as impact factors, to reveal the effect of decaying U. prolifera on marine economic benthic species, Paralichthys olivaceus, using both field and laboratory simulation experiments. Field simulation experiment showed higher mortality rates and serious histopathological damage than the laboratory simulation experiment. And both the decaying U. prolifera and the sediment containing U. prolifera were harmful to P. olivaceus. Genome-wide DNA methylation and transcription correlation analyses showed that the response of P. olivaceus to green tide stress and bacterial infection was mainly mediated by immune signaling pathways such as PI3K-Akt signaling pathway. DNA methylation regulates the expression of immune-related genes involved in the PI3K-Akt signaling pathway, which enables P. olivaceus to adapt to the adverse environmental stresses by resisting apoptosis. In summary, this research analyzed the potential role of P. olivaceus in decaying U. prolifera, which is of great significance for understanding the impact of decaying green tide on marine commercial fish and also provides some theoretical guidance for the proliferation and release of fish seedlings.

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

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

Spatial and diel variations of bacterioplankton and pico-nanoeukaryote communities and potential biotic interactions during macroalgal blooms.

We investigated spatial heterogeneity and diel variations in bacterioplankton and pico-nanoeukaryote communities, and potential biotic interactions at the extinction stage of the Ulva prolifera bloom in the Jiaozhou Bay, Yellow Sea. It was found that the presence of Ulva canopies significantly promoted the cell abundance of heterotrophic bacteria, raised evenness, and altered the community structure of bacterioplankton. A diel pattern was solely significant for pico-nanoeukaryote community structure. >50 % of variation in the heterotrophic bacterial abundance was accounted for by the ratio of Bacteroidota to Firmicutes, and dissolved organic nitrogen effectively explained the variations in cell abundances of phytoplankton populations. The factors representing biotic interactions frequently contributed substantially more than environmental factors in explaining the variations in diversity and community structure of both bacterioplankton and pico-nanoeukaryotes. There were higher proportions of eukaryotic pathogens compared to other marine systems, suggesting a higher ecological risk associated with the Ulva blooms.

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

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

Physiological cost of Ulva lactuca under combined pressure of nutrient-oxytetracycline: One potential formation and maintenance mechanism of Ulva sp. green tide.

Environmental pollution is considered to lead to Ulva sp. green tides. Nevertheless, nutrients with high concentrations inhibit algae which may be damaged by antibiotics, such as OTC (oxytetracycline). Thus, Ulva sp. algae might pay a physiological cost under nutrient-OTC combined pressures. If this hypothesis is confirmed, Ulva sp. algae cannot easily form green tides, or green tides are difficult to maintain. To test this hypothesis, an uniform design experiment during which OTC, ammonia (NH4-N) and phosphate (PO4-P) were factors was set to simulate nutrient-OTC combined pressures, and Ulva lactuca was exposed to the pressures for 96 h. The TN (total nitrogen, CTN) or TP (total phosphorus, CTP) content in U. lactuca increased with increasing nutrient concentrations, as CTN = 21.206±1.000+ 1.227±0.418NH4-N × PO4-P (R2 = 0.282, p < 0.05) and CTP = 1.886±0.266+ 0.877±0.126PO4-P (R2 = 0.689, p < 0.05), respectively. The increase in dry weight of U. lactuca (Wdry) had a relationship with combined pressures, Wdry = 0.011±0.029 - 0.036±0.014PO4-P (R2 = 0.243, p < 0.05), i.e., the algal growth was inhibited by increasing PO4-P concentration. The SOD (Superoxide dismutase) activity (ASOD) was stimulated by OTC, as ASOD = 127.868±8.741+9.587±4.179 OTC (R2 = 0.193, p < 0.05). The contents of Chl a and b (Ca and Cb) were negatively affected by OTC or PO4-P with high concentration, as Ca = 0.566±0.042 - 0.024±0.022 OTC × PO4-P (R2 = 0.179, p < 0.05) and Cb = 0.512±0.043-0.044±0.020PO4-P (R2 = 0.180, p < 0.05). Thus, too high concentrations of PO4-P or OTC may hinder the formation and maintenance of Ulva sp. green tides.

Revolutionizing early-stage green tide monitoring: eDNA metabarcoding insights into Ulva prolifera and microecology in the South Yellow Sea.

Green tides, characterized by excessive Ulva prolifera blooms, pose significant ecological and economic challenges, especially in the South Yellow Sea. We successfully employed 18S environmental DNA (eDNA) metabarcoding to detect Ulva prolifera micropropagules, confirming the technique's reliability and introducing a rapid green tide monitoring method. Our investigation revealed notable disparities in the eukaryotic microbial community composition within Ulva prolifera habitats across different regions. Particularly, during the early stages of the South Yellow Sea green tide outbreak, potential interactions emerged between Ulva prolifera micropropagules and certain previously undocumented microorganisms from neighboring waters. These findings enhance our comprehension of early-stage green tide ecosystem dynamics, underscoring the value of merging advanced molecular techniques with conventional ecological methods to gain a comprehensive understanding of the impact of green tide on the local ecosystem. Overall, our study advances our understanding of green tide dynamics, offering novel avenues for control, ecological restoration, and essential scientific support for sustainable marine conservation and management.

Metatranscriptomic insights into the microbial metabolic activities during an Ulva prolifera green tide in coastal Qingdao areas.

Green tide, a typical marine environmental disaster that profoundly influenced the coastal areas, has been occurred consecutively in the South Yellow Sea of China since 2007. Herein, the active microbial community structure and metabolic pathways in Qingdao offshore during an Ulva prolifera green tide were investigated by using metatranscriptomic approach. The dominant active microbial taxa at the outbreak phase were primarily a functional group that can utilize organic matters derived from U. prolifera, such as Lentibacter, Polaribacter and Planktomarina. While the taxa involved in biogeochemical cycles, including Phaeobacter, Pseudomonas and Marinobacterium, dominated the active microbial communities at the decline phase. The expression level of enzymes involved in U. prolifera polysaccharides degradation was significantly higher at the outbreak phase compared to the decline phase. At the same time, the main players Glaciecola and Polarbacter showed similar trends, suggesting that the low competitiveness for nutrients of related microorganisms at this phase made them degrade more U. prolifera polysaccharides to meet their own nutrient needs, thereby accelerating the degradation of U. prolifera. According to KEGG annotation, the biogeochemical pathways including nitrogen cycle, sulfur cycle and methane oxidation altered during the green tide, with thiosulfate oxidation and methane oxidation probably being the crucial pathways at the outbreak and the decline phase respectively. The coupling of sulfide oxidation and denitrification was also observed in this study. Furthermore, the green tide in Qingdao offshore might impact the greenhouse effects induced by CH4 and N2O through influencing the related microbial processes.

Morphological characteristics and genetic diversity of floating and attached Ulva prolifera--A case study in the Yellow Sea, China.

Green tides in the Yellow Sea have occurred periodically since 2007, impacting the ecological environment and green algal communities along the coasts of Jiangsu and Shandong provinces in China. To investigate the morphological characteristics and genetic diversity of Ulva prolifera, we conducted surveys and comparative analyses of both its floating and attached forms along the coastal areas of Jiangsu and Shandong. The results revealed that the external morphology of the floating U. prolifera was multibranched. The attached U. prolifera displayed significant morphological variation among individuals. Based on the analysis of the amplified characteristic bands of the chloroplast gene, it was shown that both floating and attached U. prolifera could hybridize with Ulva linza. The genetic diversity of U. prolifera was studied using mitochondrial and chloroplast genome fragments. All floating U. prolifera and three strains of attached U. prolifera belonged to the same haplotype. The genetic diversity of floating U. prolifera was low, and there were some genetic differences with attached U. prolifera. The attached U. prolifera displayed a higher level of genetic diversity with abundant sites of variation and haplotypes.

Use of citric acid-activated chlorine dioxide to control Ulva prolifera.

Since 2007, green tides have occurred almost every year in the Yellow Sea, and a method to prevent them and to control levels of attached Ulva prolifera is urgently needed. In this study, we measured the effects of different concentrations of citric acid-activated chlorine dioxide solution (0, 50, 100, 150, 200, and 250 mg/L of chlorine dioxide) on the morphology (macrostructure and microstructure), chlorophyll a content, chlorophyll b content, carotenoid content, and chlorophyll fluorescence parameters (Fv/Fm, Y (II), NPQ, and ETRmax) of U. prolifera. Micropropagules in the treatment filtrate were cultured to determine whether the solution reduced the number of micropropagules released during the treatment process. The results showed that citric acid-activated chlorine dioxide at the appropriate concentration can be applied to remove U. prolifera from Neopyropia cultivation rafts. Because U. prolifera and its micropropagules died in the 250 mg/L chlorine dioxide group, we recommend that the appropriate concentration of chlorine dioxide for removing green macroalgae is ≥250 mg/L. Our results provide a scientific basis for convenient collection of accurate data for the U. prolifera prevention trial organized by the Ministry of Natural Resources of the People's Republic of China.