Periodic Reporting for period 1 - SCaLED (Seagrass Carbon cycling and Local Environmental Drivers)
Reporting period: 2023-03-01 to 2025-02-28
One of the principal forms of combatting climate change is through biological carbon sequestration. Biological carbon sequestration occurs when carbon dioxide (CO2) is drawn down from the atmosphere into photosynthesising organisms, such as plants and algae, where it is converted into organic carbon. For the organic carbon to be considered sequestered on time-scales relevant to humans and global climate change, it must avoid degradation for thousands of years. Coastal seagrass meadows have been identified as hotspots for carbon sequestration. This is because of the high productivity of seagrasses, which results in large amounts of CO2 being incorporated into their leaves and shoots as organic carbon. The dense flexible canopy of seagrass meadows attenuates flow and stabilises the seafloor, promoting the entrapment and accumulation of organic carbon from both the seagrasses themselves and other external sources of organic carbon throughout the coastal zone. This organic carbon accumulates on the seafloor where it eventually gets buried within the seafloor sediments, a place where degradation is slower due to low oxygen, anoxic conditions. Assessing the ability of coastal habitats to sequester carbon, and thus, their impact on the global carbon cycle, provides the opportunity to further promote carbon sequestration and minimise events that might result in the unintentional degradation of organic carbon and subsequent release of CO2.
Up to now, research has focussed on assessing the total organic carbon stocks within seagrass meadows and their associated sediments. Stock-type measurements, however, vary from site to site depending on local factors of water motion and organic carbon supply. To be able to reliably assess the carbon sequestration capacity of an ecosystem and gain the power to promote further carbon sequestration, there is a need to understand the processes governing organic carbon capture and burial in marine sediments. Combining expertise from biogeochemists and ecologists the SCaLED project’s overall objective was to quantify organic carbon transformation within the coastal zone under varying environmental factors. This involved measuring the productivity of the seagrasses and the rates of CO2 incorporation into their biomass across the temperate seasons, folllowed by quantifying the degradation processes of the organic carbon as it moves through the coastal zone and gets buried within the seafloor sediments. With these measurements, we aimed to provide the necessary data to highlight the overall impact that seagrass meadows have on the coastal carbon cycle and to detail the conditions required to promote carbon sequestration within the coastal zone. Furthermore, this data will be utilised by ocean modellers to examine varying scenarios of global change and its impact on the marine carbon cycle.
Up to now, research has focussed on assessing the total organic carbon stocks within seagrass meadows and their associated sediments. Stock-type measurements, however, vary from site to site depending on local factors of water motion and organic carbon supply. To be able to reliably assess the carbon sequestration capacity of an ecosystem and gain the power to promote further carbon sequestration, there is a need to understand the processes governing organic carbon capture and burial in marine sediments. Combining expertise from biogeochemists and ecologists the SCaLED project’s overall objective was to quantify organic carbon transformation within the coastal zone under varying environmental factors. This involved measuring the productivity of the seagrasses and the rates of CO2 incorporation into their biomass across the temperate seasons, folllowed by quantifying the degradation processes of the organic carbon as it moves through the coastal zone and gets buried within the seafloor sediments. With these measurements, we aimed to provide the necessary data to highlight the overall impact that seagrass meadows have on the coastal carbon cycle and to detail the conditions required to promote carbon sequestration within the coastal zone. Furthermore, this data will be utilised by ocean modellers to examine varying scenarios of global change and its impact on the marine carbon cycle.
During the SCaLED project a seasonal field campaign was conducted within a seagrass meadow in Denmark. During each field campaign, measurements of ecosystem productivity, biomass production, organic carbon stocks within sediments and degradation rates of the organic carbon under varying conditions were completed. The degradation rates were measured in laboratory experiments lasting 2 weeks, where the degradation rates of organic carbon were measured in undisturbed sediment using a sulphur tracer method, and in resuspended conditions where the sediment was placed in oxic and anoxic seawater and kept suspended. These activities resulted in a large dataset detailing the source of organic carbon and how it degrades as it encounters different conditions within the coastal environment.
A seasonal study of a seagrass meadow in Denmark quantified how the productivity of the seagrasses varies greatly throughout the year with the meadow drawing down the most CO2 in spring and summer, followed by a large die-back of the seagrass throughout autumn. This trend results in large amounts of organic carbon being introduced into the seafloor sediments throughout autumn and winter. Although this result is not novel on its own, it is important to consider when assessing the rates of degradation of the organic carbon throughout the year. In summer and autumn there is a high amount of microbial activity within the seagrass sediments due to the warmer sea temperatures, this microbial activity results in larger rates of organic carbon degradation within the sediments. While in winter and early spring, degradation rates are low. These fluctuations in organic carbon degradation rates over the seasons are important to consider when identifying carbon sequestration hotspots.
During a disturbance event, such as a storm or through the removal of seagrasses, the sediment within a seagrass meadow gets resuspended in the water column. This resuspension exposes the buried organic carbon to oxic conditions which cause it to be degraded at 3-5 times the rate than when it remains undisturbed in the sediment. Like the degradation rates within the sediment, the rate of this degradation during resuspension is highly dependent upon the season and sea temperature. Disturbance events that cause sediments in the coastal zone to be resuspended in summer and autumn result in more organic carbon degradation to CO2 than disturbance events in winter and spring. However, very little organic carbon actually gets sequestered at anthropogenically relevant time-scales (1000’s years), and sediment disturbance events often only quicken the natural degradation of the organic carbon stored within coastal sediments.
The SCaLED project highlights the dynamic nature of carbon sequestration in the coastal zone. Studies that account for natural degradation proccesses in other coastal systems are required to improve our understanding of the coastal carbon cycle and to identify important habitats contributing to long-term carbon sequestration. Data that considers the degradation processes of organic carbon as it moves through coastal habitats is crucial for quantifying the marine carbon cycle and fully understanding the extent of human activities in the coastal zone.
During a disturbance event, such as a storm or through the removal of seagrasses, the sediment within a seagrass meadow gets resuspended in the water column. This resuspension exposes the buried organic carbon to oxic conditions which cause it to be degraded at 3-5 times the rate than when it remains undisturbed in the sediment. Like the degradation rates within the sediment, the rate of this degradation during resuspension is highly dependent upon the season and sea temperature. Disturbance events that cause sediments in the coastal zone to be resuspended in summer and autumn result in more organic carbon degradation to CO2 than disturbance events in winter and spring. However, very little organic carbon actually gets sequestered at anthropogenically relevant time-scales (1000’s years), and sediment disturbance events often only quicken the natural degradation of the organic carbon stored within coastal sediments.
The SCaLED project highlights the dynamic nature of carbon sequestration in the coastal zone. Studies that account for natural degradation proccesses in other coastal systems are required to improve our understanding of the coastal carbon cycle and to identify important habitats contributing to long-term carbon sequestration. Data that considers the degradation processes of organic carbon as it moves through coastal habitats is crucial for quantifying the marine carbon cycle and fully understanding the extent of human activities in the coastal zone.