Herbivores strongly affect the ecological structure and functioning in seagrass bed ecosystems, but may exhibit density-dependent effects on primary producers and carbon sequestration. This study examined the effects of herbivorous snail (Cerithidea rhizophorarum) density on snail intraspecific competition and diet, dominant seagrass (Thalassia hemprichii) and epiphyte growth metrics, and sediment organic carbon (SOC). The growth rates of the herbivorous snail under low density (421 ind m−2) and mid density (842 ind m−2) were almost two times of those at extremely high density (1684 ind m−2), indicating strong intraspecific competition at high density. Herbivorous snails markedly reduced the epiphyte biomass on seagrass leaves. Additionally, the seagrass contribution to herbivorous snail as food source under high density was about 1.5 times of that under low density, while the epiphyte contribution under low density was 3 times of that under high density. A moderate density of herbivorous snails enhanced leaf length, carbon, nitrogen, total phenol and flavonoid contents of seagrasses, as well as surface SOC content and activities of polyphenol oxidase and β-glucosidase. However, high density of herbivorous snails decreased leaf glucose, fructose, detritus carbon, and total phenols contents of seagrasses, as well as surface SOC content and activities of polyphenol oxidase and β-glucosidase. Therefore, the effects of herbivorous snail on seagrass, epiphyte and SOC were density-dependent, and moderate density of herbivorous snail could be beneficial for seagrasses to increase productivity. This provided theoretical guidance for enhancing carbon sink in seagrass bed and its better conservation.

Seagrass-herbivore interactions play a principal role in regulating the structure and function of coastal food webs, which were affected by nutrient enrichment. Seawater nutrient enrichment might change seagrass palatability by altering seagrass physical and chemical traits, consequently modulating herbivory patterns, but this remains elusive. In this study, the dominant tropical seagrass Thalassia hemprichii was cultured in different ammonium concentrations to examine the response of seagrass nutritional quality, deterrent secondary metabolites, and leaf toughness, as well as the subsequent effect of the changed physical (e.g., leaf toughness) and chemical traits (e.g., nitrogen content; total phenol) on the grazing activity of the herbivorous snail Cerithidea rhizophorarum. Ammonium enrichment enhanced seagrass nutritional quality and decreased physical defence. Low ammonium enrichment increased total phenol content, while high ammonium enrichment reduced it. Both low and high ammonium enrichment enhanced the grazing intensity of C. rhizophorarum on seagrass. Interestingly, nutritional quality mostly determined the herbivory preference of C. rhizophorarum on the intact seagrass having physical structure, with a chemical deterrent (total phenol) playing a secondary role. In contrast, chemical deterrent mainly determined the grazing intensity on agar seagrass food which was made artificially to exclude physical structure. This indicated that seagrass leaf physical structure might hinder phenol compounds from deterring herbivores. Overall, the results presented here demonstrate that ammonium enrichment remarkably increased seagrass palatability and subsequently induced higher susceptibility to herbivory, which might induce seagrass loss.

Background Sediment is crucial for the unique marine angiosperm seagrass growth and successful restoration. Sediment modification induced by eutrophication also exacerbates seagrass decline and reduces plantation and transplantation survival rates. However, we lack information regarding the influence of sediment on seagrass photosynthesis and the metabolics, especially regarding the key secondary metabolic flavone. Meanwhile, sulfation of flavonoids in seagrass may mitigate sulfide intrusion, but limited evidence is available. Results We cultured the seagrass Thalassia hemprichii under controlled laboratory conditions in three sediment types by combining different ratios of in-situ eutrophic sediment and coarse beach sand. We examined the effects of beach sand mixed with natural eutrophic sediments on seagrass using photobiology, metabolomics and isotope labelling approaches. Seagrasses grown in eutrophic sediments mixed with beach sand exhibited significantly higher photosynthetic activity, with a larger relative maximum electron transport rate and minimum saturating irradiance. Simultaneously, considerably greater belowground amino acid and flavonoid concentrations were observed to counteract anoxic stress in eutrophic sediments without mixed beach sand. This led to more positive belowground stable sulfur isotope ratios in eutrophic sediments with a lower Eh. Conclusions These results indicated that coarse beach sand indirectly enhanced photosynthesis in T. hemprichii by reducing sulfide intrusion with lower amino acid and flavonoid concentrations. This could explain why T. hemprichii often grows better on coarse sand substrates. Therefore, it is imperative to consider adding beach sand to sediments to improve the environmental conditions for seagrass and restore seagrass in eutrophic ecosystems.

Eutrophication, dredging, agricultural and urban runoffs, and epiphyte overgrowth could reduce light availability for seagrass. This may affect "blue carbon" stocks in seagrass beds. However, little research is available on the effect of light intensities on carbon sequestration capacity in seagrass beds, especially small-bodied seagrasses. The dominant seagrass Halophila beccarii, a vulnerable species on the IUCN Red List, was cultured in different light intensities to examine the response of vegetation and sediment carbon in seagrass beds. The results showed that low light significantly reduced leaf length and above-ground biomass, while carbon content in both above-ground and below-ground tissues were not affected. Low light reduced both the above-ground biomass carbon and the total biomass carbon. Interestingly, while under saturating light conditions, the subsurface and surface carbon content was similar, under low light conditions, subsurface sediment carbon was significantly lower than the surface content. The reduction of subsurface sediment carbon might be caused by less release flux of dissolved organic carbon from roots in low light. Taken together, these results indicate that reduced light intensities, to which these meadows are exposed to, will reduce carbon sequestration capacity in seagrass beds. Measures should be taken to eliminate the input of nutrients on seagrass meadows and dredging activities to maintain the "blue carbon" storage service by enhancing light penetration into seagrass.

Zn, Pb, Cr, Cu, Ni, Cd concentration and ecological risk were studied in three bays to evaluate the heavy metal (HM) contamination of seagrasses. Seasonal HM accumulation varied according to locations, seagrass species and tissues. Halophila beccarii had much higher HM concentrations except for Cr in Zhelin Bay, however, bioconcentration factors (BCF) of Cu, Ni, Pb, Zn were higher in Liusha than Zhelin Bay. Cr was much enriched in Thalassia hemperichii and Enhalus acoroides than Halophila beccarii and Halophila ovalis. Cr, Cu, Ni, Pb were easy to accumulate in belowground tissues in Halophila ovalis. In contrast, almost all HM were more enriched in aboveground tissues in other species. Generally, BCF exceeding 1 and high metal pollution index suggested HM had potential ecological risk on seagrasses. The results provide the reference for managing and protecting seagrass ecosystem in South China, and are significant to expand the global seagrass detection network.

Abstract Aims Sediment composition is highly crucial for seagrass growth and survival. Eutrophication has been suggested a major cause of seagrass decline globally. We investigated the effects of beach sand supplementation to natural sediments under eutrophic condition on the growth and survival of tropical dominant seagrass Thalassia hemprichii. Methods We cultured seagrass T. hemprichii under the controlled laboratory conditions in three sediment types by combining different ratio of in-situ eutrophic sediment and coarse beach sand. We examined the effect of beach sand mixing to natural eutrophic sediments on the growth of seagrass using photobiology, metabolomics and isotope labeling approaches. Results Seagrass grown in eutrophic sediments mixed with sand exhibited significantly higher photosynthetic activity with high relative maximum electron transport rate and minimum saturating irradiance. Simultaneously, considerably greater belowground amino acid and flavonoid concentrations were observed to counteract anoxic stress in eutrophic sediment without mixing sand. This led to more positive belowground stable sulfur isotope in the eutrophic sediment with lower Eh. Conclusions These results indicated coarse beach sand indirectly enhanced photosynthesis and growth for T. hemprichii by reducing sulfide intrusion with lower concentrations of amino acid and flavonoid. This could possibly explain why T. hemprichii often grow better in the coarse sand substrate. Therefore, it is imperative to consider adding sand soil in the sediments to improve the growth condition for seagrass and restoring the seagrass shoots during transplantation in eutrophicated ecosystem.

Seagrass beds act as blue carbon sinks globally; however, little attention has been given to carbon dynamics in the seagrass rhizosphere. Hence, in this study, the quantity and characteristics of dissolved organic carbon (DOC) from root exudation of the three dominant tropical seagrasses (Thalassia hemprichii, Enhalus acoroides, and Cymodocea rotundata) and sediment pore water beneath them were compared, to examine their interspecific differences, and to establish a connection between seagrass root exudation and sediment carbon. The rate of root-exuded DOC from T. hemprichii (2.15 ± 1.06 mg g DW root-1 h-1) was significantly higher (p < 0.05) than that from E. acoroides (0.72 ± 0.39 mg g DW root-1 h-1) and C. rotundata (0.46 ± 0.25 mg g DW root-1 h-1). Root exudation rates were more affected by root hair density and root hair length than by root carbon, nitrogen, and soluble sugar content. Simultaneously, DOC concentrations of the sediment pore water beneath T. hemprichii, E. acoroides and C. rotundata were 22.05 ± 11.61 mg l-1, 15.55 ± 2. 66 mg l-1, and 14.32 ± 1.82 mg l-1, respectively. The corresponding absorption coefficients at 254 nm (a254) were 30.53 ± 18.00, 17.31 ± 2.24, and 14.07 ± 2.03, respectively, while the relevant specific ultraviolet absorbances at 254 nm (SUVA254) were 1.38 ± 0.29, 1.19 ± 0.26 and 1.03 ± 0.28, respectively. Therefore, the roots of T. hemprichii exuded DOC at a higher rate, leading to a higher pore-water DOC pool in the sediment. This suggests that T. hemprichii played a greater role in the sediment carbon pool through root exudation. Thus, it can be considered as the priority species for transplantation to promote the carbon sink function of seagrass beds.

Seagrass beds are highly productive ecosystems that provide habitat and food sources for fishes and invertebrates. Herbivores are the critical link between primary producers and secondary consumers and maintain the biodiversity and complexity of food web structures in seagrass beds. Herbivore habitat selection in different seagrass habitats is influenced by food source composition, predation risks, and other factors; however, research is needed to better understand how habitat selection influences the biodiversity maintenance mechanism in seagrass beds. In this study, the distribution and predation rate of Cerithidea rhizophorarum, an important herbivorous sea snail found in tropical seagrass beds, was studied using field investigations and in-situ experiments. Three seagrass habitats were selected, which included the seagrass species Enhalus acoroides, Thalassia hemprichii, and Cymodocea rotundata. Dual stable isotope techniques were also used to investigate the food source composition of C. rhizophorarum. The results showed that the leaf area of E. acoroides, T. hemprichii, and C. rotundata were (124.76±19.72) cm2, (10.25±5.54) cm2, and (5.98±1.06) cm2, respectively. Leaf toughness was measured to be (1.41±0.34) N, (0.51±0.14) N, and (0.18±0.03) N, respectively, and the corresponding leaf epiphyte biomasses were (16.34±6.79) g/m2, (4.29±1.98) g/m2, and (1.93±0.68) g/m2, respectively. The distribution density of C. rhizophorarum showed a trend of E. acoroides<T. hemprichii<C. rotundata habitats, while the predation rate exhibited the opposite trend. Also, the δ13C values of E. acoroides, T. hemprichii and C. rotundata were (−8.26±0.25)‰, (−8.45±0.18)‰, and (−8.81±0.26)‰, respectively. The corresponding δ15N values were (11.11±0.18)‰, (10.98±0.87)‰, and (8.11±0.74)‰, respectively. The δ13C values of C. rhizophorarum were (−9.35±0.60)‰, (−8.84±0.35)‰, and (−8.20±0.25)‰, respectively. The δ15N values were (11.76±0.27)‰, (11.91±0.51)‰, and (11.10±0.48)‰, respectively. Based on Bayesian Mixed Model (MixSIAR) analysis, we found that the diet contribution of E. acoroides, T. hemprichii, and C. rotundata to C. rhizophorarum were 14.8%, 30.3%, and 61.8%, respectively. The diet contributions of epiphytes were 48.3%, 26.8%, and 3.1%, respectively. The contribution of macroalgae ranged consistently from 27.2% to 32.2%. The contribution of particulate organic matter and sediment organic matter was relatively small. Overall, there were significant differences in the distribution density, predation rate, and food source composition of C. rhizophorarum among the three seagrass habitats. Food accessibility and the threat from natural predators were the two key factors that affected habitat selection.

Seagrass meadows provide important habitats and rich organic carbon sources for consumers at different trophic levels but are threatened by accelerating eutrophication in coastal waters. Nevertheless, at present, carbon transfer processes throughout the food web and trophic pathways in eutrophic seagrass meadows are still poorly known. To resolve this issue, carbon sources of different trophic communities in a eutrophic tropical seagrass meadow [Xincun (XC) bay, South China Sea] under eutrophication were examined in summer and winter using dual stable isotopes. The δ 13 C value of omnivores and carnivores overlapped more with that of herbivores and planktivores/filter feeders, which mainly overlapped with that of epiphytes in summer and macroalgae in winter. Meanwhile, epiphytes and macroalgae exhibited high biomass and corresponding highest contribution to herbivores, omnivores, and carnivores in summer and winter, respectively. These results suggest that the grazing food chain was the main trophic pathway in this eutrophic seagrass meadow, and that the transfer of carbon flow in the grazing food chain was mainly dominated by the proliferating epiphytes or macroalgae carbon. In contrast, the contribution of seagrass to detritivores in both seasons was higher than that of other food sources. Our findings suggest that in eutrophic tropical seagrass meadows, the proliferation of epiphytes or macroalgae induced by high nutrient loading, as well as their seasonal changes, has a greater impact on the transfer of carbon in the grazing food chain than that in the detritus food chain, and the seagrass fueled the food web mainly through the detritus food chain.

Seagrass beds provide nursery habitats for marine species. Seagrass leaves, in particular, are used as spawning grounds and as a food source for fish and invertebrates, but direct evidence of spawning in seagrass leaves is rare. It is also very challenging to identify eggs through morphological analysis, since the eggs of many marine species appear similar. To accurately identify the eggs on the leaves of the dominant seagrass species in the South China Sea, and evaluate seasonal contribution of seagrass leaves as a food source, DNA barcoding and stable isotope technique were conducted. Interestingly, Monetaria annulus was found to spawn on the leaf sheath of Thalassia hemprichii in Li’an gang, Hainan Island. This choice of oviposition site might increase embryo survival compared to the other parts of the seagrass leaves. Meanwhile, the eggs of Stethojulis trilineata were deposited on the entire leaf of Halophila ovalis in Liusha Bay, Guangdong Province. The small fingernail-shaped leaves of H. ovalis might be beneficial for S. trilineata to lay eggs as they are closer to the sediment surface than other seagrass leaves. Based on the stable carbon and nitrogen isotope analyses, the primary food source of M. annulus in summer and in winter were particulate organic matter and seagrass, respectively. The results suggest that seagrass leaves are not only important for fish and invertebrates as a spawning site, but also as a food source. The findings of the present study may support the urgent requirement of the conservation of seagrass beds for sustaining the productivity of marine fisheries.

Seagrasses are submerged marine macrophytes, they form highly productive ecosystems that play a critical role in climate change mitigation. Seagrass meadows are known to stabilise coastal sediments, recycle nutrients, and provide nursery grounds and food sources for fishes and megafauna. However, nearly 30% of seagrass habitats have been lost worldwide because of global climate change and sustained negative pressure from coastal developments, as well as eutrophication and dredging. This global decline and ongoing threat from climate change have weakened their capacity to support coastal productivity. This alarming decline highlights a global urgency for the implementation of effective seagrass management strategies to prevent their decline. Until now, the physiological indicators used to monitor seagrass health, including seagrass species composition, above ground biomass, and spatial coverage, have failed to provide any insights into the intensity of environmental stress at the physiological and/or molecular level, which has led to their decline. Therefore, seagrass restoration efforts require highly accurate and novel tools for monitoring seagrass health and effective management strategies. In the era of systems biology, omics (transcriptomics, proteomics, and metabolomics) and their integration have shown promise in the identification of biomarkers of phenotypic plasticity of an organism under specific environmental conditions. Recently, with the influx of genomic information regarding seagrasses, omics have emerged as a new frontier in seagrass research and have deepened our understanding of their adaptation and underlying tolerance mechanisms to environmental stresses. This review article provides a state-of-art on seagrass’ omics and discusses their potential to discover stress biomarkers, and thereby aid in developing robust diagnostic tools for monitoring seagrass’ health and their management for protection and conservation. We discuss how omics platforms are advancing in technology, such as whole-genome shotgun, RNA-seq, 2-DE, and multidimensional GC- and MS-based platforms, which helps in developing high-throughput protocols for possible implementation in seagrass research. This article further discusses how such innovations can provide knowledge on: (1) Molecular adaptation and acclimatisation mechanisms in seagrass responses to light, salinity, thermal, carbon dioxide, eutrophication, and heavy metal concentration, (2) their genetic diversity and evolution, and (3) seagrass reproduction and development. Moreover, we discuss the organelle, such as the chloroplast, based omics frontier to explore its potential in seagrass research to better understand the biological processes at the organelle level and how it may help determine the role of different organelles in providing tolerance against various environmental stresses. Lastly, we discuss the do’s and don’ts of the experimental designs, methodological procedures, and analytical tools learned from higher plant studies to facilitate multi-omics studies and data integration in seagrass research. Creating an online user friendly database sourced from multi-omics studies and a suite of tools for analysis will attract seagrass molecular biologists and ecologists working on seagrass ecology, restoration, development, and evolution, and will improve the molecular horizons of seagrass research.

Rapid urbanization leads to an accelerating decline of seagrass beds. The status of seagrass beds along the entire coastline of a rapidly urbanizing area, Guangdong Province, was examined to document the change in seagrass beds and to explore the determinants of seagrasses characteristics and their plasticity. Thirteen seagrass beds were newly discovered with a total area as 679.04 ha, whereas eleven known seagrass beds have decreased from 972.55 ha to 858.67 ha with seven of them having disappeared in recent decade primarily due to exacerbated construction of artificial shorelines and beach dams, increased nutrient inputs from fish caging and shrimp pond culture, oyster culture, mangrove planting and shellfish collection. The leaf nitrogen content of Halophila ovalis, which dominated the largest beds, increased from (2.09 ± 0.24)% in 2011 to (3.39 ± 0.18)% in 2017, indicating enhanced eutrophication. The optimum seawater dissolved inorganic nitrogen and dissolved inorganic phosphorus levels for Halophila beccarii were 40 μmol/L and 2.5 μmol/L, respectively. The standing stock and plant dimensions of H. beccarii were positively correlated with sediment mud content. Longer, wider leaves, and greater aboveground and belowground biomass were observed at lower salinities, indicating that H. beccarii prefers hyposaline habitats. High shoot density could induce intraspecific competition followed by self-thinning in H. beccarii, leading to reduced leaf area, aboveground and belowground biomass, and root length. Thus, long-term monitoring of seagrass beds along the rapidly urbanizing coastline of Guangdong Province is needed to unravel the mechanisms of decline and to develop effective management strategies.