Seagrass Species Research Articles

Correlative adaptation between Rubisco and CO2-concentrating mechanisms in seagrasses.

Submerged angiosperms sustain some of the most productive and diverse ecosystems worldwide. However, their carbon acquisition and assimilation mechanisms remain poorly explored, missing an important step in the evolution of photosynthesis during the colonization of aquatic environments by angiosperms. Here we reveal a convergent kinetic adaptation of Rubisco in phylogenetically distant seagrass species that share catalytic efficiencies and CO2 and O2 affinities up to three times lower than those observed in phylogenetically closer angiosperms from terrestrial, freshwater and brackish-water habitats. This Rubisco kinetic convergence was found to correlate with the effectiveness of seagrass CO2-concentrating mechanisms (CCMs), which probably evolved in response to the constant CO2 limitation in marine environments. The observed Rubisco kinetic adaptation in seagrasses more closely resembles that seen in eukaryotic algae operating CCMs rather than that reported in terrestrial C4 plants. Our results thus demonstrate a general pattern of co-evolution between Rubisco function and biophysical CCM effectiveness that traverses distantly related aquatic lineages.

Chemotaxis is important for fine scale habitat selection of early juvenile Panulirus cygnus

The western rock lobster (Panulirus cygnus [George 1962]) is the basis for Australia's most valuable single species wild caught fishery. The ability of early juvenile western rock lobster to select and settle onto suitable habitats is critical for the recruitment of this commercially important species. Many marine organisms, including the western rock lobster, have highly complex olfactory systems that allow for various behaviours including habitat selection and the location of food. We tested the active habitat selection of early juvenile western rock lobsters using only chemotaxis in scent based trials. We used a Y-maze bio-assay with which we were able to present individuals with several different natural scent stimuli. This study used three commonly observed broad habitat complexes followed by three fine scale seagrass assemblages to provide different scent stimuli. Seagrass habitats were chosen significantly more often by the juvenile lobsters in the broad habitat scale trials over bare sand and turf habitats. At a finer scale, lobsters showed a mixed response but tended to choose the canopy forming seagrass species, Amphibolis antarctica [(Labillardière) Sonder & Ascherson ex Ascherson, 1868] and Posidonia australis [Hooker, 1858] significantly more often than the other scents provided. This study developed a Y-maze chemotaxis assay for P. cygnus and confirmed that they respond to habitat related scent cues. These findings suggest the importance of seagrass habitats to early juvenile western rock lobsters and the potential impacts that habitat change may have on the successful recruitment of this commercially important species.

Assessing the Effects of Ocean Warming and Acidification on the Seagrass Thalassia hemprichii

Seagrass beds serve as important carbon sinks, and it is thought that increasing the quantity and quality of such sinks could help to slow the rate of global climate change. Therefore, it will be important to (1) gain a better understanding of seagrass bed metabolism and (2) document how these high-productivity ecosystems are impacted by climate change-associated factors, such as ocean acidification (OA) and ocean warming (OW). A mesocosm-based approach was taken herein in which a tropical, Western Pacific seagrass species Thalassia hemprichii was cultured under either control or OA-simulating conditions; the temperature was gradually increased from 25 to 31 °C for both CO2 enrichment treatments, and it was hypothesized that this species would respond positively to OA and elevated temperature. After 12 weeks of exposure, OA (~1200 ppm) led to (1) increases in underground biomass and root C:N ratios and (2) decreases in root nitrogen content. Rising temperatures (25 to 31 °C) increased the maximum quantum yield of photosystem II (Fv:Fm), productivity, leaf growth rate, decomposition rate, and carbon sequestration, but decreased the rate of shoot density increase and the carbon content of the leaves; this indicates that warming alone does not increase the short-term carbon sink capacity of this seagrass species. Under high CO2 and the highest temperature employed (31 °C), this seagrass demonstrated its highest productivity, Fv:Fm, leaf growth rate, and carbon sequestration. Collectively, then, it appears that high CO2 levels offset the negative effects of high temperature on this seagrass species. Whether this pattern is maintained at temperatures that actually induce marked seagrass stress (likely beginning at 33–34 °C in Southern Taiwan) should be the focus of future research.

Foraging habitat characterization of green sea turtles, Chelonia mydas, in the Cenderawasih Bay, Papua, Indonesia: Insights from satellite tag tracking and seagrass survey

Abstract. Tapilatu RF, Wona H, Mofu B, Kolibingso D, Alzair N, Erdmann M, Marunaya B. 2022. Foraging habitat characterization of green sea turtles, Chelonia mydas, in the Cenderawasih Bay, Papua, Indonesia: Insights from satellite tag tracking and seagrass survey. Biodiversitas 23: 2783-2789. The distribution area of green turtles is not yet known in Cenderawasih Bay. On 20 November 2020, two satellite transmitters were attached to two female green turtles (Chelonia mydas Linnaeus, 1758) on Kalilemon, Southwest Cenderawasih Bay, Papua Province, Indonesia. Kalilemon area and Mangga Island as part of Kwatisore waters are the major foraging areas for this species and are under threat of turtle opportunistic poachers. The results show that the two turtles (Mary and Nona) dispersed and used limited but consistent and different foraging grounds during the tracking period through the satellite tags between November 2020-April 2021 at Mangga Island and Kalilemon. The composition of seagrass at both stations consisted of the same five species belonging to the families Cymodoceae and Hydrocharitaceae, with relatively different qualitative abundances. Overall, the mean chlorophyll-a content during the study period from November 2020-April 2021 ranged from 0.25-0.55mg/m3 at both foraging grounds (Mangga Island and Kalilemon). The mean SST during the study period ranged from 30.61-31.77oC in both locations. Before the next tracking attempt, more data on seasonal foraging and nesting activities are required. The result of this research is vital to determine the foraging grounds for green turtles related to the seagrass species with its abundance and key environmental factors in the management and conservation of sea turtles in the Cenderawasih Bay area.

Seagrass Diversity and Distribution in Maribojoc Bay, Bohol, Philippines

Seagrasses are major parts of coastal and marine biodiversity. Unfortunately, these aquatic plants and their ecological values are virtually unknown to many Filipinos. This study assessed the seagrasses in Maribojoc Bay, particularly in the coastal areas of the three municipalities, namely Maribojoc, Dauis, Panglao, and the City of Tagbilaran. Ecological assessments were conducted to determine the composition, abundance, distribution, percent cover, diversity, dominance, and evenness of seagrass species. Eight sampling sites were surveyed from October to December 2020. Seven seagrass species were identified, with Thalassia hemprichii as the most abundant (52.79%). There was a significant difference (p>0.05) in seagrass species relative abundance. The Shannon diversity index implies low diversity (H’=1.40) of seagrass species. High dominance (2.98) and low evenness (0.72) were attributed to the high abundance of T. hemprichii in the seagrass beds. The seagrass coverage was characterized by patchy and continuous meadows, with percentage cover ranging from 17.45% (poor) – 60% (good). Maribojoc Bay had a seagrass percentage cover of 38.65%, which can be classified under “fair” conditions. Seagrass community structure implies sparse coverage and low diversity, probably due to the deterioration of once-continuous meadows. However, further studies concerning seagrass communities are recommended in order to implement rehabilitation program or improve current management in Maribojoc Bay.

Phytochemical Screening and Antimicrobial Potentialo of Halodule Pinifolia

The aim of this work was to study the phytochemical and antimicrobial activity of seagrass extracts of Halodulepinifolia on clinical isolated pathogens. Seagrass species have very potential groups producing several secondary metabolites. The bioactive potential of seagrass species viz., Halodulepinifolia occurring commonly along the Thanjavur coastal area was selected. We evaluate the phytochemical and antimicrobial potential of different extracts of Halodulepinifolia. The extract of the seagrass was tested against E.coli, B.subtilis, A.nigerand C.albicans by agar diffusion method. Phytochemical screening revealed the presence of carbohydrates, reducing sugars, alkaloids, saponins, phenolic compounds and flavonoids in aqueous seagrass extract. The results of the present study conclude that the studied plant possesses broad-spectrum antimicrobial properties and may act as a potent antioxidant for biological systems susceptible to free radical-mediated reactions.

Evaluating Seagrass Meadow Dynamics by Integrating Field-Based and Remote Sensing Techniques.

Marine phanerogams are considered biological sentinels or indicators since any modification in seagrass meadow distribution and coverage signals negative changes in the marine environment. In recent decades, seagrass meadows have undergone global losses at accelerating rates, and almost one-third of their coverage has disappeared globally. This study focused on the dynamics of seagrass meadows in the northern Adriatic Sea, which is one of the most anthropogenically affected areas in the Mediterranean Sea. Seagrass distribution data and remote sensing products were utilized to identify the stable and dynamic parts of the seagrass ecosystem. Different seagrass species could not be distinguished with the Sentinel-2 (BOA) satellite image. However, results revealed a generally stable seagrass meadow (283.5 Ha) but, on the other hand, a stochastic behavior in seagrass meadow retraction (90.8 Ha) linked to local environmental processes associated with anthropogenic activities or climate change. If systemized, this proposed approach to monitoring seagrass meadow dynamics could be developed as a spatial decision support system for the entire Mediterranean basin. Such a tool could serve as a key element for decision makers in marine protected areas and would potentially support more effective conservation and management actions in these highly productive and important environments.

The relationship between seagrass abundance, distribution and the abiotic factors in Big Gifton and Abu Minqar islands, Red Sea, Egypt

Abstract. Ebead MAS, Hussein HNM, Gomaa H, Abdelmottaleb M, Mahdy A. 2022. The relationship between seagrass abundance, distribution and the abiotic factors in Big Gifton and Abu Minqar islands, Red Sea, Egypt. Biodiversitas 23: 2295-2303. Seagrass communities represent spatially complex and biomass-producing systems. The current research aims to understand the relationship between the abundance and distribution of seagrass communities and the abiotic factors in Big Gifton Island and Abu Minqar Island, Red Sea, Egypt. Seasonal seagrass samples were collected during 2019-2020 using line-transect and quadrat techniques. Canonical Correspondence Analysis was used to determine the effect of environmental factors on the occurrence and abundance of seagrass species. Three seagrass species, i.e., Halophila stipulacea, Halodule uninervis, and Halophila ovalis were recorded in the two islands. The most dominant seagrass species in both locations was H. stipulacea, which accounted for 54% in Big Gifton and 45% in Abu Minqar Island, while H. uninervis and H. ovalis were less numerous, with a percentage cover of 23 and 24%; and 8 and 5% in Big Gifton and Abu Minqar island, respectively. Halophila stipulacea was highest in the winter season with a percentage of 62 % and lowest in the spring (47%) in Big Gifton, whereas it was highest in the spring (62%) and lowest in the winter (47%) on Abu Minqar Island (22%). Statistical analysis showed highly significant differences between both sites and seasons in terms of conductivity, total dissolved solids, salinity, and the presence of H. uninervis and H. ovalis. On the other hand, there were non-significant differences between both sites and seasons in terms of temperatures, dissolved oxygen, oxidation-reduction potential, nitrite, nitrate, ammonia, phosphorus and the presence of H. stipulacea. Throughout the year, Big Gifton had a higher mean phosphate value than Abu Minqar Island and this may be related to the highest abundance of H. stipulacea in Big Gifton Island. The findings of this study may aid decision-makers in the management of natural resources, particularly the seagrass beds of Big Gifton and Abu Minqar Island.

Assessing the seagrasses meadows status and condition: A case study of Wori Seagrass Meadows, North Sulawesi, Indonesia

Abstract. Sondak CFA, Kaligis EY. 2022. Assessing the seagrasses meadows status and condition: A case study of Wori Seagrass Meadows, North Sulawesi, Indonesia. Biodiversitas 23: 2156-2166. Seagrass meadows play an important role in the coastal ecosystem through the services they provide, however, they are one of the most threatened ecosystems in the coastal environment due to anthropogenic influences and rapid environmental changes. Seagrass meadows extent is not validated and the health condition is left unmonitored and unreported. In Indonesia, the information about the current status of seagrass meadows’ extent and health conditions is limited. We assess the seagrass meadow abundance, status, and health condition. We provide information about species abundance, density threats, and the health status of seagrass meadows in North Sulawesi, Indonesia, particularly in Wori District. This district has a vast extent of seagrass meadows but has been affected by the expansion of the industrial/tourism area within the region. This study found that seagrass meadows’ coverages were moderate, but their health status was unhealthy. Six seagrass species are found in this area, including Enhalus acoroides (L.f.) Royle, Thalassia hemprichii (Ehrenb.) Aschers, Cymodocea rotundata (Asch. & Schweinf.), Halodule pinifolia (Miki) Hartog, Halophila ovalis (R.Br.) Hook. F., and Syringodium isoetifolium (Asch.) Dandy. Our results highlight the communities’ activities, including gleaning, boat propeller, anchoring, mooring, sedimentation, and the effects of climate change on the environment as factors that could affect seagrass meadows' extent and health condition in the study sites.

14-3-3 gene of Zostera japonica ZjGRF1 participates in gibberellin signaling pathway.

Zostera japonica is a unique seagrass species in Asia with important ecological value. Gibberellin (GA) is a plant hormone crucial in the regulation of plant growth and development, including seed longevity, seed germination, anti-aging, hypocotyl elongation, leaf development, reproductive organ development, and abiotic stress responses. However, the role of 14-3-3 gene of Z. japonica in GA signaling pathway remains unclear. Herein, gibberellin content and expression of GA synthesis genes were lower in Arabidopsis overexpressing ZjGRF1, 14-3-3 gene of Z. japonica, than in wild type (WT). Moreover, the expression level of GA receptors was lower in transgenic ZjGRF1 Arabidopsis than in WT. The expression level of GA response gene expansin8 (EXP8) was lower in transgenic ZjGRF1 Arabidopsis than in WT. In contrast, the expression levels of PACLOBUTRAZOL RESISTANT1 (PRE1), PRE5 and SCARECROW-LIKE 3 (SCL3) were higher in the transgenic ZjGRF1 Arabidopsis than in WT. Transgenic ZjGRF1 Arabidopsis has decreased sensitivity to paclobutrazol, an inhibitor of GA synthesis. The expression level of flowering regulatory genes was lower in transgenic ZjGRF1 Arabidopsis than in WT. This is the first study to report the function of 14-3-3 gene family in seagrass. These findings can be used in future studies on the regulation role of 14-3-3 gene on plant development.

Nutrient loading decreases blue carbon by mediating fungi activities within seagrass meadows

Coastal pollution, including nutrient loading, can negatively impact seagrass health and cover and may consequently alter soil organic carbon (SOC) accumulation and preservation. Key to understanding how eutrophication impacts SOC cycling in seagrass ecosystems is how nutrient loading changes the sources of carbon being deposited and how these changes in resources, both nutrients and carbon availability, influence soil microbiota community and activity. Currently, the direction and magnitude of nutrient loading impacts on seagrass SOC dynamics are poorly understood at a meadow scale, limiting our ability to reveal the driving mechanisms of SOC remineralisation. The purpose of this study was to assess the response of surface SOC and soil microbiomes to nutrient loading within tropical seagrass meadows. To achieve this, we quantified both total SOC and recalcitrant soil organic carbon (RSOC) concentrations and sources, in addition to the composition of bacterial and fungal communities and soil extracellular enzyme activities. We found that nutrient loading elevated SOC and RSOC content, mainly facilitated by enhanced algal growth. There was no nutrient effect on the soil prokaryotic communities, however, saprotrophic fungi groups (i.e. Trapeliales, Sordaridales, Saccharomycetales and Polyporales) and fungal activities were elevated under high nutrient conditions, including extracellular enzyme activities linked to seagrass-based cellulose and lignin decomposition. This relative increase in RSOC transformation may decrease the relative contribution of seagrass carbon to RSOC pools. Additionally, significantly different fungal communities were observed between adjacent T. hemprichii and E. acoroides areas, which coincided with elevated RSOC-decomposing enzyme activity in T. hemprichii meadows, even though the mixed seagrass meadow received allochthonous SOC and RSOC from the same sources. These results suggest that nutrient loading stimulated fungal activity and community shifts specific to the local seagrass species, thereby causing fine-scale (within-meadow) variability in SOC cycling in response to nutrient loading. This study provides evidence that fungal composition and activity, mediated by human activities (e.g. nutrient loading), can be an important influence on seagrass blue carbon accumulation and remineralisation.

Species-specific phenotypic plasticity of two tropical seagrass species in response to in situ fertilisation under different trophic conditions

Seagrasses are considered one of the most rapidly declining ecosystems in the world. One of the main reasons for this decline is eutrophication during which anthropogenic impacts, including light deprivation and nutrient enrichment often result in cumulative stress for seagrasses. This study aims to investigate the physiological and morphological responses of seagrasses to nutrient enrichment and eutrophication in Hainan, China, to understand the processes leading to seagrass losses from eutrophication. Therefore, we tested how a four-week in situ fertilisation affected traits of a multi-species seagrass community in two seasons with different trophic conditions. The species investigated in this study are Cymodocea serrulata, Cymodocea rotundata, Halodule uninervis and Thalassia Hemprichii. The dry season was characterised by high light availability and low nutrient availability. Enhanced seagrass leaf growth rates and leaf area of all species as a response to fertilisation indicate that seagrass growth is nutrient-limited in the dry season. In contrast, under eutrophic conditions in the wet season, growth and leaf area of all species were lower than in the dry season and fertilisation had no effect. Thalassia hemprichii showed signs of morphological adaption under eutrophic conditions, suggesting a higher species-level phenotypic plasticity and resilience to the stressors involved. As expressed in the fast response under eutrophic conditions, our experiment indicates synergetic adverse effects of light reduction and nutrient enrichment on seagrasses. An improved understanding of species-specific physiological and morphological response mechanisms of seagrasses to eutrophication may help to identify thresholds of exposure to stressors early on, and hence to be able to predict and possibly avoid an upcoming large-scale loss. It also has implications for aquaculture and coastal zone management.

Temperature Effects on Leaf and Epiphyte Photosynthesis, Bicarbonate Use and Diel O2 Budgets of the Seagrass Zostera marina L.

Ocean warming along with nutrient enrichment are major stressors causing global seagrass decline. While the effects of global warming on metabolic parameters in seagrasses are well described, the effect of increasing temperature on the epiphytic overgrowth of seagrass leaves and the consequences for the seagrass plant are poorly understood. Here, we investigated the effects of elevating temperature on the photosynthetic efficiency of the seagrass species Zostera marina L. and its associated epiphytes, to explore how ocean warming might affect epiphytism in seagrasses. Gas exchange and final pH measurements on bare seagrass leaves, leaves with epiphytes, and epiphytes separated from seagrass leaves were used to quantify photosynthesis and respiration rates, and the inorganic carbon extraction capacity of leaves and epiphytes as a function of photon scalar irradiance and temperature (12, 17, 22, and 27°C). Seagrass without epiphytic biofilm had a high ability to exploit the incoming irradiance regardless of the light intensity and temperature, shown as continuously high light use efficiency and maximum net photosynthesis rates. The presence of epiphytic biofilm on the seagrass leaves impaired plant photosynthesis by increasing light requirements and reducing the photosynthetic efficiency (especially at 27°C). Epiphytes showed the lowest respiration rates in darkness and had the highest oxygen surplus over diel cycles up to 22°C, whereas bare leaves had the highest diel oxygen surplus at 27°C. Both bare leaves and epiphytes lost the ability to utilize bicarbonate at 27°C, and epiphytes also did not show use of bicarbonate at 12°C. Our results indicate a competitive advantage for epiphytes in cold CO2-rich environments, whereas seagrass with bare leaves could be less affected under elevated seawater temperatures.

Thermal Performance of Seaweeds and Seagrasses Across a Regional Climate Gradient

Comparative patterns in thermal performance between populations have fundamental implications for a species thermal sensitivity to warming and extreme events. Despite this, within-species variation in thermal performance is seldom measured. Here we compare thermal performance both within-species and between-species, for two species of seagrass (Posidonia oceanica and Cymodocea nodosa) and two species of seaweed (Padina pavonica and Cystoseira compressa) across the Mediterranean Sea. Experimental populations from four locations representing between 75 and 99% of each species thermal distribution and a 6°C gradient in summer temperatures, were exposed to 10 temperature treatments between 15 and 36°C. Experimental thermal performance displayed the greatest variability between species, with optimal temperatures differing by over 10°C within the same location. Within-species differences in thermal performance were also important for P. oceanica which displayed large thermal safety margins within cool and warm-edge populations and small safety margins within central populations. Our findings suggest patterns of thermal performance in Mediterranean seagrasses and seaweeds retain deep “pre-Mediterranean” evolutionary legacies, suggesting marked differences in sensitivity to warming within and between benthic marine communities.

Seagrass Meadows under the Changing Climate: A Review of the Impacts of Climate Stressors

Seagrass meadows provide important ecological functions, particularly by serving as carbon sinks and breeding grounds for marine species. Climate change has threatened seagrass communities, causing their replacement, loss and increased vulnerability. This review examined scholarly articles published between 2010-2021 to comprehensively present the impacts of climate change on seagrass meadows. It shows that ocean warming negatively affects seagrass communities by favouring communities of lower structuring capacities, thus reducing the effectiveness of their ecological functions. Ocean warming also promotes the propagation and spread of invasive species, and changes the trophic structures leading to further loss of seagrasses of value. Higher seawater temperature is associated with shoot mortality and retarded growth of certain seagrasses. Sea level rise causes more wave energy to be received by coastal seagrass communities, thus, creating more damage to the communities. Deepening sea limits light penetration and alters distribution of seagrass meadows. Carbon dioxide enrichment of seawater increases photosynthetic rate of seagrasses but ocean warming and acidification counteract this beneficial effect. Carbon dioxide enrichment affects different seagrass species and different parts of a seagrass species differently, and, where beneficial to seagrass communities, could enhance their ecological services. Temperature extremes could kill seagrasses while marine heatwaves and flooding could act synergistically to increase carbon demand of certain seagrasses and unfavourably change their biomass. These impacts are often aggravated by anthropogenic activities. This review calls for more studies and conservation efforts to understand the impacts of climate change on seagrass communities and future-proof them against the changing climate.

The success of seagrass restoration using Enhalus acoroides seeds is correlated with substrate and hydrodynamic conditions

The extent of seagrass areas and their associated ecosystem functions and services have been declining due to many factors. Seagrass restoration is important to mitigate such declines. Seagrass restoration using seeds can be a viable method due to the high seed availability of some seagrass species and could enhance seagrass resilience to climate change stress. However, this method sometimes has low success rates due to high seed predation and seeds being washed away by wave action or substrate movement. The research was conducted to compare the settlement of Enhalus acoroides seeds and the establishment of seedlings on different sediment types (fine sand, coarse sand, and hard substrate with rubble) combined with different wave exposure levels (high and moderate). This is the first study to observe seed survival and seedling establishment of the tropical seagrass E. acoroides in the wild. On average, 64% of seeds dispersed on fine sand substrate at a moderate exposure site survived and developed into established seedlings by the end of the 40 days observation period, but the survival of seeds dispersed on coarse sand at high exposure and hard substrate at moderate exposure only remained above 50% for up to 3 days, and had declined to 2% and 1.4%, respectively, by day 40. Six years later, surviving E. acoroides sample from the coarse sand and hard substrate both had well-developed rhizomes but fewer roots than the plant from the fine sand site, these rhizome and roots characteristics were likely adaptations to increase anchoring capacity in the specific site. The results indicate that Enhalus seed settlement and seedling establishment can readily occur at sites with fine sand substrate and lower wave exposure; however, additional measures may be required at more exposed sites with mobile or hard substrates until seedlings become established.

Fish Assemblages in Subtidal Seagrass Meadows Surrounding the West Sand, South China Sea

Both beach seine (BS) and line transect-based underwater visual census (UVC) methods were applied to investigate fish assemblages of the West Sand (Xisha Islands, South China Sea) seagrass meadows and their relationship with seagrass coverage. The West Sand seagrass meadows were divided into four survey areas, two (areas A and B) to the north and two (areas C and D) to the south. The average seagrass coverages (%) in the north were significantly higher than those in the south (p < 0.05). The dominant seagrass species is Thalassia hemprichii, contributing to over 70% substrate coverage in all four areas. A total of 843 individuals from 25 fish species belonging to 22 genera, 11 families, and ten orders were recorded, with five species obtained by both BS and UVC methods. The labrids Halichoeres trimaculatus and Stethojulis strigiventer were the only two species that dominated (index of relative importance IRI > 500) in both BS and UVC methods. Fish assemblages among the four areas were significantly different (BS, p = 0.002; UVC, p = 0.002); area A was significantly different from areas B−D (p < 0.05), which was mainly driven by the variation in H. trimaculatus density. Furthermore, fish assemblages of the four areas (abundance data) revealed by both BS (r2 = 0.8455) and UVC (r2 = 0.9354) methods showed high correlation with the seagrass coverage. The majority of the individuals (>93%) recorded by BS and UVC methods were shorter than the total length at maturity (TLm), with 80.36 and 46.13% of individuals less than half the sizes of their TLm, respectively. Our results, for the first time, proved the importance of seagrass meadows at the West Sand as fish nursery grounds. The ecological roles of seagrass habitats in South China Sea merit further investigation.

Profiling the cell walls of seagrasses from A (Amphibolis) to Z (Zostera)

BackgroundThe polyphyletic group of seagrasses shows an evolutionary history from early monocotyledonous land plants to the marine environment. Seagrasses form important coastal ecosystems worldwide and large amounts of seagrass detritus washed on beaches might also be valuable bioeconomical resources. Despite this importance and potential, little is known about adaptation of these angiosperms to the marine environment and their cell walls.ResultsWe investigated polysaccharide composition of nine seagrass species from the Mediterranean, Red Sea and eastern Indian Ocean. Sequential extraction revealed a similar seagrass cell wall polysaccharide composition to terrestrial angiosperms: arabinogalactans, pectins and different hemicelluloses, especially xylans and/or xyloglucans. However, the pectic fractions were characterized by the monosaccharide apiose, suggesting unusual apiogalacturonans are a common feature of seagrass cell walls. Detailed analyses of four representative species identified differences between organs and species in their constituent monosaccharide composition and lignin content and structure. Rhizomes were richer in glucosyl units compared to leaves and roots. Enhalus had high apiosyl and arabinosyl abundance, while two Australian species of Amphibolis and Posidonia, were characterized by high amounts of xylosyl residues. Interestingly, the latter two species contained appreciable amounts of lignin, especially in roots and rhizomes whereas Zostera and Enhalus were lignin-free. Lignin structure in Amphibolis was characterized by a higher syringyl content compared to that of Posidonia.ConclusionsOur investigations give a first comprehensive overview on cell wall composition across seagrass families, which will help understanding adaptation to a marine environment in the evolutionary context and evaluating the potential of seagrass in biorefinery incentives.

Seagrass restoration trials in tropical seagrass meadows of Kenya

The degradation of seagrasses is becoming prevalent in the Western Indian Ocean (WIO) region due to anchor damage, sea urchin herbivory, extreme events such as cyclones and floods and anthropogenic factors such as pollution and sediment inflows. Consequently, there have been numerous efforts to advance the restoration of degraded seagrass beds in several countries in the region. In Kenya, experimental restoration efforts were started in 2007 in response to seagrass habitat degradation due to sea urchin herbivory. Although the initial efforts experienced challenges, there were lessons learned which provided insights into subsequent restoration work using different techniques. In this paper, insights are provided into three types of restoration techniques; the sod technique, the seagrass mimic technique, and the Hessian bag technique. In the case of the sod technique, Thalassodendron ciliatum showed a decline from 20 ± 1.7 shoots sod-1 in the first three weeks to 7 ± 4.4 shoots sod-1 at the end of the experimental period of the study, while Thalassia hemprichii sods showed an increase from 28 ± 3.4 shoots sod-1 to 32 ± 2.7 shoots sod-1 over the same period. For the Hessian bag method, the expectation was that the pilot site would be filled with the transplanted seagrass species, Thalassia hemprichii, but the findings showed that different species including Halodule uninervis, Syringodium isoetifolium, Halophila stipulacea, Cymodocea rotundata, and Cymodocea serrulata colonized the area. This indicated that it was not possible to restore the area to its original status, but that the area could be rehabilitated. The costs of restoration have also been assessed as well as community participation in such initiatives. These findings provide insights for restoration efforts in Kenya and provide a baseline for future work.

Drone-Based Characterization of Seagrass Habitats in the Tropical Waters of Zanzibar

Unmanned automatic systems (UAS) are increasingly being applied as an alternative to more costly time-consuming traditional methods for mapping and monitoring marine shallow-water ecosystems. Here, we demonstrate the utility of combining aerial drones with in situ imagery to characterize the habitat conditions of nine shallow-water seagrass-dominated areas on Unguja Island, Zanzibar. We applied object-based image analysis and a maximum likelihood algorithm on the drone images to derive habitat cover maps and important seagrass habitat parameters: the habitat composition; the seagrass species; the horizontal- and depth-percent covers, and the seascape fragmentation. We mapped nine sites covering 724 ha, categorized into seagrasses (55%), bare sediment (31%), corals (9%), and macroalgae (5%). An average of six seagrass species were found, and 20% of the nine sites were categorized as “dense cover” (40–70%). We achieved high map accuracy for the habitat types (87%), seagrass (80%), and seagrass species (76%). In all nine sites, we observed clear decreases in the seagrass covers with depths ranging from 30% at 1–2 m, to 1.6% at a 4–5 m depth. The depth dependency varied significantly among the seagrass species. Areas associated with low seagrass cover also had a more fragmented distribution pattern, with scattered seagrass populations. The seagrass cover was correlated negatively (r2 = 0.9, p < 0.01) with sea urchins. A multivariate analysis of the similarity (ANOSIM) of the biotic features, derived from the drone and in situ data, suggested that the nine sites could be organized into three significantly different coastal habitat types. This study demonstrates the high robustness of drones for characterizing complex seagrass habitat conditions in tropical waters. We recommend adopting drones, combined with in situ photos, for establishing a suite of important data relevant for marine ecosystem monitoring in the Western Indian Ocean (WIO).