Seagrass meadow carbon sequestration capacity and blue carbon storage estimates

Seagrass ecosystems rank among the most efficient natural carbon sinks on the planet, yet they remain poorly quantified in many tropical coastal regions. This research measured above-ground biomass, sediment organic carbon content, and carbon sequestration rates in seagrass meadows along the Egyptian Red Sea coast near Hurghada. Fieldwork ran from April 2021 to September 2022 across three dominant species zones: Halophila stipulacea, Thalassodendron ciliatum, and Cymodocea serrulata. Sediment cores (0-30 cm depth) and biomass quadrats were sampled at 45 stations distributed along 12 km of coastline. H. stipulacea meadows stored the highest sediment carbon, averaging 247.3 ± 38.6 g C m⁻² in the upper 15 cm, while C. serrulata meadows held the least at 89.4 ± 21.7 g C m⁻². Carbon stocks declined by 28-41% between the shallow (0-15 cm) and deeper (15-30 cm) sediment layers across all species. Above-ground biomass correlated positively with sediment carbon stocks (r = 0.72 for H. stipulacea, r = 0.61 for T. ciliatum). Estimated annual sequestration capacity for the surveyed area reached 1, 340 tonnes CO₂-equivalent, comparable to published values from Indo-Pacific meadows. These data provide the first systematic blue carbon inventory for the Egyptian Red Sea coastline and support the inclusion of seagrass conservation in regional climate mitigation strategies.

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SOC 0-15 cm (g C m⁻²): H. stipulacea vs T. ciliatum vs C. serrulataClose ×

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Introduction

Seagrass meadows capture and store carbon at rates up to 35 times faster than tropical rainforests on a per-area basis a fact that has reshaped how coastal ecosystems are valued in climate policy discussions [1] . These submerged flowering plants occupy less than 0.2% of the ocean floor but are responsible for an estimated 10-18% of total oceanic carbon burial [2] . The carbon they sequester ends up locked in dense, anoxic sediments where decomposition proceeds slowly, potentially preserving organic matter for centuries to millennia [3] . The concept of "blue carbon" carbon stored in coastal and marine ecosystems including seagrasses, mangroves, and salt marshes gained formal scientific and policy recognition over the past decade [4] . International frameworks such as the UNFCCC now acknowledge blue carbon habitats as eligible for inclusion in nationally determined contributions. But this recognition depends on credible, site-specific data, and for many tropical coastlines that data simply doesn't exist. The Red Sea, despite hosting extensive seagrass beds along its Egyptian, Saudi, and Sudanese shores, has been largely overlooked in blue carbon assessments [5] . The Egyptian Red Sea coast supports at least seven seagrass species, with H. stipulacea, T. ciliatum, and C. serrulata forming the dominant meadows between Hurghada and Marsa Alam [6] . These meadows face mounting pressure from coastal development, boat anchoring, and sedimentation linked to tourism infrastructure. Without baseline carbon storage data, the ecological and climatic cost of meadow loss can't be estimated and conservation arguments lack the quantitative backing they need. This research aimed to fill that gap by measuring above-ground biomass, sediment carbon stocks to 30 cm depth, and annual sequestration rates across three seagrass species zones along the Hurghada coast. The goal was to produce the first site-level blue carbon inventory for this section of the Red Sea [7] . [8] .

Material and Methods Material

This research was carried out at the Department of Coastal Biology, Red Sea University of Marine Biology, Hurghada, Egypt, between April 2021 and September 2022. Sampling stations (n = 45) were distributed across the three dominant seagrass species zones: 15 stations in H. stipulacea beds, 15 in T. ciliatum beds, and 15 in C. serrulata beds. Station locations were selected using a stratified random design based on preliminary habitat mapping from satellite imagery (Sentinel-2, 10 m resolution). At each station, above-ground biomass was harvested from a 25 × 25 cm quadrat, rinsed to remove sediment and epiphytes, oven-dried at 60°C for 72 hours, and weighed. Below-ground biomass was not included to avoid destructive sampling in protected areas.

Methods

Sediment cores were collected using PVC push-corers (inner diameter 5 cm, length 35 cm) driven to 30 cm depth. Each core was sectioned into two layers: 0-15 cm and 15-30 cm. Sections were dried at 60°C, ground, and passed through a 2 mm sieve. Sediment Organic Carbon (SOC) was measured by the Walkley-Black wet oxidation method with a correction factor of 1.32, and validated against Loss-on-Ignition (LOI) at 450°C for 4 hours on a 20% subsample [9] . Carbon stocks were calculated by multiplying SOC concentration by dry bulk density and layer thickness. Dry bulk density was determined from the dry mass of a known sediment volume. Annual carbon sequestration rates were estimated using published sedimentation rates for Red Sea seagrass beds (1.8-3.2 mm yr⁻¹) combined with measured SOC concentrations in the surface sediment layer [10] . CO₂equivalent values used a conversion factor of 3.67

Comprehensive interpretation

The combined data paint a consistent picture: seagrass meadows along the Hurghada coast function as meaningful carbon sinks, with H. stipulacea emerging as the most productive species both in terms of biomass and sediment carbon accumulation. The strong positive correlation between biomass and carbon stocks suggests that aboveground productivity serves as a reasonable proxy for belowground carbon storage in these meadows a relationship that could simplify future monitoring using remote sensing of canopy cover.

Carbon and greenhouse gas accounting

Total sediment organic carbon stored in the upper 30 cm across the 368.4 ha surveyed area was estimated at 9, 640 tonnes C (35, 380 tonnes CO₂-equivalent). If these meadows were lost entirely through coastal development or physical disturbance the resulting carbon release would be equivalent to the annual emissions of approximately 7, 600 passenger vehicles (using a global average of 4.6 tonnes CO₂ per vehicle per year). The annual sequestration rate of 798.2 tonnes CO₂-equivalent represents continued climate mitigation value that would cease upon meadow destruction. These figures, while modest in global carbon accounting terms, are significant at the regional scale and provide a quantitative basis for arguing that seagrass conservation in the Egyptian Red Sea should be formally incorporated into national climate commitments under the Paris Agreement framework [11] .

Discussion

The sediment carbon stocks measured here averaging 399.7 g C m⁻² in H. stipulacea zones sit within the range reported for Indo-Pacific seagrass meadows (typically 100-800 g C m⁻² in the top 25-30 cm) but are lower than values from Mediterranean Posidonia oceanica beds, which can exceed 1, 000 g C m⁻² [12] . This is expected given the smaller stature and shorter root systems of Red Sea species compared to P. oceanica. The depth-related carbon decline of 28-41% between the upper and lower sediment layers is consistent with organic matter degradation during burial, though the relatively high retention at depth suggests that the anoxic conditions common in seagrass sediments limit decomposition effectively [3] .

The per-area sequestration rate of 83.6 g C m⁻² yr⁻¹ for H. stipulacea exceeds many published estimates for this species elsewhere in the Red Sea and Mediterranean. This may partly reflect the high productivity of the Hurghada coastline, where nutrient inputs from nearby reef systems could fertilize adjacent seagrass beds [13] . However, the use of literature-derived sedimentation rates introduces uncertainty direct measurement of sediment accumulation using ²¹⁰Pb dating would strengthen future estimates. The finding that above-ground biomass correlates well with carbon stocks offers practical value: remote sensing approaches that quantify canopy coverage could potentially be calibrated to estimate carbon stores without extensive coring campaigns [14] . But this relationship may not hold across all regions, and local calibration would be needed wherever it's applied.

Conclusion

This research provides the first systematic blue carbon inventory for seagrass meadows along the Egyptian Red Sea coast near Hurghada. H. stipulacea meadows emerged as the dominant carbon reservoir, storing roughly 2.8 times the sediment carbon of C. serrulata beds in the top 30 cm. The 368.4 hectares of surveyed seagrass sequester an estimated 798 tonnes CO₂-equivalent per year and hold over 35, 000 tonnes CO₂-equivalent in their sediments.

These numbers, modest by global standards, carry real weight at the national level particularly as Egypt works to define and expand its climate commitments. Losing these meadows to coastal development wouldn't just eliminate a carbon sink; it would release centuries of accumulated sediment carbon back into the water column and atmosphere. The data from this research support formal inclusion of Red Sea seagrass habitats in Egypt's blue carbon inventory and argue for stronger enforcement of existing marine protected area regulations along the Hurghada coast. Future work should extend the sampling to deeper sediment layers (>30 cm) and include direct sedimentation rate measurements to refine sequestration estimates.

Figures and tables from the paper

Species zone	Biomass (g DW m⁻²)	SOC 0-15 cm (g C m⁻²)	SOC 15-30 cm (g C m⁻²)	Total 0-30 cm (g C m⁻²)
H. stipulacea	412.8 ± 87.3	247.3 ± 38.6	152.4 ± 29.1	399.7 ± 61.4
T. ciliatum	278.4 ± 63.7	168.9 ± 31.2	114.7 ± 22.8	283.6 ± 48.3
C. serrulata	163.2 ± 41.6	89.4 ± 21.7	52.8 ± 14.3	142.2 ± 33.7
ANOVA F (p)	F=47.3 (<0.001)	F=62.1 (<0.001)	F=38.9 (<0.001)	F=54.7 (<0.001)

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Sediment carbon stocks and above-ground biomass by seagrass species zone (mean ± SD)

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Species zone	Area (ha)	Seq. Rate (g C m⁻² yr⁻¹)	Annual (t C yr⁻¹)	CO₂-eq (t yr⁻¹)
H. stipulacea	127.4	83.6 ± 14.2	106.5	390.8
T. ciliatum	98.7	62.4 ± 11.8	61.6	226.1
C. serrulata	142.3	34.7 ± 8.9	49.4	181.3
Total surveyed	368.4	—	217.5	798.2

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Estimated annual carbon sequestration rates and CO₂-equivalent storage by species zone

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Relationship between above-ground seagrass biomass and sediment carbon stock (0-15 cm layer) across the three species zones.

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Box plots of sediment organic carbon concentration (%) at two depth intervals (0-15 cm and 15-30 cm) across the three seagrass

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