A catchy name and an apparently easy translation into economical terms has made Blue Carbon (BC) research an unstoppable phenomenon. The capacity of coastal BC ecosystems (CBCE) to capture and store organic carbon in sediments over hundreds or thousands of years is granting these habitats a purportedly relevant place in the context of biospheric sinks and, whether good, bad or both, in carbon trading schemes. Even though mangrove forests, tidal marshes, and seagrass meadows are already recognized in the major climate conventions and frameworks as potentially valuable contributors to climate change mitigation and adaptation, we are still a long way from having the necessary evidence to rigorously assess that potential. Major knowledge gaps are as fundamental as knowing the past and present size of these CBCEs (mapping) and their associated carbon stocks (plant biomass and soil), associated carbon fluxes (through accretion rates), conservation status, restoration potential, and vulnerability (and risk) from ongoing climate and anthropogenic pressures (e.g., global warming, eutrophication, or habitat mechanical destruction). Other key knowledge gaps challenge the very heart of the BC concept, such as whether or not to include algal forests into the BC family, quantifying and adding or not the BC-derived “off-site” stocks, resolving the controversy of a possible double accounting, or the long-standing critical debate on the role of carbonates, and the quantification of non-CO 2 GHG emissions from CBCEs. Technical challenges remain, including the lack of consensus on the application of standardized methodologies. Together with the complexity of the certification procedures and the lack of clear policies at regional, national, and international scales, these knowledge gaps pose significant barriers for BC projects to readily access the voluntary and, additionally, the regulated carbon markets. The challenges above have been identified during the first scoping meeting of the Blue Carbon group of the Joint Programming Initiative Healthy and Productive Seas and Oceans (JPI Oceans), a pan-European intergovernmental platform that increases the efficiency and impact of research and innovation for sustainably healthy and productive seas and oceans. The session welcomes contributions reporting advances which will help to close these knowledge gaps; we equally welcome contributions which challenge the actual soundness of CBCEs as relevant global carbon sinks. Contributions presenting successful or unsuccessful examples of carbon offset projects based on BC ecosystems, as well as initiatives to incorporate other ecosystem services alongside BC would be welcome. New perspectives from modern economics, marketing, or social science approaches are also welcomed. Those contributions with a special emphasis on the leitmotiv of this year’s ASLO ASM, i.e., resilience and recovery, will be prioritized.
Lead Organizer: Miguel-Ángel Mateo, Consejo Superior de Investigaciones Científicas (mateo@ceab.csic.es)
Co-organizers:
William Austin, University of St. Andrews (wena@st-andrews.ac.uk)
Grace Cott, University College Dublin (grace.cott@ucd.ie)
Paul Steven Lavery, Edith Cowan University (p.lavery@ecu.edu.au)
Presentations
10:30 AM
Methane emission in seagrass (Posidonia oceanica) mattes (6607)
Primary Presenter: Eugenia Apostolaki, Hellenic Centre for Marine Research (eapost@hcmr.gr)
Seagrass meadows have a substantial capacity to store carbon, but their cover loss may pose a significant risk to that capacity and increase carbon emissions from underlying sediments. Here we aimed to determine if seagrass cover loss could affect carbon flow in a Posidonia oceanica meadow. The study was conducted at Pefki Bay, Greece (36o 04.08 N, 28o 02.99 E), which hosts a meadow intermixed with dead matte, representing 46 % and 6 % of the sea bottom, respectively, between 5 and 30 m of water depth. We estimated organic carbon (Corg) stocks in 1 m cores, accumulation rates through sediment dating by 210Pb and fluxes of methane (CH4) following a 24-h cycle using in situ benthic chambers, in living and adjacent dead P. oceanica mattes. Based on the 210Pbxs inventories, seagrass loss was estimated to have occurred approximately 32 years before our sampling (i.e., 2021). Corg stock in top meter was significantly lower at P. oceanica than dead matte (7.5 ± 2.4 kg m-2 and 10.2 ± 0.4 kg m-2, respectively), while CH4 efflux showed the reverse pattern (172.2 ± 162.1 and 88.3 ± 81.1 μg m-2 h-1, respectively). After seagrass loss, the accumulation rate was similar between P. oceanica and dead matte (24.4 ± 1.8 g m-2 y-1 and 25.8 ± 13.2 g m-2 y-1, respectively), but the abundance of mud increased at the latter. Overall, CH4 efflux was very low (0.07 ± 0.06 and 0.03 ± 0.03 g CO2equivalent m-2 y-1 at P. oceanica and dead matte, respectively) compared to the rate of carbon accumulation at both mattes, suggesting that cover loss does not necessarily relate to considerable CH4 emissions.
10:45 AM
The climate benefit of seagrass blue carbon is reduced by methane fluxes and enhanced by nitrous oxide fluxes – a synthesis (5554)
Primary Presenter: Bradley Eyre, Southern Cross University (bradley.eyre@scu.edu.au)
Blue carbon refers to carbon stored long-term (>100 years) in vegetated coastal and marine ecosystems, which constitutes an important sink for atmospheric carbon dioxide. However, during organic matter diagenesis methane and nitrous oxide can be produced or consumed. Methane and nitrous oxide have a global warming potential that is, depending on the 20-year or 100-year time-horizon, 81 to 28 for methane or 273 for nitrous oxide times respectively more powerful than carbon dioxide. As such, the production and release of methane and nitrous oxide from seagrass sediments has the potential to reduce the climatic benefit of carbon sequestered as blue carbon. Similarly, an uptake of methane and nitrous oxide has the potential to enhance the climate benefit of seagrass communities. In this presentation we will present a synthesis of seagrass methane and nitrous oxide fluxes and organic carbon burial rates, and use these data to estimate the reduced climate benefit (offsets) of seagrass blue carbon. Three upscaling approaches will be presented (1) offsets for individual seagrass species, (2) a globally offset and (3) an offset for the Australian region. This presentation will highlight the importance of using long-term organic carbon burial rates and accounting for both methane and nitrous oxide fluxes in future seagrass blue carbon assessments.
11:00 AM
Burial flux estimates from sediment cores suggest carbonate production partially offsets blue carbon potential of seagrass meadows across South Florida seascape (7106)
Primary Presenter: Johannes Krause, Florida International University (jkrause@fiu.edu)
Vegetated coastal ecosystems, including seagrass meadows, were proposed as globally significant sites for carbon (C) sequestration, because of their potential to accumulate organic carbon (OC)-rich sediments. However, many seagrass meadows are found in tropical to subtropical latitudes, where sediment formation can be dominated by carbonates (IC). The CO2 production resulting from IC formation (dissolution) within seagrass meadows may offset (enhance) their potential as C sinks. Here, we examine OC and IC burial fluxes of South Florida seagrass meadows using 18 210Pb-dated sediment cores spanning a range of geomorphic settings from estuarine to backreef environments. We find considerable variability in sediment C stocks across the seascape, with spatial decoupling of C stocks and C burial rate hotspots. Through analysis of C stable isotopes and other auxiliary data, we estimate the proportion of autochthonous OC and IC production and derive first-order estimates of net benthic C fluxes. We find that particularly meadows in Florida Bay are likely net C sources to the atmosphere, supporting recent findings from eddy covariance measurements. We go on to evaluate the relative importance of seagrass condition versus seascape configuration for benthic OC, IC, and net C burial fluxes, adding to the conceptual framework for blue carbon potential in carbonate-dominated seagrass meadows.
11:15 AM
Degradation pathways are key elements for understanding macroalgal blue carbon (6294)
Primary Presenter: Lydia White, Tvärminne Zoological Station, University of Heksinki (lydia.white@helsinki.fi)
Seaweed forests exhibit exceptionally high per-area production rates, however measuring this productivity in situ can be challenging. We have developed novel methods to quantify net community metabolism in macroalgal forests, demonstrating that these habitats draw the largest carbon flux by any vegetated habitat in the coastal ocean and are indeed net autotrophic systems. The assumption that these habitats are therefore carbon sinks, however, requires further dissection. Macroalgae release up to 35% of their net primary productivity as dissolved organic carbon and these habitats can also release methane emissions which may counterbalance atmospheric CO2 uptake. Importantly, macroalgal forests contribute substantial detritus which can reach marine sediments where it may potentially be sequestered. For macroalgae to contribute meaningfully to carbon sequestration, however, it’s detritus must be under environmental conditions such that it decomposes slowly and/or incompletely, increasing the probability of permanent burial in sediments. There have been numerous estimates of algal degradation rates in the shallows, and some limited estimates of carbon loading rates to deep sea sediments. But we lack a mechanistic understanding of how the degradation of algal detritus varies with depth and how it might degrade during its journey. We are exploring whether macroalgae in the Baltic Sea, with cold temperatures and low oxygen levels, relative to other regions, can serve as a useful model system for understanding macroalgal carbon turnover and potential sequestration.
11:30 AM
Making the case for including air-water CO2 exchanges in Blue Carbon accounting (4656)
Primary Presenter: Bryce Van Dam, Helmholtz-zentrum Hereon (Bryce.Dam@hereon.de)
Coastal “blue carbon” (BC) habitats like seagrass meadows can mitigate anthropogenic carbon emissions by sequestering CO2 in their sediments. However, air-water greenhouse gas exchange, especially CO2 fluxes can alter net carbon sequestration, complicating BC mitigation. A complex suite of biogeochemical and physical drivers challenge the prediction of CO2 fluxes, presenting a significant challenge especially for BC monitoring, reporting, and verification. I first present a case study in a subtropical seagrass meadow, examining the "bottom-up" biogeochemical forcing and "top-down" physical forcing, which together cause air-water CO2 exchanges to diverge from what might be expected. Eddy covariance measurements reveal net CO2 emissions of 700 ± 660 µmol m−2 hour−1 (6.1 mol m−2 year−1) over an annual cycle, with CO2 flux greater during the day than at night. Net alkalinity consumption by ecosystem calcification explains >95% of the observed CO2 emissions, far exceeding organic carbon burial and anaerobic alkalinity generation. Water temperature was an important driver, with solubilty explaining ~23% of the diurnal CO2 flux excursion, including the observed night-time convective enhancement of gas transfer rates. These results are placed into the context of a global synthesis of direct CO2 flux measurements over seagrass meadow, in which we found that CO2 fluxes are large, relative to carbon burial. Factors like light availability, tidal mixing and temperature were key drivers of global variability. Taken together, we argue that efforts to monitor, report and verify BC sequestration projects will be incomplete if they do not include assessments of air-water CO2 (and other GHG) exchange.
11:45 AM
sea4soCiety - searching for solutions for Carbon sequestration in coastal ecosystems (5278)
Primary Presenter: Martin Zimmer, Leibniz-Centre for Marine Tropical Research (ZMT) (martin.zimmer@leibniz-zmt.de)
Climate change mitigation requires immediate and long-lasting drastic reductions in anthropogenic greenhouse gas emissions. As some of these emissions cannot easily be avoided, the net-zero emissions aim can only be reached through carbon dioxide removal (CDR) strategies. Many of these rely on technical, physical or chemical approaches that are promising but not yet fully implementable nor fully accepted by society (https://cdrmare.de/en). Nature-based Solutions (NbS), by contrast, may be less efficient but enjoy high societal desirability and methodological feasibility. sea4soCiety aims at developing innovative approaches to enhance the potential for Blue Carbon sequestration in mangrove forests and other blue carbon ecosystems (saltmarshes, seagrass meadows, kelp beds), e.g., through expanding their spatial extent into new areas, if and where ecologically feasible, environmentally sound, legally and ethically unobjectionable, socially acceptable, and economically viable. I will critically shed light on the general concept, progress and first findings specifically with respect to methodology, organic matter stability and origin, faunal effects on carbon fluxes, and societal perception and acceptance.
SS093B Coastal Blue Carbon Ecosystems: Advances and Challenges
Description
Time: 10:30 AM
Date: 9/6/2023
Room: Sala Palma