Volume 61, Issue 3 - Special issue: Seagrass research in Southeast Asia / Issue editors: Siti Maryam Yaakub, Jillian Lean Sim Ooi, Pimchanok Buapet and Richard K.F. Unsworth

A wide variety of organisms are known to graze on seagrasses and their associated epiphytes, and this plant-animal interaction can affect the health of seagrass meadows. Grazing patterns tend to vary across meadows and faunal groups, and little is known regarding how gastropod grazers influence meadows in the tropics. To better understand this interaction, we surveyed the gastropod diversity in five meadows in Singapore. Further, grazing potential (i.e. potential food sources and feeding rates) of common gastropod species was quantified through ex situ grazing experiments, while their diet compositions were elucidated using dual δ 13 C and δ 15 N stable isotope analyses. The surveys revealed a high diversity of 274 gastropod species/morphospecies while PERMANOVA and SIMPER analyses showed that communities differed significantly among sites but not among seagrass species. Diet composition analysis indicated that seagrass leaves were the main food source for most gastropod species examined while epiphytes were important for microsnail (shell size <5 mm) species. However, all the gastropod species tested fed on epiphytes in the ex situ experiments. These findings contribute new insights into grazing by marine gastropods on tropical seagrass meadows, and highlight the potential importance of both direct grazing and epiphyte removal on tropical meadows.

Dependence on seafood across Southeast Asia is extensive. Myanmar is no exception, but the country’s provisioning marine ecosystems are threatened. Seagrass is one habitat that is frequently overlooked in management as an important fisheries resource, despite its nursery function. In Myanmar, research on seagrass habitats is particularly sparse, and as a result, our understanding of seagrass exploitation remains limited. In this study, we provide a baseline assessment of the seagrass-associated fish assemblages at four locations in the Myeik Archipelago in southern Myanmar using mono Baited Remote Underwater Video systems. Across the sites surveyed only 12 taxa of motile fauna were recorded. Relative to other regional and global studies, this figure is meagre. Our data adds to a growing literature suggesting that the marine ecosystems of Myanmar are in a worrying state. Despite the lack of recorded seagrass associated fauna, our study revealed minimal impacts to seagrass meadows from eutrophication or sedimentation, and the meadows included appeared to be healthy. The sites with the highest number of motile fauna were within Myanmar’s only National Marine Park offering some optimism for the effectiveness of protection, but further assessments are required to allow targeted management of Myanmar’s seagrass meadows.

Seagrass restoration is necessary to provide the critical ecosystem functions that are being lost with seagrass decline. Seagrass restoration of mixed species seagrass beds, especially in the Indo-Pacific Ocean, is poorly understood. Here, we transplanted random combinations of 1, 2, 4 and 5 seagrass species common in Indonesia to determine the effect of multispecies plantings on expansion of rhizomes. We measured seagrass vegetative expansion outside of the transplantation plots after 1 year post-transplantation for 5 months. The expansion rate increased with species richness. This result indicates that a multispecies approach to seagrass restoration in Indonesia would benefit restoration efforts. Further, for multispecies restoration we suggest using five-species combinations because they expanded the fastest of all species richness levels by the end of the experiment (on average 15.4 cm 2 day −1 ).

Submerged aquatic vegetation (SAV) is considered as a keystone habitat, contributing significantly to structure and function of coastal lagoons. However, limited understanding of the factors driving SAV distribution and abundance across a wide range of salinity in tropical coastal lagoons has restricted the effectiveness of managing and preserving the ecosystem services in coastal lagoon habitats. This study examined the distribution and abundance of SAV species in the growing season in relation to water physico-chemical variables and grain sizes of sediment types in a tropical lagoon in Viet Nam. The results revealed that Najas indica and Halophila beccarii were the dominant species in the community of 7 SAV species, accounting for 70% of the total cover and 55% of the total biomass sampled. Variation partitioning showed that both water and sediment variables were important in explaining spatial distribution and abundance of SAV species across the coastal lagoon. Salinity was the most significant predictor variable that accounted for the variation of SAV species data. The study implied that changes of salinity and silt (versus sand) particles can lead to different SAV assemblages in the lagoon.

A survey was conducted with the objective to study seagrass distribution, bed size and shoot density in different substrata of the South Andaman Islands. A total of 231 seagrass sites were observed during the study, in which a total of eight species were found. The South Andaman region exhibited the highest species diversity followed by Havelock, Little Andaman and Neil Islands. Eleven continuous seagrass bed were noted during the study which were confined to three different types of substrata: sand only; sand mixed with coral sand and coral rubble; sand mixed with mud. Thalassia hemprichii and Cymodocea rotundata were found in dense canopies and covered large areas associated with sand and coral rubble. Halophila beccarii showed moderate abundance in muddy substrata and Halodule spp. grew in fine sand. The highest shoot density was recorded at Haddo, followed by Vijaya Nagar and Chidyatapu, contributed mostly by Halodule spp. In general, seagrass diversity and distribution were mainly influenced by the percentage composition of sand. The authors recommend that the seagrass meadow of Haddo should be considered for conservation priority because of multiple anthropogenic pressures, such as sewage input, dumping of solid wastes on the shoreline and anchoring of boats.

The population genetics of two seagrasses, Syringodium isoetifolium and Thalassia hemprichii were assessed throughout the Indonesian Archipelago. We genotyped 257 blades of S . isoetifolium at 15 microsatellite loci collected from 14 sampling sites and 406 blades of T. hemprichii at 17 microsatellite loci from 16 sampling sites. Once clones were removed, 165 individuals of S . isoetifolium and 389 of T. hemprichii were used in downstream analyses. Bayesian clustering methods revealed two genetically distinguishable clusters in each species, although the geographic boundaries of these clusters differed. Syringodium isoetifolium has a cluster that is located exclusively on the shallow Sunda Shelf and appears to follow the demarcation defined by Wallace’s line. Thalassia hemprichii , however, has a genetically distinguishable cluster located within the Banda Sea. The Banda Sea has unique physical oceanographic conditions, which may be driving the observed differentiation in T. hemprichii . We speculate that historical sea level changes exposing the Sunda Shelf, and the unique oceanographic conditions of the Banda Sea are contributing to the observed population differentiation. The observed genetic differentiation in both species is a possible starting point for speciation.

This study investigated photosynthetic and oxidative stress-related responses to moderate (500 μmol photons m −2 s −1 ) and high irradiances (1500 μmol photons m −2 s −1 ) in two tropical intertidal seagrasses, Halophila ovalis and Thalassia hemprichii . Exposure to high irradiance for 3 h resulted in a large decline in the maximum quantum yield of photosystem II (Fv/Fm) and in the effective quantum yield of photosystem II (ΦPSII) but induced non-photochemical quenching (NPQ) and chloroplast clumping in H. ovalis . Thalassia hemprichii , however, showed only a minor decrease in Fv/Fm and ΦPSII, and no change in either NPQ or the intracellular arrangement of chloroplasts. The irradiance did not affect reactive oxygen species (ROS) or superoxide dismutase (SOD) activity in either seagrass, whereas high irradiance enhanced ascorbate peroxidase (APX) activity in H. ovalis . The results from a subsequent experiment using an oxidative stress inducer, methyl viologen (MV), also showed a higher degree of Fv/Fm inhibition in H. ovalis . These results show that H. ovalis is much more physiologically responsive to high irradiances and oxidative stress than T. hemprichii and that its photoprotective mechanisms involve NPQ, APX and reduction of the amount of light absorbed through the clumping of its chloroplasts.

This study aims to examine photophysiological and osmotic responses in seedlings of the seagrass Enhalus acoroides after exposure to different salinity levels. Seagrass seedlings were grown for 20 days in control (salinity 30), hyposaline (salinity 10 and 20) and hypersaline (salinity 40 and 50) conditions. The present study showed that both hypo- and hypersaline conditions affected the photophysiology of E. acoroides seedlings, reducing the maximum quantum yield of photosystem II ( F v / F m ) and total chlorophyll content. The photosynthetic system appeared to be more sensitive to hyposaline than to hypersaline conditions as shown by immediate declines in F v / F m and total chlorophyll content. Hyposaline conditions increased the water content in roots. The increase in tissue Na + content induced by hypersalinity did not affect photosynthetic integrity and was more pronounced in leaves than in roots. It is concluded that the ionic homeostasis of E. acoroides seedlings is less affected by short-term hypersalinity than by hyposalinity. The K + /Na + ratios in leaves with hypersalinity decreased by 20 days after treatment. Additionally, the photosynthetic efficiency ( F v / F m and total chlorophyll content) is highly sensitive to salinity shifts and can be used as a marker for short-term acclimation to salinity stress in this seagrass species.

Southeast Asia has the highest diversity of seagrass species and habitat types, but basic information on seagrass habitats is still lacking. This review examines the known distribution, extent, species diversity, and research and knowledge gaps of seagrasses in Southeast Asia by biogeographic region of the Marine Ecoregions of the World (MEOW). The extent of seagrass meadows in Southeast Asia is ~36,762.6 km 2 but this is likely an underestimate as some ecoregions were not well-represented and updated information was lacking. There is a paucity of information from the Western Coral Triangle Province, with no areal extent data available for the Indonesian regions of Kalimantan, Central and Southeast Sulawesi, the Maluku Islands, and West Papua. Regional research output has increased in the last two decades, with a trend towards more experimental, rather than descriptive research. However, there are knowledge gaps in socio-cultural-economic themed research, despite growing awareness of the importance of seagrass-human relationships in this region. Obstacles to advancing seagrass research, knowledge and conservation are rooted in either lack of expertise and training or the failure of effective management and policies. We propose a roadmap for seagrass conservation, with suggested solutions, including 1) encouraging collaboration between research institutions and scientists in the region to build capacity and share knowledge; 2) engaging with policymakers and governments to encourage science-based policies; 3) engaging with communities to raise awareness and foster stewardship of seagrass in the region.

Due to the dearth of information regarding current and changing health of seagrass habitat in the Indo-Pacific region, prior research into global trends of seagrass habitat health has included little data from this region, even though it contains the highest abundance and species diversity of seagrass globally. This study evaluates the suitability of four satellite sensors [Worldview-2 (WV2), Advanced Land Imager (ALI), Enhanced Thematic Mapper+ (ETM+), Operational Land Imager (OLI)] for determining trends in seagrass habitat extent over the past decade in Singapore’s largest seagrass meadow, and thus contributes incrementally to the data available for regional or global analyses of seagrass habitat health. Using all four sensors, we find that seagrass bed extent at Pulau Semakau, Singapore, declined 37% from 2001 to 2015 at an average rate of 3.9% year −1 . Using very high spatial resolution satellite images, we calculate that, although bed extent decreased 18% from April 2011 to June 2013, median meadow biomass increased, indicating that complex meadow dynamics may be mediating seagrass response to anthropogenic and environmental pressures. From a technological perspective, we find that, despite their lower spatial resolution, freely available satellite images can be used to measure the extent of a narrow, multi-species seagrass bed and to determine decadal trends reliably.

Seagrass habitats provide a range of goods and benefits to coastal communities by supporting ecosystem functioning, food provisioning, and cultural values. However, they are at risk worldwide from anthropogenic activities, climate change impacts and limited resources, which hamper efforts to protect them. Effective conservation planning requires prioritisation of sites based on multiple factors, including their relative value and vulnerability (VU). The current study examines the efficacy of combining two well-established approaches – ecosystem service assessments and habitat VU analyses – to test a method for determining conservation priority for selected seagrass meadows in Southeast Asia. Seven seagrass meadows around Southeast Asia were selected and scored by the authors based on (1) the cumulative contribution of 12 ecosystem services (ES) and the condition of the meadows, (2) VU from 16 anthropogenic threats, which were each ranked against five factors (spatial scale, frequency, functional impact, resistance and recovery time) and (3) the combination of the two approaches to create a single numerical index. Prioritisation of sites differed when meadows were scored solely for the ES they provide, compared to how vulnerable they are to anthropogenic threats. The new combined analysis brought the two metrics together to give conservation priority to sites where management intervention will yield maximum conservation benefits from conservation effort, and resulted in the prioritisation of three sites: Barrang Lompo (Indonesia), Cape Bolinao (Philippines) and Beting Tanjung Kupang (Malaysia). This method is easy to use and does not require great technical expertise. It is also auditable, allowing for clear and transparent understanding of decision-making process.