Unusual distribution of floating seaweeds in the East China Sea in the early spring of 2012

Frontal processes as drivers of floating marine debris in coastal areas

The influence of floating marine debris (FMD) on coastal and marine communities and ecosystems is undeniable, and attention is increasingly focused on ecologically and biologically important coastal areas. To protect marine life and valuable resources from FMD pollution, identifying FMD accumulation zones is recognized as a priority. One of the coastal ocean processes found governing the distribution of FMD is water convergence (frontal zones). These fronts are driven by various oceanographical factors. To date, the transport and accumulation of FMD in relation to fronts in coastal areas is poorly understood. To address this knowledge gap, we reviewed various types of ocean fronts as well as FMD accumulation along frontal zones in coastal areas defined as the region between the coastline and the shelf break. Frontogenesis (physical processes related to frontal formation) were reviewed alongside studies on FMD accumulation in frontal zones to identify physical factors that drive the pathways and accumulation in these areas. This review will contribute to our understanding of accumulation hotspots of FMD within ocean fronts and identify gaps for further research on developing a proxy for FMD hotspot identification in ecologically important coastal areas.

Where does floating Sargassum in the East China Sea come from?

Floating macroalgae of Sargassum horneri (S. horneri) in the East China Sea (ECS) has increased in recent years, with ocean warming being one of the driving factors. Yet their possible origins, based on a literature review, are unclear. Here, using multi-sensor high-resolution remote sensing data and numerical experiments for the period of 2015-2023, we show two possible origins of the ECS floating S. horneri, one being local near the Zhejiang coast with initiation in January-February and the other being remote (> 800 km from the first) in the Bohai Sea with initiation in June-November. While their drifting pathways are revealed in the sequential remote sensing imagery, numerical experiments suggest that S. horneri from the remote origin (Bohai Sea) can hardly meander through the strong Yangtze River frontal zone, which may serve as a "wall" to prevent trespassing of surface floating seaweed to the south of the frontal zone, where S. horneri has a local origin. PLAIN LANGUAGE SUMMARY: Sargassum horneri (S. horneri) is a brown macroalgae (seaweed) abundant in surface waters of the East China Sea (ECS), which can serve as a moving habitat, but can also cause major beaching events and environmental problems. Knowledge of its origins is important to help implement mitigation strategies and understand possible ecological impacts along its drifting pathways. Using high-resolution remote sensing images and numerical experiments, we track floating S. horneri in space and time between 2015 and 2023. Two possible origins are identified, one being far away from the ECS and the other being local, both of which are known to have benthic S. horneri. The study also reveals how S. horneri are transported from their source regions resulting in large-scale distributions previously observed in medium-resolution satellite imagery.

Interannual variations of Sargassum blooms in the Yellow Sea and East China Sea during 2017-2021

Golden tide, caused by Sargassum horneri, is becoming another periodic and trans-regional harmful macroalgal bloom in the Yellow Sea (YS) and East China Sea (ECS) other than the green tide. In this study, we employed high-resolution remote sensing, field validations, and population genetics to investigate the spatiotemporal development pattern of Sargassum blooms during the years 2017 to 2021 and explore the potential environmental factors that influence them. Sporadic floating Sargassum rafts could be detected in the middle or northern YS during autumn and the distribution area then occurred sequentially along the Chinese and/or western Korean coastlines. The floating biomass amplified significantly in early spring, reached its maximum in two to three months with an evident northward expansion, and then declined rapidly in May or June. The scale of the spring bloom was much larger than the winter one in terms of coverage, suggesting an additional local source in ECS. The blooms were mostly confined to waters with a sea surface temperature range of 10-16℃, while the drifting pathways were consistent with the prevailing wind trajectory and surface currents. The floating S. horneri populations exhibited a homogenous and conservative genetic structure among years. Our findings underscore the year-round cycle of golden tides, the impact of physical hydrological environments on the drifting and blooming of pelagic S. horneri, and provide insights for monitoring and forecasting this emerging marine ecological disaster.

A comparison of physiological responses between attached and pelagic populations of Sargassum horneri under nutrient and light limitation

Large-scale Sargassum blooms have been increasingly observed in coastal zones in recent years. Sargassum horneri (Turner) C. Agardh blooms (pelagic) have been observed in Jeju Island (Korea) and the southwest of the Korean Peninsula, causing serious problems for seaweed and abalone farms as well as for fisheries, tourism and recreational industries. The present study explored the physiological responses of attached and pelagic S. horneri populations cultivated under different nutrient concentrations (HN: 50 μM of nitrogen and 5 μM of phosphorus; LN: 5 μM of nitrogen and 0.5 μM of phosphorus) and photosynthetically active radiation (PAR) (H-PAR: 250; M-PAR: 150; L-PAR: 50 μmol photons m-2 s-1) for 25 days. Relative growth rates (RGR) were significantly lower in the pelagic population than that in the attached population. All thalli from the pelagic population died within 20 days. Chlorophyll a and c, and carotenoids were significantly higher at HN than at LN, and decreased as PAR increased for both populations. For the attached population, photosynthetic rate, tissue nitrogen, and carbon and nitrogen removal were also significantly higher at HN than at LN. These results suggest that high nutrient and lower PAR increased the biomass accumulation of attached populations in coastal areas. Nutrient limitation and high PAR may accelerate senescence of the pelagic populations while traveling on the sea surface from their point of origin.

Harmful macroalgal blooms (HMBs) in China's coastal water: Green and golden tides

Harmful macroalgal blooms (HMBs) have been increasing along China's coasts, causing significant social impacts and economic losses. Besides extensive eutrophication sustaining coastal seaweed tides, the stimuli and dynamics of macroalgal blooms in China are quite complex and require comprehensive studies. This review summarizes the distinct genesis, development and drifting patterns of three HMBs that have persistently occurred in China's coastal waters during recent years: transregional green tides of drifting Ulva prolifera in the Yellow Sea (YS), local green tides of multiple suspended seaweeds in the Bohai Sea and large-scale golden tides of pelagic Sargassum horneri in the YS and East China Sea. While specific containment measures have been developed and implemented to effectively suppress large-scale green tides in the YS, the origin and blooming mechanism of golden tides remain unclear due to lack of field research. With the broad occurrence of HMBs and their increased accumulation on beaches and coastal waters, it is necessary to investigate the blooming mechanism and ecological impacts of these HMBs, especially with the growing stresses of climate change and anthropogenic disturbances.

Sargassum blooms in the East China Sea and Yellow Sea: Formation and management

Large-scale Sargassum blooms, known as golden tides, have been occurring along the coast of the Yellow Sea in recent years, resulting in an enormous loss of Pyropia yezoensis production. To locate the source of the blooms, we performed large-scale spatio-temporal sampling in the South Yellow Sea, East China Sea, and Jeju Island, South Korea. Based on morphology and molecular traits, the attached and floating Sargassum samples collected from the three regions were all identified as Sargassum horneri, although slight differences were observed in morphology among samples. Genetic distance and automatic barcode gap discovery analysis revealed very low genetic diversity among the three regions. The 33 samples from 12 sites were divided into six haplotypes, and the samples from the ECS shared more haplotypes than samples from other two regions. Our results suggested that S. horneri in the ECS was responsible for the formation of blooms in the Yellow Sea.

Comparative Analysis of Sequence Polymorphism in Complete Organelle Genomes of the ‘Golden Tide’ Seaweed Sargassum horneri between Korean and Chinese Forms

Drifting and inundating brown seaweed Sargassum horneri biomass is called “golden tide”, as it resembles golden massive algal blooms like green tides. This phenomenon occurs globally and its serious ecological impacts on coastal ecosystems have recently begun to be paid attention to. In the present study, by sequencing whole organelle genomes of Korean indigenous S. horneri, we aimed to develop novel molecular markers that can be used for differentiating indigenous from nonindigenous individuals. To this end, we analyzed sequence polymorphisms in mitochondrial (mt) and chloroplast (cp) genomes of two Korean benthic samples in comparison to Chinese ones as a reference. We mapped mt genomes of 34,620~34,628 bp and cp genomes of 123,982~124,053 bp for the Korean samples. In comparative analyses, mtDNA cytochrome c oxidase subunit II (cox2) gene showed the highest number of single nucleotide polymorphisms (SNPs) between Korean and Chinese individuals. NADH dehydrogenase subunit 7 (Nad7)-proline tRNA (trnP) intergenic spacer (IGS) in the mt genome showed a 14 bp insertion or deletion (indel) mutation. For the cp genome, we found a total of 54 SNPs, but its overall evolution rate was approximately four-fold lower than the mt genome. Interestingly, analysis of Ka/Ks ratio in the cp genome revealed a signature of positive selection on several genes, although only negative selection prevalent in mt genome. The ‘candidate’ genetic markers that we found can be applied to discriminate between Korean indigenous and nonindigenous individuals. This study will assist in developing a molecular-based early detection method for effectively managing nonindigenous S. horneri in Korean waters.

Limited Genetic Connectivity Among Sargassum horneri (Phaeophyceae) Populations in the Chinese Marginal Seas Despite Their high Dispersal Capacity

Sargassum horneri is a habitat-forming species in the Northwest Pacific and an important contributor to seaweed rafts. In this study, 131 benthic samples and 156 floating samples were collected in the Yellow Sea and East China Sea (ECS) to test the effects of seaweed rafts on population structure and connectivity. Our results revealed high levels of genetic diversity in both benthic and floating samples based on concatenated mitochondrial markers (rpl5-rps3, rnl-atp9, and cob-cox2). Phylogenetic analyses consistently supported the existence of two lineages (lineages I and II), with divergence dating to c. 0.692 Mya (95% HPD: 0.255-1.841 Mya), indicating that long-term isolation may have occurred during the mid-Pleistocene (0.126-0.781 Mya). Extended Bayesian skyline plots demonstrated a constant population size over time in lineage I and slight demographic expansion in lineage II. Both lineages were found in each marginal sea (including both benthic and floating samples), but PCoA, FST , and AMOVA analyses consistently revealed deep genetic variation between regions. Highly structured phylogeographic pattern supports limited genetic connectivity between regions. IMA analyses demonstrated that asymmetric gene flow between benthic populations in the North Yellow Sea (NYS) and ECS was extremely low (ECS→NYS, 2Nm = 0.6), implying that high dispersal capacity cannot be assumed to lead to widespread population connectivity, even without dispersal barriers. In addition, there were only a few shared haplotypes between benthic and floating samples, suggesting the existence of hidden donors for the floating masses in the Chinese marginal seas.

An anomalous bi-macroalgal bloom caused by Ulva and Sargassum seaweeds during spring to summer of 2017 in the western Yellow Sea, China

An unprecedented bi-macroalgal bloom caused by Ulva prolifera and Sargassum horneri occurred from spring to summer of 2017 in the western Yellow Sea (YS) of China, where annual large-scale green tides have prevailed for a decade. The distinct genesis and blooming dynamics of the two seaweed species were detected and described. Unlike the consistent raft-origin of the floating Ulva biomass, the massive pelagic S. horneri was derived from multiple sources (residual seaweeds from the previous winter bloom and those drifting from offshore water in the south). The scale of the green tide in 2017 was found smaller than the previous four years. We then discussed a number of hypotheses attributing to this reduction, including reduced epiphytic green algae from aquaculture rafts and the influences of the massive pelagic S. horneri. However, further research is needed to identify the origin of the pelagic S. horneri in the western YS and any affiliations with the benthic populations, and to elucidate the interactions of this species with the annual green tides and the ensuing consequences.

The protective effect of Sargassum horneri against particulate matter-induced inflammation in lung tissues of an in vivo mouse asthma model

Sargassum horneri is an edible brown seaweed with potential anti-inflammatory properties. The present study was designed to evaluate the anti-inflammatory properties of S. horneri using an in vivo mouse asthma model following exposure to particulate matter (PM). 7-8 week old BALB/c mice (20-25 g) were randomly divided into seven groups (n = 4) as follows: 1: no treatment, 2: OVA (ovalbumin) + PM, 3: OVA + PM + SHE (S. horneri ethanol extract) 200 mg kg-1, 4: OVA + PM + SHE 400 mg kg-1, 5: OVA + PM + prednisone 5 mg kg-1, 6: OVA only, and 7: PM only. All mice (except healthy controls) were sensitized on the first day by intraperitoneal injection of 10 μg OVA and 2 mg Al(OH)3 in 200 μL of saline. Starting from day 15, mice (except groups 1 and 6) were exposed to sonicated PM (5 mg m-3, 30 min day-1) through a nebulizer daily for 7 consecutive days. Mice exposed to PM and OVA showed up-regulated expression of MAPKs and pro-inflammatory cytokine production in the lungs. Furthermore, PM-exposed lungs had significantly reduced expression of Nrf2 and HO-1 genes. However, oral administration of the SHE reduced the phosphorylation levels of MAPKs, iNOS and COX2 expression levels, and mRNA expression levels of pro-inflammatory cytokines. In addition, SHE treated group mice had up-regulated anti-oxidant gene expression levels in the lungs compared to group 2. These findings demonstrate that oral administration of the SHE re-establishes PM-induced inflammation and oxidative stress in the lungs. Taken together, the SHE has therapeutic potential in preventing PM-induced inflammation and oxidative stress.

Effects of increased CO2 and temperature on the physiological characteristics of the golden tide blooming macroalgae Sargassum horneri in the Yellow Sea, China

The golden tide, caused by the brown algae Sargassum horneri, exerts severe influences on the Pyropia aquaculture of Jiangsu coast, China. To study the outbreak of the golden tide in response to increasing greenhouse gas emissions, S. horneri was cultured under four conditions: ambient condition (10 °C, 400 μatm), elevated temperature condition (14 °C, 400 μatm), elevated CO2 level (10 °C, 1000 μatm), and potential greenhouse condition (14 °C, 1000 μatm). The growth, photosynthetic performances, and inorganic carbon affinity of S. horneri were studied. The results showed that elevated temperature exerted a more pronounced positive influence on S. horneri growth, photosynthesis, and carbon assimilation than CO2 enrichment. The growth of S. horneri was significantly improved by moderately elevated temperatures, especially under concurrently elevated CO2 levels. This suggests that the greenhouse effect will benefit growth and carbon sequestration of S. horneri, which may enhance the frequency and scale of golden tides.

Community Structure of Bacteria Associated With Drifting Sargassum horneri, the Causative Species of Golden Tide in the Yellow Sea

Golden tides dominated by Sargassum spp. are occurring at an accelerated rate worldwide. In China, Sargassum has started to bloom in the Yellow Sea and led to tremendous economic losses, but the underlying biological causes and mechanisms are still unclear. Although algae-associated bacteria were suggested to play crucial roles in algal blooms, the profiles of bacteria associated with drifting Sargassum remain unexplored. In this study, the community structures and functions of Sargassum-associated bacteria were analyzed using the high-throughput sequencing data of the V5–V7 hypervariable region of the 16S rRNA gene. Molecular identification revealed that the golden tide analyzed in the Yellow Sea was dominated by a single species, Sargassum horneri. They were a healthy brown color nearshore but were yellow offshore with significantly decreased chlorophyll contents (P < 0.01), which indicates that yellow S. horneri was under physiological stress. The structural and functional analyses of bacterial communities indicated that the drifting S. horneri had an obvious selectivity on their associated bacteria against surrounding seawater. Although the bacterial communities phylogenetically differed between brown and yellow S. horneri (P < 0.01), their dominant functions were all nitrogen and iron transporters, which strongly indicates microbial contribution to blooming of the algal host. For the first time, potential epiphytic and endophytic bacteria associated with Sargassum were independently analyzed by a modified co-vortex method with silica sand. We showed that the composition of dominant endophytes, mainly Bacillus and Propionibacterium, was relatively consistent regardless of host status, whereas the epiphytic operational taxonomic units (OTUs) greatly varied in response to weakness of host status; however, dominant functions were consistent at elevated intensities, which might protect the host from stress related to nitrogen or iron deficiency. Thus, we propose that host physiological status at different intensities of functional demands, which were related to variable environmental conditions, may be a critical factor that influences the assembly of epiphytic bacterial communities. This study provided new insight into the structure and potential functions of associated bacteria with golden tide blooms.

The origin and population genetic structure of the 'golden tide' seaweeds, Sargassum horneri, in Korean waters

In recent years, drifting and inundating brown seaweed (Sargassum horneri) biomass, called 'golden tides', has frequently drifted and accumulated along the southern coastlines of Korea, causing devastating impacts on the local economy and coastal ecosystems. In this study, based on combined analyses of mitochondrial DNA cox3 gene and seven microsatellites, we investigated the genetic makeup of the floating S. horneri populations (N = 14) in comparison to Korean benthic populations (N = 5), and tracked their genetic sources. Given a shared mtDNA haplotype and oceanic circulation systems, the floating populations may have been originated from the southeastern coast of China (e.g. Zhoushan, Zhejiang province). Population structure analyses with microsatellites revealed two distinct genetic clusters, each comprising floating and benthic populations. High levels of inter-population differentiation were detected within Korean benthic samples. The floating populations from the same periods during a 2015-2018 year were genetically more different from one another than those from different periods. These results suggest that the floating populations might be of multiple genetic sources within geographic origin(s). This study will inform management efforts including the development of "S. horneri blooming forecasting system", which will assist in mitigating ecological and economic damages on the Korean coastal ecosystems in the future.

Sargassum horneri C. Agardh space capacity estimation reveals that thallus surface area varies with wet weight

Sargassum horneri C. Agardh is an important commercial edible seaweed species in east Asia. Benthic beds and floating rafts in coastal areas make excellent habitats for marine organisms to feed, hide, and spawn. Many commercially important fish species such as Japanese anchovy (Engraulis japonicus), yellowtail (Seriola quinqueradiata), and Japanese horse mackerel (Trachurus japonicus) live in seaweed beds. Chinese and Japanese fisherman rely on S. horneri beds as productive fish harvest areas. The Zhejiang government in China set a total allowable catch standard, to preserve the Ma'an Islands ecosystem, which is a marine protected area. In this study we analysed the association between weight and one-sided surface area of S. horneri beds, and calculated the ratio of one-sided surface area to change in wet weight over time. We collected samples from December 2014 to May 2015. Approximately 1 g of S. horneri biomass provided ~15 cm2 of one-sided surface area available to marine organisms. These calculations can be used as a reference regarding potential space to improve total allowable catch standard management in S. horneri beds, through the estimation of space capacity of seaweed beds.

Historical isolation and contemporary gene flow drive population diversity of the brown alga Sargassum thunbergii along the coast of China

BACKGROUND

Long-term survival in isolated marginal seas of the China coast during the late Pleistocene ice ages is widely believed to be an important historical factor contributing to population genetic structure in coastal marine species. Whether or not contemporary factors (e.g. long-distance dispersal via coastal currents) continue to shape diversity gradients in marine organisms with high dispersal capability remains poorly understood. Our aim was to explore how historical and contemporary factors influenced the genetic diversity and distribution of the brown alga Sargassum thunbergii, which can drift on surface water, leading to long-distance dispersal.

RESULTS

We used 11 microsatellites and the plastid RuBisCo spacer to evaluate the genetic diversity of 22 Sargassum thunbergii populations sampled along the China coast. Population structure and differentiation was inferred based on genotype clustering and pairwise F ST and allele-frequency analyses. Integrated genetic analyses revealed two genetic clusters in S. thunbergii that dominated in the Yellow-Bohai Sea (YBS) and East China Sea (ECS) respectively. Higher levels of genetic diversity and variation were detected among populations in the YBS than in the ECS. Bayesian coalescent theory was used to estimate contemporary and historical gene flow. High levels of contemporary gene flow were detected from the YBS (north) to the ECS (south), whereas low levels of historical gene flow occurred between the two regions.

CONCLUSIONS

Our results suggest that the deep genetic divergence in S. thunbergii along the China coast may result from long-term geographic isolation during glacial periods. The dispersal of S. thunbergii driven by coastal currents may facilitate the admixture between southern and northern regimes. Our findings exemplify how both historical and contemporary forces are needed to understand phylogeographical patterns in coastal marine species with long-distance dispersal.

The variable routes of rafting: stranding dynamics of floating bull kelp Durvillaea antarctica (Fucales, Phaeophyceae) on beaches in the SE Pacific

Dispersal on floating seaweeds depends on availability, viability, and trajectories of the rafts. In the southern hemisphere, the bull kelp Durvillaea antarctica is one of the most common floating seaweeds, but phylogeographic studies had shown low connectivity between populations from continental Chile, which could be due to limitations in local supply and dispersal of floating kelps. To test this hypothesis, the spatiotemporal dynamics of kelp strandings were examined in four biogeographic districts along the Chilean coast (28°-42°S). We determined the biomass and demography of stranded individuals on 33 beaches for three subsequent years (2013, 2014, 2015) to examine whether rafting is restricted to certain districts and seasons (winter or summer). Stranded kelps were found on all beaches. Most kelps had only one stipe (one individual), although we also frequently found coalesced holdfasts with mature males and females, which would facilitate successful rafting dispersal, gamete release, and reproduction upon arrival. High biomasses of stranded kelps occurred in the northern-central (30°S-33°S) and southernmost districts (37°S-42°S), and lower biomasses in the northernmost (28°S-30°S) and southern-central districts (33°S-37°S). The highest percentages and sizes of epibionts (Lepas spp.), indicative of prolonged floating periods, were found on stranded kelps in the northernmost and southernmost districts. Based on these results, we conclude that rafting dispersal can vary regionally, being more common in the northernmost and southernmost districts, depending on intrinsic (seaweed biology) and extrinsic factors (shore morphology and oceanography) that affect local supply of kelps and regional hydrodynamics.

Attachment strength of the subtidal seaweed Sargassum horneri (Turner) C. Agardh varies among development stages and depths

Sargassum horneri is one of the most important contributors to the rafts of floating seaweed in the waters off the coasts of northeastern Asia. These rafts serve as spawning and nursery grounds for many marine organisms, including Japanese saury and yellowtail. Thus, the details of the attachment/detachment mechanisms of S. horneri are of commercial significance for the aquaculture industry. Here, we describe variations in the attachment strength of S. horneri as it relates to its developmental stage and depth along a bottom gradient. We measured the attachment strength/dislodgement force of S. horneri samples with holdfast detachment in Shidagaura Cove, Shimoda, Japan, from December 2014 to May 2015. After we had determined the dislodgement forces required to detach thalli from the substratum (using a spring scale device) in the field, we transferred released individuals to the laboratory and measured selected morphological traits. Attachment strength was linearly related to the holdfast basal area when thalli were immature (prior to mid-March), but not when they were mature (mid-March to May). Thus, attachment strength was maintained through the reproductive phase and declined thereafter, allowing released thalli to join the drifting raft community. Rafting may be a mechanism by which the species expands its distribution range, as floating thalli continuing to shed germlings that are able to recruit when suitable habitat is encountered. Attachment strengths were greater in the shallows than in deeper water, reflecting the differences in wave forces experienced at different depths.