The influence of hydrodynamic exposure on carbon storage and nutrient retention in eelgrass (Zostera marina L.) meadows on the Swedish Skagerrak coast

Seagrass decline weakens sediment organic carbon stability

Seagrass meadows are globally recognized as critical natural carbon sinks, commonly known as 'blue carbon'. However, seagrass decline attributed to escalating human activities and climate change, significantly influences their carbon sequestration capacity. A key aspect in comprehending the impact of seagrass decline on carbon sequestration is understanding how degradation affects the stored blue carbon, primarily consisting of sediment organic carbon (SOC). While it is widely acknowledged that seagrass decline affects the input of organic carbon, little is known about its impact on SOC pool stability. To address this knowledge, we examined variations in total SOC and recalcitrant SOC (RSOC) at a depth of 15 cm in nine seagrass meadows located on the coast of Southern China. Our findings revealed that the ratio of RSOC to SOC (RSOC/SOC) ranged from 27 % to 91 % in the seagrass meadows, and the RSOC/SOC increased slightly with depth. Comparing different seagrass species, we observed that SOC and RSOC stocks were 1.94 and 3.19-fold higher under Halophila beccarii and Halophila ovalis meadows compared to Thalassia hemprichii and Enhalus acoroides meadows. Redundancy and correlation analyses indicated that SOC and RSOC content and stock, as well as the RSOC/SOC ratio, decreased with declining seagrass shoot density, biomass, and coverage. This implies that the loss of seagrass, caused by human activities and climate change, results in a reduction in carbon sequestration stability. Further, the RSOC decreased by 15 %, 29 %, and 40 % under unvegetated areas compared to adjacent Halophila spp., T. hemprichii and E. acoroides meadows, respectively. Given the anticipated acceleration of seagrass decline due to climate change and increasing coastal development, our study provides timely information for developing coastal carbon protection strategies. These strategies should focus on preserving seagrass and restoring damaged seagrass meadows, to maximize their carbon sequestration capacity.

Loss of POC and DOC on seagrass sediments by hydrodynamics

Coastal development and climate change are sparking growing concern about the vulnerability of the organic carbon (OC) stocks in marine sediments to remineralization, especially in high threaten coastal ecosystems like seagrass meadows. Uncertainties still exist regarding the role played by hydrodynamics, seagrass canopies and sediment properties in OC resuspension and remineralization. A set of laboratory experiments were conducted to assess, for the first time, the mechanisms by which the particulate and dissolved organic carbon (POC and DOC) may be released and remineralized under hydrodynamic conditions (i.e., unidirectional and oscillatory flows) in two eelgrass densities and sediments properties (i.e., grain size and OC content). After a gradually increase in hydrodynamic forces, our results demonstrated that the presence of eelgrass reduced sediment erosion and OC loss in high-density canopies, while low-density canopies promote OC resuspension (on average, 1.8-fold higher than high-density canopies). We also demonstrated that unidirectional and oscillatory flows released similar DOC from surface sediments (on average, 15.5 ± 1.4 and 18.4 ± 1.8 g m-2, respectively), whereas oscillatory flow released significantly more POC than unidirectional flows (from 10.8 ± 1.1 to 32.1 ± 5.6 g m-2 for unidirectional and oscillatory flows, respectively). POC and DOC released was strongly influenced by both seagrass meadow structure (i.e., lower eelgrass density and shoot area) and sediment properties (i.e., lower mud and higher sediment water content). We found that, although >74 % of OC in upper sediments was remineralized within 30 days, a relatively high amount of OC in high-density canopies is recalcitrant, highlighting its potential for the formation of blue carbon deposits. This study highlights the vulnerability of OC deposits in seagrass sediments to resuspension if the meadow is degraded and/or the climate change yield stronger storms, which could potentially weaken the seagrass meadows' service as blue carbon ecosystem in the future.

Current and future carbon stocks in coastal wetlands within the Great Barrier Reef catchments

Australia's Great Barrier Reef (GBR) catchments include some of the world's most intact coastal wetlands comprising diverse mangrove, seagrass and tidal marsh ecosystems. Although these ecosystems are highly efficient at storing carbon in marine sediments, their soil organic carbon (SOC) stocks and the potential changes resulting from climate impacts, including sea level rise are not well understood. For the first time, we estimated SOC stocks and their drivers within the range of coastal wetlands of GBR catchments using boosted regression trees (i.e. a machine learning approach and ensemble method for modelling the relationship between response and explanatory variables) and identified the potential changes in future stocks due to sea level rise. We found levels of SOC stocks of mangrove and seagrass meadows have different drivers, with climatic variables such as temperature, rainfall and solar radiation, showing significant contributions in accounting for variation in SOC stocks in mangroves. In contrast, soil type accounted for most of the variability in seagrass meadows. Total SOC stock in the GBR catchments, including mangroves, seagrass meadows and tidal marshes, is approximately 137 Tg C, which represents 9%-13% of Australia's total SOC stock while encompassing only 4%-6% of the total extent of Australian coastal wetlands. In a global context, this could represent 0.5%-1.4% of global SOC stock. Our study suggests that landward migration due to projected sea level rise has the potential to enhance carbon accumulation with total carbon gains between 0.16 and 0.46 Tg C and provides an opportunity for future restoration to enhance blue carbon.