Contrasting root length, nutrient content and carbon sequestration of seagrass growing in offshore carbonate and onshore terrigenous sediments in the South China Sea

Utilities of environmental radioactivity tracers in assessing sequestration potential of carbon in the coastal wetland ecosystems

Demand for accurate estimation of coastal blue carbon sequestration rates in a regular interval has recently surged due to the increasing awareness of nature-based climate solutions to alleviate adverse impacts stemming from the recent global warming. The robust estimation method is, however, far from well-established. The international community requires, moreover, to quantify its effect of "management." This article tries to provide the environmental isotope community with basic biophysical features of coastal blue carbon ecosystems to identify a suitable set of environmental isotopes for promoting coastal ocean-based climate solutions. This article reviews (i) the primary biophysical characteristics of coastal blue carbon ecosystems and hydrology, (ii) their consequential impact on the accumulation and preservation of organic carbon (OC) in the sediment column, (iii) suitable environmental isotopes to quantifying the sedimentary organic carbon accumulation, outwelling of the carbon-containing byproducts of decomposition of biogenic organic matter and acid neutralizing alkalinity produced in situ sediment to the offshore. Above-ground biomass is not cumulative over the years except for mangrove forests within coastal blue carbon systems. Non-gaseous carbon sequestration and loss occur mainly as a form of sediment organic carbon (SOC) and dissolved carbon in an intertidal and subtidal bottom sediment body in a slow, patchy, and dispersive way, on which this article focuses. Investigating environmental radionuclides is probably the most cost-effective effort to contribute to defining the offshore spatial extent of coastal blue carbon systems except for seagrass beds (e.g., Ra isotopes), to quantify millimeter per year scale carbon accretion and loss within the systems (e.g., 7Be, 210Pb) and a liter per meter of coastline per a day scale water movement from the systems (Ra isotopes). A millimeter-scale spatial and an annual (or less) time-scale resolution offered by the use of environmental isotopes would equip us with a novel tool to enhance the carbon storage capacity of the coastal blue carbon system.

Effects of herbivore on seagrass, epiphyte and sediment carbon sequestration in tropical seagrass bed

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.

Two tropical seagrass species show differing indicators of resistance to a marine heatwave

Marine heatwaves (MHWs) are a growing threat to marine species globally, including economically and ecologically important foundation species, such as seagrasses. Seagrasses in tropical regions may already be near their thermal maxima, and, therefore, particularly susceptible to increases in temperature, such as from MHWs. Here, we conducted a 10-day MHW experiment (control +4°C) to determine the effects of such events on the two tropical seagrasses Halophila beccarii and Halophila ovalis. We found that both species were largely resistant to the MHW, however, there were differences between the species' responses. For H. beccarii, the surface area of existing leaves was smaller under MHW conditions, yet a substantial increase in the number of new leaves under the MHW indicated its tolerance to-or even increased performance under-the MHW. While there was no direct effect of the MHW on H. ovalis, this species saw less epiphyte biomass and percentage cover on its leaves under the MHW. While a lower epiphyte cover can potentially increase the health and ecophysiological performance of the seagrass, the change of epiphytes can lead to bottom-up trophic implications via the influence on mesograzer feeding. Together, the results of this study demonstrate the species-specific responses of seagrasses of the same genus to a warming event. With the current global decline of seagrasses, our results are encouraging for these important habitat formers as we show that anomalous warming events may not necessarily lead to ecosystem collapse.

A multi-approach inventory of the blue carbon stocks of Posidonia oceanica seagrass meadows: Large scale application in Calvi Bay (Corsica, NW Mediterranean)

In Mediterranean, Posidonia oceanica develops a belowground complex structure ('matte') able to store large amounts of carbon over thousands of years. The inventory of blue carbon stocks requires the coupling of mapping techniques and in situ sediment sampling to assess the size and the variability of these stocks. This study aims to quantify the organic (C_org) and inorganic (C_inorg) carbon stocks in the P. oceanica matte of the Calvi Bay (Corsica) using sub-bottom profiler imagery and biogeochemical analysis of sediment cores. The matte thicknesses map (average ± SD: 2.2 m ± 0.4 m) coupled with marine benthic habitat cartography allows to estimate matte volume at 12 473 352 m³. The cumulative stocks were assessed at 20.2-50.3 kg C_org m^-2 and 26.6-58.7 kg C_inorg m^-2 within the first meter of depth on matte (3632 ± 486 cal yr BP). The data contributed to estimate the overall carbon stocks at 389 994 t C_org and 615 558 t C_inorg, offering a new insight of the heterogeneity of blue carbon stocks in seagrass meadows. Variability of carbon storage capacity of matte influenced by substrate is discussed.

Eutrophication reduced the release of dissolved organic carbon from tropical seagrass roots through exudation and decomposition

Seagrass bed ecosystem is one of the most effective carbon capture and storage systems on earth. Seagrass roots are the key link of carbon flow between leaf-root-sediment, and the release of dissolved organic carbon (DOC) from seagrass roots through exudation and decomposition are vital sources to the sediment organic carbon (SOC) in the seagrass beds. Unfortunately, human-induced eutrophication may change the release process of DOC from seagrass roots, thereby affecting the sediment carbon storage capacity. However, little is known about the effect of nutrient enrichment on the release of DOC from seagrass roots, hindering the development of seagrass underground ecology. Therefore, we selected Thalassia hemprichii, the tropical dominant seagrass species, as the research object, and made a comparison of the release of DOC from roots through exudation and decomposition under different nitrate treatments. We found that under control, 10 μmol L^-1, 20 μmol L^-1 and 40 μmol L^-1 nitrate treatments, soluble sugar of T. hemprichii roots were 71.37 ± 3.43 mg g^-1, 67.03 ± 5.33 mg g^-1, 49.14 ± 3.48 mg g^-1, and 18.51 ± 2.09 mg g^-1, respectively, while the corresponding root DOC exudation rates were 7.00 ± 0.97 mg g DW root^-1 h^-1, 5.11 ± 0.42 mg g DW root^-1 h^-1, 4.08 ± 0.23 mg g DW root^-1 h^-1, and 3.78 ± 0.74 mg g DW root^-1 h^-1, respectively. There was a significant positive correlation between root soluble sugar and DOC exudation rate. DOC concentration of sediment porewater and SOC content also decreased under nitrate enrichment (though not significantly), which were both significantly positively correlated with the rate of root exuded DOC. Meanwhile, nitrate enrichment also reduced the release rate of DOC from seagrass roots during initial decomposition, and the release flux of DOC from decomposition. Therefore, nutrient enrichment could decrease nonstructural carbohydrates of seagrass roots, reducing the rate of root exuded DOC, thereby lowered SOC, as well as the DOC release from seagrass root decomposition. In order to increase the release of DOC from seagrass roots and improve the carbon sequestration capacity of seagrass beds, effective measures should be taken to control the coastal nutrients input into seagrass beds.

Sand supplementation favors tropical seagrass Thalassia hemprichii in eutrophic bay: implications for seagrass restoration and management

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.

Interspecific differences in root exudation for three tropical seagrasses and sediment pore-water dissolved organic carbon beneath them

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.

Selection of parameters for seagrass management: Towards the development of integrated indicators for French Antilles

Seagrass beds are increasingly impacted by human activities in coastal areas, particularly in tropical regions. The objective of this research program was to study seagrass beds characteristics under various environmental conditions in the French Antilles (FA, Caribbean Sea). A total of 61 parameters, from plant physiology to seagrass ecosystem, were tested along a gradient of anthropogenic conditions, distributed across 11 sites and 3 islands of the FA. A selection of 7 parameters was identified as relevant for the monitoring of seagrass meadows in the framework of public policies. They combined "early warning indicators" (e.g. nutrients and some trace metals) and long-term responding parameters (e.g. shoot density) adapted to management time scales. The ecological status of seagrass meadows was evaluated using a PCA. This work is a first step towards monitoring and management of seagrass meadows in the FA.

Seagrass Cymodocea nodosa across biogeographical regions and times: Differences in abundance, meadow structure and sexual reproduction

Seagrasses are key habitat-forming species of coastal areas. While previous research has demonstrated considerable small-scale variation in seagrass abundance and structure, studies teasing apart local from large-scale variation are scarce. We determined how different biogeographic scenarios, under varying environmental and genetic variation, explained variation in the abundance and structure (morphology and biomass allocation), epiphytes and sexual reproduction intensity of the seagrass Cymodocea nodosa. Regional and local-scale variation, including their temporal variability, contributed to differentially explain variation in seagrass attributes. Structural, in particular morphological, attributes of the seagrass leaf canopy, most evidenced regional seasonal variation. Allocation to belowground tissues was, however, mainly driven by local-scale variation. High seed densities were observed in meadows of large genetic diversity, indicative of sexual success, which likely resulted from the different evolutionary histories undergone by the seagrass at each region. Our results highlight that phenotypic plasticity to local and regional environments need to be considered to better manage and preserve seagrass meadows.

Coastal and estuarine blue carbon stocks in the greater Southeast Asia region: Seagrasses and mangroves per nation and sum of total

Climate Change solutions include CO₂ extraction from atmosphere and water with burial by living habitats in sediment/soil. Nowhere on the planet are blue carbon plants which carry out massive carbon extraction and permanent burial more intensely concentrated than in SE Asia. For the first time we make a national and total inventory of data to date for "blue carbon" buried from mangroves and seagrass and delineate the constraints. For an area across Southeast Asia of approximately 12,000,000 km², supporting mangrove forests (5,116,032 ha) and seagrass meadows (6,744,529 ha), we analyzed the region's current blue carbon stocks. This estimate was achieved by integrating the sum of estuarine in situ carbon stock measurements with the extent of mangroves and seagrass across each nation, then summed for the region. We found that mangroves ecosystems regionally supported the greater amount of organic carbon (3095.19Tg C_org in 1st meter) over that of seagrass (1683.97 Tg C_org in 1st meter), with corresponding stock densities ranging from 15 to 2205 Mg ha^-1 and 31.3 to 2450 Mg ha^-1 respectively, a likely underestimate for entire carbon including sediment depths. The largest carbon stocks are found within Indonesia, followed by the Philippines, Papua New Guinea, Myanmar, Malaysia, Thailand, Tropical China, Viet-Nam, and Cambodia. Compared to the blue carbon hotspot of tropical/subtropical Gulf of Mexico's total carbon stock (480.48 Tg Corg), Southeast Asia's greater mangrove-seagrass stock density appears a more intense Blue Carbon hotspot (4778.66 Tg Corg). All regional Southeast Asian nation states should assist in superior preservation and habitat restoration plus similar measures in the USA & Mexico for the Gulf of Mexico, as apparently these form two of the largest tropical carbon sinks within coastal waters. We hypothesize it is SE Asia's regionally unique oceanic-geologic conditions, placed squarely within the tropics, which are largely responsible for this blue carbon hotspot, that is, consistently high ambient light levels and year-long warm temperatures, together with consistently strong inflow of dissolved carbon dioxide and upwelling of nutrients across the shallow geological plates.

The end of resilience: Surpassed nitrogen thresholds in coastal waters led to severe seagrass loss after decades of exposure to aquaculture effluents

Although eutrophication is considered a major driver for global seagrass loss with aquaculture effluents being a main factor, little is known about the effect on seagrass meadows in eastern Asia and their resilience to long-term nutrient impact. Seagrass meadows impacted by land-based aquaculture since the 1990s, were visited in 2008/2009 and revisited after another 9 years of effluent exposure. During that period seagrass aboveground biomass declined by 87%. Species diversity decreased with increasing effluent exposure. A δ¹⁵N of 9.0‰ of seagrass leaves and additional biogeochemical and biological indicators identify pond effluents as the driver of the observed eutrophication. When continuously exposed to dissolved inorganic nitrogen (DIN) concentrations exceeding a calculated threshold of 8 μM DIN seagrass meadows will disappear. Chronic nutrient pollution from aquaculture effluents can lead to a reduction of biodiversity and ultimately to a complete loss of seagrasses along the aquaculture-dominated coasts in E and SE Asia.