James Warren Fourqurean

Crucial for coastal carbon sequestration, global seagrass organic carbon (OC) burial rates remain uncertain as early estimates were derived from mixed methodologies across geographically and taxonomically biased datasets. Here, we present a data-driven reassessment from a synthesis of 332 sediment cores spanning the last century. Results show that seagrasses bury OC at 25 ± 3 g OC m ⁻² yr ⁻¹ (geometric mean), with an area-weighted global average of 33 ± 9 g OC m ⁻² yr ⁻¹ . Rates reflect a balance between sediment OC content and sediment accumulation, which share a negative exponential relationship driven by mineral dilution. Based on a revised mapped seagrass extent (247,700–366,400 km ² ), global burial scales to 6–15 Tg C yr ⁻¹ . Although one-fourth of earlier benchmarks, seagrasses account for 3–4% to 3–6% of total oceanic OC burial despite covering <0.1% of the seafloor. Combined with mangroves and tidal marshes, vegetated coastal ecosystems contribute 8–13% of oceanic OC burial.

The active removal of carbon dioxide (CO2) from the atmosphere may be the only viable means of mitigating the worst effects of global warming. Known as carbon dioxide removal (CDR), these approaches involve a variety of techniques to sequester atmospheric CO2. One technique is marine enhanced rock weathering (mERW), which deploys natural silicate minerals (olivine) along the coastal zones, that dissolve, generate alkalinity and induce atmospheric CO2 drawdown upon air-sea equilibration. While the geochemistry of olivine dissolution is well known, the effects on foundation species are relatively unexplored. Here, we established a 16-week ecotoxicology experiment that exposed two tropical seagrasses (Thalassia testudinum and Syringodium filiforme) to one of three treatments: (1) surficial dose of fine-grained olivine; (2) surficial dose of clean aragonite sand; (3) control - no dose. During treatment exposure, seagrass structural and physiological characteristics were measured. Olivine or sand dosing had no effect on shoot density, leaf morphometrics, or photosynthetic yield. Leaf productivity, belowground biomass and carbon / nitrogen content were also unaffected. However, final aboveground biomass was lower in the olivine treatment relative to sand, but only for T. testudinum. Tissue trace metal concentrations (nickel and manganese) were elevated with olivine dosing for both species, while other metals showed mixed results. Neither olivine nor sand dosing influenced air-water CO2 flux. These results suggest that while olivine dosing had minimal effects on tropical seagrasses, elevated trace metals pose a concern, thus it may be necessary to regulate olivine dosing to avoid potential impacts on higher trophic levels.

Seagrass ecosystems are pivotal contributors to coastal carbon sequestration through the long-term burial of organic carbon (OC) in sediments. Yet global burial estimates remain uncertain, with early values of 138 ± 38 g OC m-2 yr-1 derived from a limited dataset biased toward highly depositional communities and indirect, production-based approaches. We call for a downward revision, supported by a data-driven assessment based on a global synthesis of 326 dated sediment cores integrating OC burial over the last century. We find that seagrass meadows bury OC at a geometric mean rate of 26 ± 2 g OC m-2 yr-1, with an area-weighted global average of 33 ± 10 g OC m-2 yr-1 accounting for bioregional differences in seagrass distribution. Globally, these rates scale to 6–16 Tg C yr-1, based on current mapped seagrass extent (247,800–366,200 km2), and reveal that ~15% of seagrass net community production is retained and buried locally. Although our estimate is roughly one-fourth of earlier values, seagrass sediments still account for 3–6% of total oceanic OC burial, despite occupying <0.1% of the seafloor. Combined with mangroves and tidal marshes, OC burial in vegetated coastal sediments represents an estimated 8–13% of total oceanic OC burial.

Seagrass ecosystems are recognized for their capacity to sequester and store organic carbon, but there is large variability in soil organic carbon stocks associated with plant traits and environmental conditions, making the quantification and scaling of carbon storage and fluxes needed to contribute to climate change mitigation highly challenging. Here, we provide estimates of carbon stocks associated with seagrass systems (biomass and soil) through analyses of a comprehensive global database including 2700+ seagrass soil cores. The median global soil C_org stock estimate is 24.2 (12.4 – 44.9) Mg C_org ha⁻¹ in the top 30 cm of soil, 27% lower than estimates from previous global syntheses, refining the IPCC Tier 1 soil C_org stock currently used for carbon accounting in places without local data. We estimate that seagrass carbon stocks at risk of degradation could emit 1,154 Tg (665 – 1699) CO₂ with a social cost of $213 billion (2020 US dollars), if no action is taken to conserve these habitats.

Seagrass ecosystems are recognized as important carbon sinks that accumulate organic carbon (C_org) in their biomass and underlying soils. In Bermuda, seagrass meadows have declined precipitously over the last two decades, potentially leading to loss of those C_org stocks. This study found 45% lower C_org stocks in Bermuda (ca. 32.3°N) compared to lower-latitude tropical Atlantic seagrass meadows. We also report 2–3 orders of magnitude lower long-term carbon burial rates (0.10 Mg C_orgha⁻¹y^− 1 over the last 1830 years) compared to global estimates, determined via ¹⁴C dating of mangrove peats buried 1m below the modern sediment-water interface. Historical data from 2008, relatively early in the decline of Bermuda’s seagrasses, showed that neither seagrass abundance nor life history strategy (persistent vs. ephemeral) were associated with C_org content of surficial sediments or the relative contribution of seagrass-derived organic matter to the soil C_org. Following the collapse of most of Bermuda’s seagrasses, C_org content of surficial sediments declined by 15% over the period 2008–2024, although this decline was not a function of seagrass loss at the site scale. This suggests that seagrass-derived C_org is deposited across the Bermuda Platform, regardless of seagrass presence or abundance, and that the regional loss in seagrasses has led to a regional-scale decline in C_org content of the surficial soils. We conclude that seagrass loss can have landscape-scale effects on sediment C_org stocks even without site-specific relationships, and that caution should be exercised when applying regional or global literature values for average C_org stocks to estimate carbon market valuations.

The particulate organic matter buried in carbonate-rich seagrass ecosystems is an important blue carbon reservoir. While carbonate sediments are affected by alkalinity produced or consumed in seagrass-mediated biogeochemical processes, little is known about the corresponding impact on organic matter. A portion of particulate organic matter is carbonate-associated organic matter. Here, we explore its biogeochemistry in a carbonate seagrass meadow in central Florida Bay, USA. We couple inorganic stable isotope analyses (δ³⁴S, δ¹⁸O) with a molecular characterization of dissolved and carbonate associated organic matter (21 tesla Fourier-transform ion cyclotron resonance mass spectrometry). We find that carbonate-associated molecular formulas are highly sulfurized compared to surface water dissolved organic matter, with multiple sulfurization pathways at play. Furthermore, 97% of the formula abundance of surface water dissolved organic matter is shared with carbonate-associated organic matter, indicating connectivity between these two pools. We estimate that 9.2% of the particulate organic matter is carbonate-associated, and readily exchangeable with the broader aquatic system as the sediment dissolves and reprecipitates.

Climate change is altering the functioning of foundational ecosystems. While the direct effects of warming are expected to influence individual species, the indirect effects of warming on species interactions remain poorly understood. In marine systems, as tropical herbivores undergo poleward range expansion, they may change food web structure and alter the functioning of key habitats. While this process (‘tropicalization’) has been documented within declining kelp forests, we have a limited understanding of how this process might unfold across other systems. Here we use a network of sites spanning 23° of latitude to explore the effects of increased herbivory (simulated via leaf clipping) on the structure of a foundational marine plant (turtlegrass). By working across its geographic range, we also show how gradients in light, temperature and nutrients modified plant responses. We found that turtlegrass near its northern boundary was increasingly affected (reduced productivity) by herbivory and that this response was driven by latitudinal gradients in light (low insolation at high latitudes). By contrast, low-latitude meadows tolerated herbivory due to high insolation which enhanced plant carbohydrates. We show that as herbivores undergo range expansion, turtlegrass meadows at their northern limit display reduced resilience and may be under threat of ecological collapse.