Variations of dominant free-floating Ulva species in the source area for the world's largest macroalgal blooms, China: Differences of ecological tolerance

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.

Regulation of seawater dissolved carbon pools by environmental changes in Ulva prolifera originating sites: A new perspective on the contribution of U. prolifera to the seawater carbon sink function

The Ulva prolifera bloom is considered one of the most serious ecological disasters in the Yellow Sea in the past decade, forming a carbon sink in its source area within a short period but becoming a carbon source at its destination. To explore the effects of different environmental changes on seawater dissolved carbon pools faced by living U. prolifera in its originating area, U. prolifera were cultured in three sets with different light intensity (54, 108, and 162 μmol m-2 s-1), temperature (12, 20, and 28 °C) and nitrate concentration gradients (25, 50, and 100 μmol L-1). The results showed that moderate light (108 μmol m-2 s-1), temperature (20 °C), and continuous addition of exogenous nitrate significantly enhanced the absorption of dissolved inorganic carbon (DIC) in seawater by U. prolifera and most promoted its growth. Under the most suitable environment, the changes in the seawater carbonate system were mainly dominated by biological production and denitrification, with less influence from aerobic respiration. Facing different environmental changes, U. prolifera continuously changed its carbon fixation mode according to tissue δ13C results, with the changes in the concentrations of various components of DIC in seawater, especially the fluctuation of HCO3- and CO2 concentrations. Enhanced light intensity of 108 μmol m-2 s-1 could shift the carbon fixation pathway of U. prolifera towards the C4 pathway compared to temperature and nitrate stimulation. Environmental conditions at the origin determined the amount of dissolved carbon fixed by U. prolifera. Therefore, more attention should be paid to the changes in marine environmental conditions at the origin of U. prolifera, providing a basis for scientific management of U. prolifera.

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.

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.

Effects of photoperiod on the growth and physiological responses in Ulva prolifera under constant and diurnal temperature difference conditions

Photoperiod and temperature are two main factors in the growth of macroalgae, and changes in photoperiod and diurnal temperature difference exist in natural condition. In order to study the effects of photoperiod and diurnal temperature difference on the growth of green algae Ulva prolifera, we cultured this species under three light/dark cycles (light: dark = 10:14, 12:12 and 16:08) with constant (22 °C for light and dark period, noted as 22-22 °C) and diurnal temperature difference (22 °C and 16 °C for light and dark period, respectively, noted as 22-16 °C) conditions. The results showed that: 1) Compared with 10:14 light/dark cycle, the growth of U. prolifera under 12:12 light/dark cycle was significantly enhanced by 39% and 16% for 22-22 °C and 22-16 °C treatments, respectively, while the increase proportion decreased when the daylength increase from 12 h to 16 h. 2) The enhancement in growth induced by diurnal temperature difference was observed under 10:14 light/dark cycle, but not for 12:12 and 16:08 light/dark cycle treatments. 3) The Chl a content and photosynthetic rate increased under short light period and 22-22 °C conditions, while under 22-16 °C conditions, higher photosynthetic rate was observed under 12:12 light/dark cycle and no significant difference in Chl a content was observed. 4) Under 22-22 °C conditions, compared with 10:14 (L:D) treatment, the expression levels of proteins in light-harvesting complexes, PSII and carbon fixation were down regulated, while the photorespiration and pentose phosphate pathway (PPP) were up regulated by 16:08 light dark cycle. Then we speculate that the higher photosynthetic rate may be one compensation mechanism in short photoperiod, and under long light period condition the up regulations of photorespiration and PPP can be in charge of the decrease in enhancement growth induced by longer daylength.

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.

The ecological effect of large-scale coastal natural and cultivated seaweed litter decay processes: An overview and perspective

Seaweeds are important components of marine ecosystems and can form a large biomass in a few months. The decomposition of seaweed litter provides energy and material for primary producers and consumers and is an important link between material circulation and energy flow in the ecosystem. However, during the growth process, part of the seaweed is deposited on the sediment surface in the form of litter. Under the joint action of the environment and organisms, elements enriched in seaweed can be released back into the environment in a short time, causing pollution problems. The cultivation yield of seaweed worldwide reached 34.7 million tons in 2019, but the litter produced during the growth and harvest process has become a vital bottleneck that restricts the further improvement of production and sustainable development of the seaweed cultivation industry. Seaweed outbreaks worldwide occur frequently, producing a mass of litter and resulting in environmental pollution on coasts and economic losses, which have negative effects on coastal ecosystems. The objective of this review is to discuss the decomposition process and ecological environmental effects of seaweed litter from the aspects of the research progress on seaweed litter; the impact of seaweed litter on the environment; and its interaction with organisms. Understanding the decomposition process and environmental impact of seaweed litter can provide theoretical support for coastal environmental protection, seaweed resource conservation and sustainable development of the seaweed cultivation industry worldwide. This review suggests that in the process of large-scale seaweed cultivation and seaweed outbreaks, ageing or falling litter should be cleared in a timely manner, mature seaweed should be harvested in stages, and dried seaweed produced after harvest and washed up on shore should be handled properly to ensure the benefits of environmental protection provided by seaweed growth and sustainable seaweed resource development.

Physiological responses of Ampithoe valida and its feeding potential on Ulva prolifera

Large numbers of Amphipoda feed on floating green tide macroalgae in the Yellow Sea, among which Ampithoe valida has a high abundance in the stable and decline periods. Amphipoda preferentially feed on Ulva. Under different temperatures, salinities, and pH, the physiological responses of A. valida and its feeding potential on Ulva prolifera were investigated, along with its physiological responses during green tide blooms in the Southern Yellow Sea. Ampithoe valida could survive within a temperature range of 5-30 °C, salinity of 5-40, and pH of 4-10. Optimal environmental conditions for growth were temperature 15-25 °C, salinity 10-40, and pH 6-10. At temperatures of 5-30 °C, salinities of 5-35, and pH of 4-9, A. valida could effectively reduce U. prolifera biomass. The feeding ability of A. valida was greatest in 25-30 °C, 10-25 salinity, and neutral seawater, exceeding 5 mg·ind.^-1·d^-1. During green tide outbreaks in the Southern Yellow Sea from May to August, the monthly average sea surface temperature, salinity, and pH range is 17.5-27.3 °C, 23.8-29.6, and 7.87-8.17, respectively, within which A. valida showed well growth and could effectively reduce U. prolifera biomass. Finally, this study further discussed the possibility of A. valida as a biological method to control green tide.

Temperature and high nutrients enhance hypo-salinity tolerance of the bloom forming green alga, Ulva prolifera

The response of seaweeds to environmental stressors can be population-specific, and be related to the regime of their habitats. To explore the growth and physiological responses of Ulva prolifera, two strains of this alga (Korean and Chinese strains) were studied under an interaction of temperature (20 and 25 °C), nutrients (low nutrients: 50 μM of nitrate and 5 μM of phosphate; high nutrients: 500 μM of nitrate and 50 μM of phosphate) and salinity (20, 30 and 40 psu). The lowest growth rates of both strains were observed at 40 psu of salinity, independent of temperature and nutrient levels. At 20 °C and low nutrients condition, the carbon: nitrogen (C: N) ratio and growth rate in the Chinese strain were increased by 31.1% and 21.1% at a salinity of 20 psu in comparison to the salinity of 30 psu, respectively. High nutrients decreased the ratio of C:N in both strains with increasing tissue N content. At the same time, high nutrients also increased soluble protein and pigments contents, as well as photosynthetic and growth rates in both strains at the same salinity levels at 20 °C. Under 20 °C and high nutrients conditions, the growth rates and C:N ratio of both strains were significantly decreased with increasing salinity. The pigment, soluble protein and tissue N showed an inverse trend with the growth rate at all conditions. Moreover, the higher temperature of 25 °C inhibited the growth in both strains regardless of nutrients levels. The temperature of 25 °C enhanced the contents of tissue N and pigments in the Chinese strain only at the low nutrients level. The interaction of high nutrients and 25 °C led to the accumulation of tissue N and pigment contents in both strains under all salinity conditions compared to the 20 °C and high nutrients level. The temperature of 25 °C and high nutrients decreased the growth rate in the Chinese strain at both salinities of 30 and 40 psu more than the 20 °C, and low nutrients level at the same salinity. These results suggest that the Ulva blooms caused by the Chinese strain were more impacted at hypo-salinity levels compared to the Korean strain. Eutrophic or high nutrients level enhanced the salinity tolerance in both strains of U. prolifera. There will be a decline of U. prolifera blooms of the Chinese strain at hyper-salinity levels.

Two bloom-forming species of Ulva (Chlorophyta) show different responses to seawater temperature and no antagonistic interaction

The generalized use of molecular identification tools indicated that multispecific green tides are more common than previously thought. Temporal successions between bloom-forming species on a seasonal basis were also revealed in different cold temperate estuaries, suggesting a key role of photoperiod and temperature controlling bloom development and composition. According to the Intergovernmental Panel on Climate Change, water temperatures are predicted to increase around 4°C by 2100 in Ireland, especially during late spring coinciding with early green tide development. Considering current and predicted temperatures, and photoperiods during bloom development, different eco-physiological experiments were developed. These experiments indicated that the growth of Ulva lacinulata was controlled by temperature, while U. compressa was unresponsive to the photoperiod and temperatures assayed. Considering a scenario of global warming for Irish waters, an earlier development of bloom is expected in the case of U. lacinulata. This could have significant consequences for biomass balance in Irish estuaries and the maximum accumulated biomass during peak bloom. The observed seasonal patterns and experiments also indicated that U. compressa may facilitate U. lacinulata development. When both species were co-cultivated, the culture performance showed intermediate responses to experimental treatments in comparison with monospecific cultures of both species.

Prevention strategies for green tides at source in the Southern Yellow Sea

As global ecological disasters, green tide outbreaks have been observed in the Southern Yellow Sea (SYS) of China since 2007, resulting in considerable economic losses and environmental damage to the coastal cities of Jiangsu and Shandong Provinces. Therefore, prevention of green tides is crucial. Previous studies have revealed that a relatively small green tide outbreak scale in the SYS was observed in 2018 and 2020, with the green tides covering areas of 193 km² and 192 km² and durations of 91 days and 64 days, respectively. Killing green macroalgae attached to cultivation ropes in Neopyropia aquaculture areas, which has been considered a primary source of the blooms, early removal of Neopyropia aquaculture rafts, and green tide prevention in the SYS are the key reasons for the decrease in green tides in 2018 and 2020. Furthermore, to address the challenges associated with the current green tide source prevention measures, we proposed a comprehensive control method that combines ecological farming, early green tide prevention, and resource utilization. Potential secondary pollution caused by the chemicals used to control Ulva prolifera can be minimized. Conversely, Neopyropia yezoensis quality may be enhanced through continuous improvement of its culturing process, which in turn, could reduce the green tide blooming scale.

Massive Ulva Green Tides Caused by Inhibition of Biomass Allocation to Sporulation

The green seaweed Ulva spp. constitute major primary producers in marine coastal ecosystems. Some Ulva populations have declined in response to ocean warming, whereas others cause massive blooms as a floating form of large thalli mostly composed of uniform somatic cells even under high temperature conditions-a phenomenon called "green tide". Such differences in population responses can be attributed to the fate of cells between alternative courses, somatic cell division (vegetative growth), and sporic cell division (spore production). In the present review, I attempt to link natural population dynamics to the findings of physiological in vitro research. Consequently, it is elucidated that the inhibition of biomass allocation to sporulation is an important key property for Ulva to cause a huge green tide.

Complete chloroplast genome of Ulva compressa (Ulvales: Ulvaceae)

Ulva compressa is one of the causal green macroalgae in many countries. In this study, complete chloroplast genome sequence of U. compressa was reported, and the total length of this species was 94,226 bp (GenBank accession number MT916929). The overall base composition of chloroplast genome was A (37.2%), T (37.0%), C (12.7%) and G (13.1%), and the percentage of A + T (74.2%) was higher than C + G (25.8%). U. compressa chloroplast genome encoded 90 genes, including 63 protein-coding genes, 23 transfer RNAs genes, and 4 ribosomal RNAs genes. The maximum likelihood phylogenetic analysis showed that U. compressa is the closest sister species of U. linza. This study will be helpful to understand the genetic diversity of Ulva species.

Characterization of the development stages and roles of nutrients and other environmental factors in green tides in the Southern Yellow Sea, China

Large-scale floating green tides in the Southern Yellow Sea (SYS) caused by the macroalgal species Ulva prolifera have been recurring for 13 years and have become one of the greatest marine ecological disasters in the world. In this study, we attempt to explore the development pattern of green tides and find its key environmental influencing factors. The satellite remote sensing data of the development process of green tides fit the logistic growth curve (R² = 0.93, P < 0.01) well, showing three distinct growth phases (lag, exponential growth, and short plateau phases). Correspondingly, the green tide-drifting area from the coast of Jiangsu to the nearshore waters of the Shandong Peninsula was divided into three sections: the lag phase zone (A), the exponential growth phase zone (B), and the plateau phase zone (C). Zone A in the south of Jiangsu coastal waters had abundant inorganic nutrients that were indispensable to the green tide initiation. Zone B was mainly located out of Haizhou Bay, south of 34.5° N and north of 35.5° N, where approximately 80% of the green tide biomass was generated. The rich bioavailable nutrient sources, suitable temperature, and irradiance in this area were the main promotion factors for the rapid growth and scale expansion of green tides. Wet precipitation in zone B in May and June also played an important role in the final scale of green tides. Zone C had poor nutrients, increasing temperature, and irradiance (high transparency), which limited the continued expansion of green tides, and organic nutrients might be an important support to green tides development in this region. The study based on the growth phases of green tides could help us further understand the eutrophication mechanism in the green tide outbreaks in SYS.

Competitive advantages of Ulva prolifera from Pyropia aquaculture rafts in Subei Shoal and its implication for the green tide in the Yellow Sea

The physiological characteristics of Ulva prolifera and Blidingia sp. during two pre-bloom stages (March & May) were compared to evaluate the competitive advantage of U. prolifera on Pyropia aquaculture rafts in Subei Shoal. (1) Compared to Blidingia sp., U. prolifera had a lower growth rate, chlorophyll content, photosynthetic efficiency, and antioxidant capacity in March. (2) In May, various indicators of U. prolifera's physiological function improved significantly, while the antioxidant capacity of Blidingia sp. decreased significantly. Large lipidic globules in U. prolifera cells became scattered small lipidic globules in May, which indicated a decrease in lipid membrane peroxidation. (3) In U. prolifera, the ratio of buoyancy to gravity of per unit volume was 1.73, and the bubbles inside the thalli provided 60% of the total buoyancy. Buoyancy generated by the inflatable structure of U. prolifera allowed this species to float after being separated from the rafts.

Role of nutrients in the development of floating green tides in the Southern Yellow Sea, China, in 2017

The largest-scale green tides in the world caused by Ulva prolifera have been recurring annually in the Southern Yellow Sea since 2007. In this study, spatio-temporal variations of green tides and nutrients were investigated in the spring and summer of 2017, and the roles of different nutrients in the development of green tides are discussed. The results showed that the development of green tides could be divided into two parts according to the distinct growth phases of green tides: (1) the development area (DA), which was located south of 35°N and characterised by the quick expansion of green tide and high-content nutrient; (2) the accumulation area (AA), which was located north of 35°N and characterised by high U. prolifera coverage area and low-content inorganic nutrients. Through calculation of nutrient reductions, we found that DA provided 96% of nitrogen and 87% of phosphorus for the development of green tides in 2017, and the dominant nutrient species were dissolved inorganic nitrogen and dissolved organic phosphorus. Regarding AA, the dominant nitrogen component was dissolved organic nitrogen. Thus, we conclude that reducing the level of nutrient input in order to alleviate the eutrophication of seawater in the Jiangsu coastal area may be an important measure for reducing the scale of green tides.

Reproductive strategy of the floating alga Ulva prolifera in blooms in the Yellow Sea based on a combination of zoid and chromosome analysis

Green algal blooms have occurred in the Yellow Sea for 13 consecutive years since 2007. However, little is known about the reproductive strategy of the dominant species Ulva prolifera in the field. In particular, it is not clear whether the floating Ulva species are sporophytes or gametophytes, and if their life history is sexual or asexual. In this study, the life history type was determined based on the size, phototactic response, and flagella number for the zoids in at least two successive generations. In addition, chromosome observations were conducted to distinguish the gametophytes and sporophytes in the floating Ulva species. The results showed that the floating Ulva species were all sporophytes with sexual reproductive patterns, thereby indicating that this Ulva species always maintains vegetative growth from April to June during the early stage of the blooms. In addition, we found that the chromosome numbers were 18 for the diploid sporophytes and nine for the haploid male and female gametophytes. These results provide useful information to help understand the explosive growth of these green algal blooms.

Buoyancy potential of dominant green macroalgal species in the Yellow Sea's green tides, China

Large-scale green tides caused by Ulva prolifera, occurred for 12 consecutive years in the Yellow Sea of China. To resolve the abrupt shift in species composition between attached and floating macroalgal assemblages, field experiments were conducted from May to July 2017 to quantify the net buoyancy force and compare the floating potential of the common green macroalgae from the red algal seaweed Pyropia yezoensis rafts. At the same time, U. prolifera from different sampling locations were tested to study variable buoyancy of this species and the associated influencing factors. Our results illustrated a stronger positive buoyant force and a proportionally greater buoyancy capacity of U. prolifera, compared to the other co-occurring species. Buoyancy is a dynamic trait and is closely correlated with light intensity, morphology and physiological status. The positive buoyancy of U. prolifera is an important factor that helps explain its predominance in the Yellow Sea's large-scale green tides.

Spatial and temporal variability of biomass and composition of green tides in Ireland

Although nutrient enrichment of estuarine and coastal waters is considered a key factor for the development of green tides, the extent, distribution, and species composition of blooms vary among systems of similar nutrient loading, which compromises our ability to predict these events based on information about nutrient status alone. Additional factors may play a role in the control and development of macroalgal blooms. The identification of relevant scales of variation is a necessary prerequisite before explanatory models can be proposed and tested. In this study spatial and temporal patterns of biomass distribution were assessed for two Ulva morphologies in two Irish estuaries heavily affected by green tides (wet biomass >1 kg m^-2 during the peak bloom). Moreover, using genetic markers, the species composition of these green tides was assessed. Results revealed that these blooms were multi-specific, with Ulva prolifera, U. compressa and U.rigida the most frequent species. The species U. prolifera and U. compressa usually showed a tubular morphology, while U. rigida was mainly laminar. A seasonal succession common to both estuaries was also identified, with the bloom dominated by tubular species during spring and early summer, and co-dominated by tubular and laminar morphologies during late summer and autumn. Moreover, tubular and laminar morphologies exhibited different distribution patterns, with tubular morphologies varying at bigger spatial scales and higher biomass than the laminar. As tubular and laminar morphologies exhibited different distribution patterns, varying tubular morphologies along bigger spatial scales with higher biomass levels than the laminar. Considering that tubular morphologies were usually anchored to the sediment, while laminar Ulva were usually observed free-floating, these differences could explain a differential influence by water motion. An important annual and decadal variability in biomass levels of Ulva was observed, in the case of the Tolka estuary a noticeable increase over the last two decades. These findings should be considered for the development of management and monitoring strategies since the different habitat of laminar and tubular morphologies (anchored vs. free-floating) may play an important role in the balance of nutrients and biomass in the estuary, or determine the response to pollutant exposure. Furthermore, the presence of different species with different ecological requirements could favour the duration and extension of the bloom though temporal and spatial successions.