Marine Floral Biodiversity, Threats, and Conservation in Vietnam: An Updated Review

Simple Summary The present review provides (1) an updated checklist of the Vietnamese marine flora, (2) a review of molecular-assisted alpha taxonomic efforts, (3) an analysis of marine floral biodiversity spatial distribution nationally and regionally (South China Sea), (4) a discussion on the impact of anthropogenic and environmental stressors on the Vietnamese marine flora, and (5) the efforts developed in the last decade for its conservation. The updated checklist consists of 878 species, including 439 Rhodophyta, 156 Ochrophyta, 196 Chlorophyta, 87 Cyanobacteria, and 15 seagrasses. The South Central Coast supports the highest species diversity of marine algae, which coincides with the largest density of coral reefs along the Vietnam coast. Vietnam holds one of the richest marine floras in the South China Sea owing to the country’s coastline length and associated marine habitat diversity. However, the Vietnamese marine floral biodiversity is facing critical threats, and present management efforts are yet insufficient for their conservation. A methodical molecular-assisted re-examination of Vietnam marine floral biodiversity is urgently needed, complemented with in-depth investigations of the main threats targeted against it; and finally, conservation measures should be urgently implemented. Abstract Part of the Indo-Chinese peninsula and located on the northwest edge of the Coral Triangle in the South China Sea, the Vietnamese coastal zone is home to a wealthy marine biodiversity associated with the regional geological setting and history, which supports a large number of marine ecosystems along a subtropical to tropical gradient. The diversity of coastal benthic marine primary producers is also a key biological factor supporting marine biological diversity. The present review provides: (1) an updated checklist of the Vietnamese marine flora, (2) a review of molecular-assisted alpha taxonomic efforts, (3) an analysis of marine floral biodiversity spatial distribution nationally and regionally (South China Sea), (4) a review of the impact of anthropogenic and environmental stressors on the Vietnamese marine flora, and (5) the efforts developed in the last decade for its conservation. Based on the studies conducted since 2013 and the nomenclatural changes that occurred during this period, an updated checklist of benthic marine algae and seagrasses consisted in a new total of 878 species, including 439 Rhodophyta, 156 Ochrophyta, 196 Chlorophyta, 87 Cyanobacteria, and 15 phanerogam seagrasses. This update contains 54 new records and 5 new species of macroalgae. The fairly poor number of new records and new species identified in the last 10 years in a “mega-diverse” country can be largely attributed to the limited efforts in exploring algal biodiversity and the limited use of genetic tools, with only 25.4% (15 species) of these new records and species made based on molecular-assisted alpha taxonomy. The South Central Coast supports the highest species diversity of marine algae, which coincides with the largest density of coral reefs along the Vietnamese coast. Vietnam holds in the South China Sea one of the richest marine floras, imputable to the country’s geographical, geological, and climatic settings. However, Vietnam marine floral biodiversity is under critical threats examined here, and current efforts are insufficient for its conservation. A methodical molecular-assisted re-examination of Vietnam marine floral biodiversity is urgently needed, complemented with in-depth investigations of the main threats targeting marine flora and vulnerable taxa, and finally, conservation measures should be urgently implemented.


Introduction
Located along the eastern margin of the Indo-Chinese Peninsula, on the northwest edge of the Coral Triangle biodiversity hotspot [1], in the South China Sea (also known as the East Vietnam Sea or Biển Ðông), the Vietnamese coastal zone is home to a remarkably rich marine biodiversity [2,3]. Vietnam has been listed among the top 25 most biologically diverse countries in the world [4] and characterized by some authors as a "mega-diverse country" [5,6]. The origin of this diversity is linked to the region's (Southeast Asia) geological and climatic history, the country's coastline length covering some 3260 km along a north-south orientation, thus with a wide latitudinal range (stretching from 21 • 30 N to 8 • 25 N), spanning a subtropical-tropical transition zone, which supports no less than 20 types of marine ecosystems [7]. It is nonetheless important to also point to the role of coastal benthic marine primary producers (e.g., algae, corals, seagrasses) as a key biological factor supporting other forms of marine biological diversity [8]. Benthic marine algae occur across virtually all marine coastal systems from intertidal zone to depths of >200 m, on soft (e.g., sandy) to hard (e.g., rocky) substrates, in a variety of habitats (e.g., lagoons, bays, islands, islets, atolls, and reefs) and ecosystems (e.g., mangroves, seagrasses beds, and coral reefs).
The marine flora of Vietnam, which includes three main classes of macroalgae (Chlorophyta, Ochrophyta, Rhodophyta), marine phanerogams or seagrasses (Alismatales), and Cyanobacteria (Cyanophyceae), has attracted the attention of marine botanists since the 1800s. The first mention of Vietnamese seaweeds appeared in the Flora Cochinchinensis [9], with the record of 11 names of marine macroalgal species, later referenced in the work of Agardh [10,11] and De Toni [12,13]. The French institution "Institut océanographique de l'Indochine", corresponding today to the "Institute of Oceanography", contributed considerably during the 1930s to the knowledge on the algal diversity of Vietnam, notably  [15], (C) Grateloupia filicina, and (D) Chnoospora implexa collected by Dawson [16] in Nha Trang.  [15], (C) Grateloupia filicina, and (D) Chnoospora implexa collected by Dawson [16] in Nha Trang.

Molecular-Assisted Alpha Taxonomy of the Vietnamese Marine Flora
The use of molecular-assisted alpha taxonomy of marine algae is very recent in Vietnam [24,93]. Molecular tools are presently needed among other purposes: (1) validate previous species identification, (2) identify new records and species, and (3) detect introduced species (e.g., [94][95][96]). Studies combining DNA-based species delimitation techniques and detailed morphological observations have refined our knowledge on Vietnamese species taxonomy and on the individual species' biogeographical ranges. Nevertheless, such efforts have been very limited in the last decade. Among the 59 new records and species made in the last 10 years, only 25.4% (15 species) were based on molecular-assisted alpha taxonomy. Hereafter, we reviewed molecular studies conducted thus far on Vietnamese marine macroalgae, identifying the taxa studied, marker used, and taxonomic results.

Molecular-Assisted Alpha Taxonomy of Rhodophyta
Molecular-assisted alpha taxonomic studies on Rhodophyta have comprised a total of four markers, analyzed individually or combined, consisting of two chloroplast genes (large subunit of ribulose-1,5-bisphosphate-carboxylase-oxygenase (rbcL); photosystem I P700 chlorophyll a apoprotein A1 (psaA)), one mitochondrial gene (cytochrome c oxidase I (cox1)), and one nuclear gene (LSU rDNA (28S)). The plastidic rbcL gene has been mostly used. The 2006 publication by Hau et al. [24] conducted one of the first molecular studies on Vietnamese Rhodophyta, analyzing the phylogenetic relationships among Gracilariaceae using rbcL, which revealed a new species of Gracilariopsis, Gracilariopsis nhatrangensis Le & Lin. Based on rbcL alone, Le et al. [49] Figure 2E); and Duy [47] reported the Rhodomelaceae species Chondrophycus tronoi (Ganzon-Fortes) Nam from Vietnam. Analyses based on cox1, psaA, and rbcL sequences allowed the discovery of the Gelidiellaceae species Perronella gracilis Boo, Nguyen, Kim & Boo from Nha Trang Bay from Southern Vietnam [53], and the transfer of Gelidiella adnata Dawson to Parviphycus adnatus (Dawson) Santelices. Analyses based on the concatenated rbcL and cox1 sequences also revealed a new record of Delesseriaceae from Vietnam, Zellera tawallina Martens (Figure 2A), previously identified as Claudea batanensis Tanaka [55]. Analyses combining rbcL and cox1 sequences allowed the identification of a new species, Meristotheca lysonensis Nguyen, Nguyen, Kittle & McDermid, collected at Ly Son Island in the South Central Coast region of Vietnam [52] (Figure 2B). A last worthy account for Rhodophyta is that of the Halymeniaceae species Halymenia dilatata Zanardini, a common species in Vietnam, previously reported in several publications [14,16]. Based on phylogenetic analyses using concatenated chloroplast and mitochondrial and nuclear markers (rbcL, cox1, and LSU rDNA (28S)), Vy et al. [50] showed that H. dilatata may have been misidentified as Halymenia malaysiana Tan, Lim, Lin & Phang, a study that confirms new distributional records of Phycocalidia tanegashimensis along the Chinese and Vietnamese coastline in the South China Sea. The study used molecular sequence data from rbcL, COI-5P, and 18S rRNA genes to place P. tanegashimensis in a clade with P. acanthophora, P. denticulata, P. suborbiculata, and P. vietnamensis as out-groups [54].

Molecular-Assisted Alpha Taxonomy of Ochrophyta
Molecular-assisted alpha taxonomic studies on Ochrophyta have comprised a total of four molecular markers, analyzed individually or combined, consisting of two chloroplast genes (rbcL and the PSII thylakoid protein D1 (psbA)), one mitochondrial gene (cytochrome c oxidase subunit III (cox3)), and one nuclear encoded ribosomal cistron (ITS 2 rDNA). Tu [98] used ITS2 rDNA and cox3 sequences to reassess Sargassum species diversity from Vietnam. The order Dictyotales has received particular attention in recent years. Using rbcL and psbA markers, Nguyen-Nhat et al. [58] newly identified Dictyota hauckiana Nizamuddin from Ninh Thuan ( Figure 2C). One additional species of Dictyota was newly recorded from Vietnam, Dictyota grossedentata De Clerck & Coppejans [55] ( Figure 2D). Molecular phylogenetic analyses based on concatenated rbcL and cox3 sequences led to the description of the new species Lobophora tsengii Tien & Sun from Bach Long Vy [60], although morphological and molecular analyses did not conclusively rule out its conspecificity with Lobophora rosacea C.W.Vieira, Payri& De Clerck.

Molecular-Assisted Alpha Taxonomy of Ochrophyta
Molecular-assisted alpha taxonomic studies on Ochrophyta have comprised a total of four molecular markers, analyzed individually or combined, consisting of two chloroplast genes (rbcL and the PSII thylakoid protein D1 (psbA)), one mitochondrial gene (cytochrome c oxidase subunit III (cox3)), and one nuclear encoded ribosomal cistron (ITS 2 rDNA). Tu [97] used ITS2 rDNA and cox3 sequences to reassess Sargassum species diversity from Vietnam. The order Dictyotales has received particular attention in recent years. Using rbcL and psbA markers, Nguyen-Nhat et al. [58] newly identified Dictyota hauckiana Nizamuddin from Ninh Thuan ( Figure 2C). One additional species of Dictyota was newly recorded from Vietnam, Dictyota grossedentata De Clerck & Coppejans [55] (Figure 2D). Molecular phylogenetic analyses based on concatenated rbcL and cox3 sequences led to the description of the new species Lobophora tsengii Tien & Sun from Bach Long Vy [60], although morphological and molecular analyses did not conclusively rule out its conspecificity with Lobophora rosacea C.W.Vieira, Payri& De Clerck.

Molecular-Assisted Alpha Taxonomy of Chlorophyta
For Chlorophyta, the only molecular-assisted alpha taxonomic study reported until now is that by Tran et al. [61], who reassessed the species diversity in Vietnam of the Ulvaceae genus Ulva based on rbcL and the elongation factor Tu (tuf A). The study revealed seven new records of Ulva from Vietnam and identified a new species, U. vietnamensis L-A. T. Tran, Leliaert & De Clerck.

Molecular-Assisted Alpha Taxonomy of Cyanobacteria and Alismatales
For Alismatales (seagrasses), the concatenated rbcL and matK were applied to assess the species diversity of Halophila [98]. Based on the genetic marker ITS, a later study by Nguyen et al. [44] showed that Halophila major was the correct name for the collections of Halophila ovalis from Nha Trang Bay. All seagrass species from Vietnam were confirmed with molecular markers, and samples previously labeled as "Halophila johnsonii" were reidentified as H. ovalis. Therefore, Halophila johnsonii was removed from the seagrass checklist of Vietnam [99]. Halophila major was found in most offshore islands, whereas H. ovalis occurred in lagoons in Vietnamese waters [100]. In contrast, no molecular-assisted alpha taxonomic study on Cyanophyceae was yet conducted in Vietnam.

Intraspecific Genetic Diversity Studies
Several DNA fingerprinting have been applied to investigate the genetic relationships among individuals within or among populations of the same species [101,102]. In a global study of Gracilaria salicornia (Agardh) Dawson from Southeast Asia, Yang et al. [103] distinguished a lineage of the Philippines from other Southeast Asian countries (e.g., Malaysia and Thailand). For another Rhodophyta, Phycocalidia acanthophora (Oliveira & Coll) Santiañez, the dataset of rbcL indicated that there is no haplotype sharing between populations in the Philippines and other nearby areas, including Taiwan, Japan, and Hong Kong [104]. In Vietnam, the red algae species in Kappaphycus and Eucheuma are important economically and were widely cultivated in the South Central. So far, based on a combined cox2-3 and rbcL dataset, Zuccarelo et al. [105] compared the genetic variation among cultivated Kappaphycus alvarezii (Doty) Liao farming worldwide, including a strain from Vietnam; the authors indicated that there is no genetic variation among samples collected in Vietnam and other Southeast Asian countries, such as the Philippines, Malaysia, and Indonesia. However, Kappaphycus alvarezii collected from Africa and Hawai'i showed significant differences from populations in Southeast Asian countries. By using random amplified polymorphic DNA (RAPD) markers, Hong et al. [106] also revealed the genetic variation among strains of Kappaphycus spp. and Eucheuma spp. in Vietnam, Kappaphycus striatus (Schmitz) Liao. The analyses of the mitochondrial cox2-3 spacer of Kappaphycus spp. and Eucheuma spp. showed that there are two haplotypes of K. alvarezii, and an unidentified Kappaphycus sp. was also found in Vietnam and the Philippines [107]. A later study by Tan et al. [108] indicated that the aring-aring strain was described as the new species Kappaphycus malesianus Tan, Lim & Phang. There is no evidence of occurrence of this species in Vietnamese waters. The biogeography of Halymenia malaysiana was studied in more detail. Our previous study showed that the common haplotype in Vietnam is R1, and three new haplotypes were added to H. malaysiana for Southeast Asia (Figure 3). There are statistically significant genetic differences between Sunda Shelf (Vietnam and Malaysia) populations and those in Philippine waters [50]. For another economic species, Gracilaria tenuistipitata Chang & Xia, Song et al. [109] found that there is only one haplotype (T5) in Vietnam. Compared with other haplotypes in Thailand, Malaysia, and Singapore, a haplotype of Gracilaria tenuistipitata collected in Vietnam showed from one to eight mutational steps. Recently, the tuf A gene was applied to find the haplotype and genetic diversity of the green algae Halimeda spp.; Nguyen et al. [110] concluded that the genetic variation in H. macroloba Decaisne is very low, and H. opuntia (Linnaeus) J.V.Lamouroux tends to form a distinct group in Vietnamese waters.

Marine Floral Biodiversity across the South China Sea
Located on the northwest edge of the Coral Triangle biodiversity hotspot, the South China Sea is one of the most productive marine regions in the world [112,113]. The sea is bordered by twelve states and territories, including Brunei, Cambodia, (mainland) China, Hong Kong, Indonesia, Macao, Malaysia, the Philippines, Singapore, Taiwan, Thailand, and Vietnam. Phang et al. [63] documented 1412 species of marine algae from the South China Sea (119 Cyanophyceae, 305 Chlorophyta, 258 Ochrophyta, and 730 Rhodophyta) from six countries bordering the South China Sea (Indonesia, Malaysia, the Philippines, Singapore, Thailand, and Vietnam). Their analyses showed similarity in the marine algal floras of Malaysia, Singapore, and Thailand and those of Vietnam and the Philippines. We present here an overview of species diversity from states and territories bordering the South China Sea based on AlgaeBase [46] for four algae classes (Cyanobacteria, Rhodophyta, Ochrophyta, and Chlorophyta) and seagrasses. No data were availa-

Marine Floral Biodiversity across the South China Sea
Located on the northwest edge of the Coral Triangle biodiversity hotspot, the South China Sea is one of the most productive marine regions in the world [111,112]. The sea is bordered by twelve states and territories, including Brunei, Cambodia, (mainland) China, Hong Kong, Indonesia, Macao, Malaysia, the Philippines, Singapore, Taiwan, Thailand, and Vietnam. Phang et al. [63] documented 1412 species of marine algae from the South China Sea (119 Cyanophyceae, 305 Chlorophyta, 258 Ochrophyta, and 730 Rhodophyta) from six countries bordering the South China Sea (Indonesia, Malaysia, the Philippines, Singapore, Thailand, and Vietnam). Their analyses showed similarity in the marine algal floras of Malaysia, Singapore, and Thailand and those of Vietnam and the Philippines. We present here an overview of species diversity from states and territories bordering the South China Sea based on AlgaeBase [46] for four algae classes (Cyanobacteria, Rhodophyta, Ochrophyta, and Chlorophyta) and seagrasses. No data were available on AlgaeBase for Brunei, Hong Kong, Macau, Cambodia, and Taiwan. Data for seagrasses were retrieved from different sources indicated in Figure 4 and Table 3. The Gulfs of Thailand and Tonkin were included in the South China Sea. It should be noted that with the exception of Vietnam, the species numbers provided here are not restricted to the South China Sea, but are all-inclusive for each country (i.e., not restricted to the South China Sea), and retrieving data restricted to the South China Sea was not possible. In comparison with other South China Sea bordering states/territories, Vietnam supports the fourth highest marine floral diversity with 877 species, according to AlgaeBase [46] (but 881 species according to our updated checklist). However, taking into account the all-inclusiveness of the number for other countries, Vietnam possibly holds the highest diversity in the South China Sea. In fact, the South China Sea coastlines of the three other species-rich states (China, the Philippines, Indonesia) represent only a fraction of these countries. China Sea bordering states/territories, Vietnam supports the fourth highest marine floral diversity with 877 species, according to AlgaeBase [46] (but 881 species according to our updated checklist). However, taking into account the all-inclusiveness of the number for other countries, Vietnam possibly holds the highest diversity in the South China Sea. In fact, the South China Sea coastlines of the three other species-rich states (China, the Philippines, Indonesia) represent only a fraction of these countries.
the fourth highest marine floral diversity with 877 species, according to AlgaeBase [46] (but 881 species according to our updated checklist). However, taking into account the all-inclusiveness of the number for other countries, Vietnam possibly holds the highest diversity in the South China Sea. In fact, the South China Sea coastlines of the three other species-rich states (China, the Philippines, Indonesia) represent only a fraction of these countries. Biodiverisity numbers should nevertheless be interpreted cautiously as they may under-represent the actual floral diversity of each country and the region, since they are for the most part established on morphological-based identification, and additionally, some countries have received much lesser attention than others (e.g., Brunei, Cambodia, and Malaysia). Notwithstanding, the high floral biodiversity in the South China Sea documented so far from Vietnam can be attributed to its geographical location, situated along the southeastern margin of the Indo-Chinese Peninsula, comprising the largest area of the peninsula and the longest coastline in the South China Sea.   [46]. * Data for the Vietnam marine flora based on the current updated checklist.** Colors mean blue is Alismatales, green is Chlorophyta, red is Rhodophyta, brown is Ochrophyta, and purple is Cyanobacteria.   [46]. * Data for the Vietnam marine flora based on the current updated checklist. Colors mean blue is Alismatales, green is Chlorophyta, red is Rhodophyta, brown is Ochrophyta, and purple is Cyanobacteria. Biodiverisity numbers should nevertheless be interpreted cautiously as they may under-represent the actual floral diversity of each country and the region, since they are for the most part established on morphological-based identification, and additionally, some countries have received much lesser attention than others (e.g., Brunei, Cambodia, and Malaysia). Notwithstanding, the high floral biodiversity in the South China Sea documented so far from Vietnam can be attributed to its geographical location, situated along the southeastern margin of the Indo-Chinese Peninsula, comprising the largest area of the peninsula and the longest coastline in the South China Sea.
We examined the similarity of the marine floras in seven of the states and territories bordering the South China Sea using a Bray-Curtis similarity index [120] multivariate analysis implemented in Primer V.6 software [121] based on compiled data for the region. Results showed that the Vietnamese marine flora was most similar to that of China, followed by those of Indonesia and the Philippines (Figure 5), and that the marine floras of Malaysia, Singapore, and Thailand were very similar ( Figure 5), consistent with previous findings by Phang et al. [63].
phyta, brown is Ochrophyta, and purple is Cyanobacteria. We examined the similarity of the marine floras in seven of the states and territories bordering the South China Sea using a Bray-Curtis similarity index [121] multivariate analysis implemented in Primer V.6 software [122] based on compiled data for the region. Results showed that the Vietnamese marine flora was most similar to that of China, followed by those of Indonesia and the Philippines (Figure 5), and that the marine floras of Malaysia, Singapore, and Thailand were very similar ( Figure 5), consistent with previous findings by Phang et al. [63].  Hierarchical clustering is based on square-root-transformed presence data and on a resemblance matrix calculated using S17 Bray-Curtis similarity.

Marine Floral Biodiversity across Vietnam Regions
The Vietnamese coastline is divided into three "Geographical Regions" (Northern Vietnam, Central Vietnam, Southern Vietnam) subdivided into eight "Administrative Regions" (Northeast, Northwest, Red River Delta, North Central Coast, South Central Coast, Central Highlands, Southeast, Mekong River Delta) ( Figure 6A). Based on our updated checklist, we show marine floral biodiversity across the "Administrative Regions", excluding the Northwest and Central Highlands regions, which have no coast-line. The geographical distribution of the marine floral biodiversity is uneven across Vietnamese regions. The South Central Coast holds the highest diversity by far, with a total of 587 species, followed by the Southeast (243), Red River Delta (210), North Central Coast (204), and Mekong River Delta (203) regions. The Northeast region is the least species region with 160 species. The high marine floral biodiversity documented in the South Central Coast coincides with the largest coral reef density along the coastline of Vietnam ( Figure 6B) and a high diversity of marine environments [3]. It is worth mentioning that Spalding [122] proposed five marine ecoregions (Gulf of Thailand, Gulf of Tonkin, Southern Vietnam, Sunda Shelf/Java Sea, South China Sea (East Vietnam Sea) Ocean Islands) along the Vietnamese coastline ( Figure 6A).

Marine Floral Biodiversity across Vietnam Regions
The Vietnamese coastline is divided into three "Geographical Regions" (Northern Vietnam, Central Vietnam, Southern Vietnam) subdivided into eight "Administrative Regions" (Northeast, Northwest, Red River Delta, North Central Coast, South Central Coast, Central Highlands, Southeast, Mekong River Delta) ( Figure 6A). Based on our updated checklist, we show marine floral biodiversity across the "Administrative Regions", excluding the Northwest and Central Highlands regions, which have no coastline. The geographical distribution of the marine floral biodiversity is uneven across Vietnamese regions. The South Central Coast holds the highest diversity by far, with a total of 587 species, followed by the Southeast (243), Red River Delta (210), North Central Coast (204), and Mekong River Delta (203) regions. The Northeast region is the least species region with 160 species. The high marine floral biodiversity documented in the South Central Coast coincides with the largest coral reef density along the coastline of Vietnam ( Figure 6B) and a high diversity of marine environments [3]. It is worth mentioning that Spalding [123] proposed five marine ecoregions (Gulf of Thailand, Gulf of Tonkin, Southern Vietnam, Sunda Shelf/Java Sea, South China Sea (East Vietnam Sea) Ocean Islands) along the Vietnamese coastline ( Figure 6A). According to present data, with only 45 endemic species, Vietnam would seem to contain a very low level of endemism of marine flora (5.01%, 45 spp.; Table 4). However, this number most likely under-represents the actual level of endemism for the marine flora of Vietnam, since molecular-assisted alpha taxonomic efforts, needed to obtain accurate taxonomic data, have been very limited.

Seaweed Biodiversity Loss, Threats, and Conservation
Some species of marine algae in Vietnam have experienced declines in their populations due to a variety of factors. Several threats to marine algae exist in Vietnam, including but not restricted to pollution, climate change, overharvesting, invasive species, and habitat destruction. Marine algae are vulnerable to pollution from a variety of sources, including agricultural runoff, industrial discharge, and sewage. Pollution can harm marine algae directly and also make their habitat less suitable for growth [123]. Marine algae are sensitive to changes in temperature, salinity, and other environmental conditions, and they may be negatively impacted by climate change [124]. Some species of marine algae are in high demand for use in food, cosmetics, and other products, and overharvesting can lead to a decline in their populations [125]. Non-native species of marine algae that are introduced to new areas can outcompete native species and reduce their populations [126]. Marine algae rely on specific types of habitat for growth, and the destruction of these habitats can negatively impact their populations [127]. The biodiversity (marine and terrestrial) of Vietnam has decreased quickly [128]. Some of the known factors in Vietnam are land conversion without a proper scientific base, quick reduction of natural forests, infrastructure developments (e.g., dams, roads, and new urban and rural human settlements), and overexploitation of natural resource/illegal exploitations in fishing, hunting, forestry [2,129,130]. It is difficult to quantify the extent of marine algal diversity loss in Vietnam, as there are limited data available on this topic. Titlyanov et al. [38] quantified seaweed community changes in Nha Trang Bay and investigated the factors associated with these changes. Collections sampled between 1953 and 1968 and 1982 and 1987 did not change significantly in either the species diversity nor the floristic composition. However, the species composition assessed between 2002 and 2010 showed changes in the species diversity composition, with an increase inf Chlorophyta and a reduction of Rhodophyta and Ochrophyta species. In Con Dao Island, significant changes in marine floral species composition were observed between 1998 and 2008, with a proportional species replacement in each taxonomic group over the last two decades [40]. Since the 1970s, several species have not been observed, such as Erythrocladia irregularis Rosenvinge, Acrochaetium crassipes (Børgesen) Børgesen, Metagoniolithon stelliferum (Lamarck) Ducker, and Exophyllum wentii Weber Bosse [41]. Similarly, the species diversity of Sargassum was previously well studied at Nha Trang Bay, with the identification of 21 species between 1950 and 1970. Between 1980 and 2000, 9 of the previously identified species were not recorded, while an additional 15 species were newly added to Sargassum. However, based on the most recent collection, in 2020 in Nha Trang Bay, Sargassum was represented by 14 species, including 7 species found in the previous two surverys and 7 new additions. Overall, 24% (149 species) of algal species in South Vietnam recorded between 1980 and 2000 could not be found between 2000 and 2020 [41]. A report by Vy et al. [131] indicated that nearly 50% of the Sargassum beds at Hon Chong (Khanh Hoa Province) have disappeared because of loss of substratum, and the species Sargassum crassifolium, once a dominant species in this site, disappeared. In Nha Trang Bay, seawater pollution resulting from dissolved organic and inorganic compounds of nitrogen and phosphorus may lead to an increasing larger number of green algae and their biomass as well as population density. The green algae may displace fleshy and foliose forms of red and brown macroalgae from communities [38].
Another threat to seaweed biodiversity is harvesting of natural stocks. Local harvesters collect large quantities of Sargassum for production of alginates, Asian herbal medicine, and various human foods [132]. Young populations of Sargassum are commonly harvested prior to reaching sexual maturity and reproduction, thus affecting natural stock renewal (authors' pers. obs.). Another case of overexploitation of natural stocks was reported in the edible red seaweed Betaphycus gelatinus, now very rare due to harvesting by locals at Ninh Thuan Province. The Vietnam Red data Book [133] shows 8 and 5 species of Rhodophyta and Ochrophyta, respectively. Among them, Crytonemia undulata is in the critically endangered category. Six species including 5 Rhodophyta and 3 Ochrophyta are in the endangered category. The 6 remaining species are in the vulnerable category. There are 12 marine protected areas (MPAs) from 10 provinces/cites in Vietnam. Large seaweed beds in Khanh Hoa, Ninh Thuan, are out of the core zone of MPAs and therefore under threat. The natural stock of Hydropuntia eucheumatoides, Betaphycus gelatinus, and Sargassum spp. (endangered category) is still collected by local people due to lack of Red Data Book. Like seagrasses, the management models of marine macroalgal ecosystems in Vietnam are mostly integrated into coastal management models to solve the problems of weaknesses that exist in the management, exploitation, and use of natural resources and environmental protection in coastal areas.

Conservation Efforts
Conservation efforts are needed to protect and conserve marine floral diversity in Vietnam. It is important to address these threats in order to maintain the health and resilience of Vietnam marine environments and the economic and cultural value of these resources. This may involve measures such as habitat conservation, sustainable harvesting, pollution reduction, and invasive species management. There are several conservation efforts underway in Vietnam to protect the marine flora and the marine environments they are a part of. Some of these efforts are reviewed below.

Habitat Conservation
Many conservation efforts in Vietnam focus on protecting and preserving the habitats that support marine flora. This may involve establishing marine protected areas (MPAs) or other types of conservation zones, which are designated areas of the ocean that are set aside for the protection and conservation of marine life. Recognizing the importance of marine protected areas in the protection of marine biodiversity, the prime minister released Decision No. 742/QD-TTg on 26 May 2010, authorizing the preparation for the marine biodiversity scheme. Marine conservation in Vietnam will continue until 2020 (this deadline has been extended), with the aim of preserving habitats and marine species of economic and scientific importance. It aimed to contribute to the development of marine economy and improve the livelihoods of fishermen communities in coastal localities. The Fisheries Law of 2017 was passed by the 14th National Assembly, which includes provisions for the protection and growth of aquatic resources, including marine conservation, in the sense of sustainable fisheries development and international integration. The Communist Party of Vietnam's Central Committee released Resolution No. 36-NQ/TW on the Strategy for Sustainable Development of Vietnam's Marine Economy to 2030 with a Vision to 2045 on 22 October 2018. The document stated on the matter: "Sustainable development of the marine economy on the basis of green growth, biodiversity protection, and marine environment conservation; ensure harmony between economic and natural environments, conservation and development, promoting the sea's potentials and advantages, and creating a driving force for national economic development"; and that the specific target was to "Well maintain and protect aquatic, coastal, and island ecosystems; raise the area of marine and coastal protected areas to at least 6% of the national marine area". Currently, the Ministry of Agriculture and Rural Development and other cities have created and operationalized 12 marine protected areas ( Figure 4B, Table 5). These 12 MPAs amount to a total of 243,023 ha (ca. 2430 km 2 ), which corresponds to ca. 0.17% of the total surface (ca.   Table 6). The Ministry of Agriculture and Rural Development has developed comprehensive plans for the establishment of four MPAs, which have been submitted to provincial people's committees for approval: Hon Me/Thanh Hoa, Nam Yet/Khanh Hoa, Phu Quy/Binh Thuan, and Hai Van-Son Cha/Da Nang-Hue. Despite the fact that the Ministry of Agriculture and Rural Development has organized a mission to inspect, guide, and have several documents to direct and inform, the provincial people's committees have not yet approved the establishment after more than 5 years of handover of Vietnam's coastal area has high biodiversity: 13 out of 28 national parks, 22 out of 55 nature reserves, and 17 out of 34 forests of cultural, historical, and environmental significance are located in coastal areas and islands.

Sustainable Harvesting
Some species of seaweed in Vietnam are harvested for use in food, cosmetics, and other products. In order to ensure the sustainable use of these resources, there are efforts to establish sustainable harvesting practices and to manage fisheries to ensure that seaweed populations are not overharvested. Vietnam adopted international standards of the sanitary and phytosanitary (SPS) agreement-based regulation, which includes seaweeds. This established regulation covers a wide range of standards, including ensuring that the seaweeds are disease-, pathogen-, toxin-free, and furthermore that seaweeds meet permissible levels for heavy metals and other contaminants (e.g., pesticides). Vietnam has national regulations for controlling the movement of aquatic aquaculture organisms (quarantine), which also includes the import of live seaweed. For example, Vietnam provided a technical guideline for importing live seaweed, e.g., Gracilaria species [135], and technical requirements for Kappaphycus alvarezii (Table 7). Table 7. Biosecurity components adopted in the national seaweed policies and regulations in the main Gracilaria and Kappaphycus alvarezii production.

Pollution Reduction
Seaweeds are vulnerable to pollution from a variety of sources, and efforts are being made to reduce pollution in Vietnam's coastal waters in order to protect these ecosystems. This may involve measures such as improving wastewater treatment, regulating industrial discharge, and reducing agricultural runoff. However, no particular reports and regulations were found on pollution reduction in Vietnam's coastal waters.

Invasive Species Management
Non-native species of seaweed that are introduced to new areas can outcompete native species and reduce their populations. To address this threat, efforts are being made to control the spread of invasive seaweed species in Vietnam. Circular No. 35/2018/TT-BTNMT dated 28 December 2018 of MONRE stipulates the criteria for the identification and promulgation of a list of invasive alien species. However, the subject only focuses on species that have been announced under the guidance of Circular 35, and the assessed ecosystems are only terrestrial and aquatic. There is almost no information about groups of marine organisms, including seaweed and seagrasses, more specifically, foreign species that are invasive in the sea and by shipping route; there has not been a specific study in Vietnam. In order to prevent the entry of alien organisms in the ballast water environment transported by ships from other sea areas, affecting the ecosystem, economy, and human health and strengthening measures to protect the marine environment, IMO ratified the BWM Convention on 13 February 2004, and the convention met the conditions to enter into force on 8 September 2017. By 8 September 2024, all ships are required to use a ballast water management system (D2). Vietnam is in the process of completing the procedures to join the convention. The basic legal documents related to the activities of dumping garbage and discharging wastewater and ballast water are specified in Article 117 of Decree 58/2017/ND-CP guiding the Vietnam Maritime Code on the management of cargo operations [136].

Conclusions: Challenges and Future Directions
Studies conducted in the last decade effectively illustrated the need to combine molecular tools with morphological observations (i.e., habit view, vegetative and reproductive morphology) in (1) the reassessment of marine floral species diversity, (2) previous species names' validation, (3) misidentification detection, and (4) new species discovery. However, the fairly poor number (15 taxa; 25.4% of the new records and species) of new records and species made over the last 10 years in a "mega-diverse" country raised worrying concerns on the efforts put into the study of marine floral biodiversity. Past molecular-assisted taxonomic efforts have been focused on a limited number of taxa and localities. Currently, three main institutions, including (1) the Institute of Marine Environment and Resources in the North, (2) the Institute of Oceanography in the Central, and (3) the Institute of Tropical Biology in the South, are conducting most studies on marine algae taxonomy nationally. Considering the important length of the Vietnamese coastline (>3200 km), an exhaustive exploration of the Vietnamese marine flora represents a Herculean task for these institutions alone. In addition, the limited number of algal taxonomists in Vietnam and limited funding availability represent a major challenge to the study of marine floral biodiversity. Methodical molecular-assisted re-examination of Vietnam marine floral biodiversity is urgently needed in order to get an accurate picture of biodiversity and endemism, and thereby obtain baseline data for the marine floral management and protection. In particular, future efforts will need to be directed towards specific taxa and regions of Vietnam. Data provided in this review on species diversity, groups targeted with molecular-assisted alpha taxonomic approaches, and spatial variation in biodiversity offer valuable data to orientate future efforts. Finally, a more in-depth investigation of the threats targeting the marine flora of Vietnam is needed, and urgent implementation of measures for its conservation is called for, in particular, the increase in marine protected areas across Vietnam, which represent now less than 1% of the Vietnamese Marine Exclusive Economic Zone.