Mapping and assessing seagrass meadows changes and blue carbon under past, current, and future scenarios

Carbon dynamics under loss and restoration scenarios in the world's largest seagrass meadow

Seagrass sediments accumulate high amounts of organic carbon, but they are threatened by human activities and their global extent continues to shrink. Simultaneously, there is interest in including seagrass carbon accumulation in countries' Nationally Determined Contributions (NDCs). We used the InVEST Coastal Blue Carbon Model to estimate sediment organic carbon (SOC) accumulation over 100 years in seagrass of the Bahama Banks, the world's largest seagrass meadow. Using seagrass maps and sediment core measurements, we modeled SOC accumulation in two scenarios: (1) 1% seagrass area loss per year, the Business As Usual scenario (BAU); (2) restoration of seagrass extent to that of 30 years prior by 2120, meeting the goals of the Kunming-Montreal global biodiversity framework. With a conservative initial seagrass extent, by 2120, the SOC accumulation was 90.6 Mt CO2 eq (24.0 autochthonous Mt CO2 eq) in the BAU and 703.7 Mt CO2 eq (186.5 autochthonous Mt CO2 eq) in the restoration scenario, and average additional SOC accumulation was 611.0 Mt CO2 eq (161.9 autochthonous Mt CO2 eq). Using a high estimate of initial seagrass extent, by 2120, the net SOC accumulation was 155.4 Mt CO2 eq (41.2 autochthonous Mt CO2 eq) in the BAU and 1058.2 Mt CO2 eq (280.4 autochthonous Mt CO2 eq) in the restoration scenario, and additional SOC accumulation was 902.8 Mt CO2 eq (239.2 autochthonous Mt CO2 eq). The potential for either SOC accumulation or losses to occur if seagrass extent continues to decline highlights uncertainty around whether Bahamian seagrass meadows will remain a net carbon sink. The additional accumulation of autochthonous carbon if seagrasses were restored was comparable in scale to the annual greenhouse gas emissions of The Bahamas, suggesting potential for seagrass restoration to contribute to the country's NDCs and broader climate mitigation strategies.

Blue carbon ecosystems for hypoxia solution: how to maximize their carbon sequestration potential

Blue carbon refers to the carbon captured and stored by coastal and oceanic ecosystems, such as mangroves, seagrasses, and salt marshes. These ecosystems are vital for biodiversity and play a crucial role in sequestering carbon dioxide from the atmosphere, helping to mitigate climate change, which can also provide economic value by evaluating payment for ecosystem services (PES) schemes. Additionally, they help regulate dissolved organic carbon, mitigate eutrophication, and improve water quality, reducing the impact of global deoxygenation. Conserving and restoring blue carbon ecosystems are vital for mitigating hypoxia, enhancing biodiversity, and supporting various ecosystem services. Moreover, genomic research on blue carbon plants and microbes reveals adaptive traits that enhance resilience to hypoxia and environmental stress. Integrating conservation, restoration, and molecular approaches will maximize their carbon sequestration potential, ensuring ecological stability and climate adaptation. This review aims to provide an overview of blue carbon and its significance, particularly in addressing hypoxia, highlighting the critical need for investigating hypoxia responses and microbial interactions to fully understand the mechanisms of carbon sequestration and hypoxia mitigation.

Ocean acidification and global warming may favor blue carbon service in a Cymodocea nodosa community by modifying carbon metabolism and dissolved organic carbon fluxes

Ocean acidification (OA) and global warming (GW) drive a variety of responses in seagrasses that may modify their carbon metabolism, including the dissolved organic carbon (DOC) fluxes and the organic carbon stocks in upper sediments. In a 45-day full-factorial mesocosm experiment simulating forecasted CO2 and temperature increase in a Cymodocea nodosa community, we found that net community production (NCP) was higher under OA conditions, particularly when combined with warming (i.e., synergistic effect). Moreover, under OA conditions, an increase in aboveground biomass and photosynthetic shoot area was recorded. Interestingly, DOC fluxes were reduced when exposed to OA; however, an increase occurred when both factors acted together (i.e., antagonistic effect), which was attributable to increased DOC release by plants. Our results suggest that C. nodosa populations in temperate latitude may favor blue carbon service in future scenarios of OA and GW by increasing the NCP, the DOC export with lower labile:recalcitrant ratio, and accumulating more organic carbon in upper sediments. These findings offer additional arguments for the urgent need to protect and conserve this valuable ecosystem.

Methods Using Marine Aquatic Photoautotrophs along the Qatari Coastline to Remediate Oil and Gas Industrial Water

Qatar and other Gulf States have a diverse range of marine vegetation that is adapted to the stressful environmental conditions of seawater. The industrial wastewater produced by oil and gas activities adds further detrimental conditions for marine aquatic photosynthetic organisms on the Qatari coastlines. Thus, these organisms experience severe stress from both seawater and industrial wastewater. This review discusses the biodiversity in seawater around Qatar, as well as remediation methods and metabolic pathways to reduce the negative impacts of heavy metals and petroleum hydrocarbons produced during these activities. The role of microorganisms that are adjacent to or associated with these aquatic marine organisms is discussed. Exudates that are released by plant roots enhance the role of microorganisms to degrade organic pollutants and immobilize heavy metals. Seaweeds may have other roles such as biosorption and nutrient uptake of extra essential elements to avoid or reduce eutrophication in marine environments. Special attention is paid to mangrove forests and their roles in remediating shores polluted by industrial wastewater. Seagrasses (Halodule uninervis, Halophila ovalis, and Thalassia hemprichii) can be used as promising candidates for phytoremediation or bioindicators for pollution status. Some genera among seaweeds that have proven efficient in accumulating the most common heavy metals found in gas activities and biodegradation of petroleum hydrocarbons are discussed.

Sonarlogger: Enabling long-term underwater sonar observations

Coastal seas are under increasing pressure from extreme weather events and sea level rise, resulting in impacts such as changing hydrodynamic conditions, coastal erosion, and marine heat waves. To monitor changes in coastal marine habitats, such as reefs and macrophytes meadows, which add to the resilience of our coasts, consistent, medium- to long-term seafloor observations are needed. This project aims to deliver repeated, high-frequency sonar surveys on a stationary seabed mooring of a specific target area over a period of up to several months. A new stand-alone subsea system, the Sonarlogger, based on a battery pack, low-power logger and a high-resolution scanning sonar, was developed. It allows for long-term deployments with a customisable battery pack, WI-FI download and configurable sleep state. The system has been tested for over 130 days in dynamic coastal environments off the Belgian coast. Combined with auxiliary sensors, such as for measuring currents, waves and turbidity, this system enables comprehensive studies of morphologic changes and changing benthic ecosystems. Moreover, this system has the capacity to provide measurements of coastal environments during storms, where conventional systems may fall short, providing insights into event-based changes of the seafloor.

Influence of seagrass meadow length on beach morphodynamics: An experimental study

A novel flume experiment was conducted to compare the sheltering effect of surrogate seagrass meadows of two different lengths against a bare beach (benchmark). The analyses focused on assessing the impact of meadow cross-shore extent on wave height attenuation, behaviour of wave orbital velocity components, sediment transport, and shoreline erosion. Throughout the tests conducted in the large-scale CIEM wave flume at LIM/UPC Barcelona, meadow density and submergence ratio remained constant, while irregular waves were run over an initial 1:15 sand beach profile. In both meadow layouts, a persistent decrease in wave height from the offshore area in front of the meadow to the breaking zone was found. This reduction was directly correlated with the length of the seagrass meadow. As a result of the reduction in wave energy, less erosion occurred at the shoreline in accordance with the decrease in wave height. The mean velocities exhibited changes in the velocity profile from the meadow area to the immediate zone behind the meadow, a phenomenon not observed in more onshoreward positions. Orbital velocities displayed a reduction exclusively for the long meadow case. This decrease was persistent up to the breaking zone. As a consequence of these changes, the long meadow layout led to a decrease in the volume of sediment transport and a breaker bar closer to the shoreline. The short meadow layout resulted in a higher volume of sediment transport compared to the long meadow layout, although still less than the benchmark layout. Furthermore, in the short meadow layout, the final bar was situated in a location similar to that observed in the benchmark layout.

Operationalizing blue carbon principles in France: Methodological developments for Posidonia oceanica seagrass meadows and institutionalization

Conservation of ecosystems is an important tool for climate change mitigation. Seagrasses, mangroves, saltmarshes and other marine ecosystems have particularly high capacities to sequester and store organic carbon (blue carbon), and are being impacted by human activities. Calls have been made to mainstream blue carbon into policies, including carbon markets. Building on the scientific literature and the French voluntary carbon standard, the 'Label Bas-Carbone', we develop the first method for the conservation of Posidonia oceanica seagrasses using carbon finance. This methodology assesses the emission reduction potential of projects that reduce physical impacts from boating and anchoring. We show how this methodology was institutionalized thanks to a tiered approach on key parameters including carbon stocks, degradation rates, and decomposition rates. We discuss future needs regarding (i) how to strengthen the robustness of the method, and (ii) the expansion of the method to restoration of seagrasses and to other blue carbon ecosystems.