Can we manage coastal ecosystems to sequester more blue carbon?

© The Ecological Society of America To promote the sequestration of blue carbon, resource managers rely on best-management practices that have historically included protecting and restoring vegetated coastal habitats (seagrasses, tidal marshes, and mangroves), but are now beginning to incorporate catchment-level approaches. Drawing upon knowledge from a broad range of environmental variables that influence blue carbon sequestration, including warming, carbon dioxide levels, water depth, nutrients, runoff, bioturbation, physical disturbances, and tidal exchange, we discuss three potential management strategies that hold promise for optimizing coastal blue carbon sequestration: (1) reducing anthropogenic nutrient inputs, (2) reinstating top-down control of bioturbator populations, and (3) restoring hydrology. By means of case studies, we explore how these three strategies can minimize blue carbon losses and maximize gains. A key research priority is to more accurately quantify the impacts of these strategies on atmospheric greenhouse-gas emissions in different settings at landscape scales

Carbon sequestration by Australian tidal marshes

Australia's tidal marshes have suffered significant losses but their recently recognised importance in CO2 sequestration is creating opportunities for their protection and restoration. We compiled all available data on soil organic carbon (OC) storage in Australia's tidal marshes (323 cores). OC stocks in the surface 1 m averaged 165.41 (SE 6.96) Mg OC ha-1 (range 14-963 Mg OC ha-1). The mean OC accumulation rate was 0.55 ± 0.02 Mg OC ha-1 yr -1. Geomorphology was the most important predictor of OC stocks, with fluvial sites having twice the stock of OC as seaward sites. Australia's 1.4 million hectares of tidal marshes contain an estimated 212 million tonnes of OC in the surface 1 m, with a potential CO2 -equivalent value of $USD7.19 billion. Annual sequestration is 0.75 Tg OC yr -1, with a CO2 -equivalent value of$USD28.02 million per annum. This study provides the most comprehensive estimates of tidal marsh blue carbon in Australia, and illustrates their importance in climate change mitigation and adaptation, acting as CO2 sinks and buffering the impacts of rising sea level. We outline potential further development of carbon offset schemes to restore the sequestration capacity and other ecosystem services provided by Australia tidal marshes

Author Correction: The future of Blue Carbon science.

An amendment to this paper has been published and can be accessed via a link at the top of the paper

Sediment properties as important predictors of carbon storage in zostera marina meadows: a comparison of four European areas

Seagrass ecosystems are important natural carbon sinks but their efficiency varies greatly depending on species composition and environmental conditions. What causes this variation is not fully known and could have important implications for management and protection of the seagrass habitat to continue to act as a natural carbon sink. Here, we assessed sedimentary organic carbon in Zostera marina meadows (and adjacent unvegetated sediment) in four distinct areas of Europe (Gullmar Fjord on the Swedish Skagerrak coast, Asko in the Baltic Sea, Sozopol in the Black Sea and Ria Formosa in southern Portugal) down to similar to 35 cm depth. We also tested how sedimentary organic carbon in Z. marina meadows relates to different sediment characteristics, a range of seagrass-associated variables and water depth. The seagrass carbon storage varied greatly among areas, with an average organic carbon content ranging from 2.79 +/- 0.50% in the Gullmar Fjord to 0.17 +/- 0.02% in the area of Sozopol. We found that a high proportion of fine grain size, high porosity and low density of the sediment is strongly related to high carbon content in Z. marina sediment. We suggest that sediment properties should be included as an important factor when evaluating high priority areas in management of Z. marina generated carbon sinks

Converting beach-cast seagrass wrack into biochar: A climate-friendly solution to a coastal problem

© 2016 Excessive accumulation of plant ‘wrack’ on beaches as a result of coastal development and beach modification (e.g. groin installation) is a global problem. This study investigated the potential for converting beach-cast seagrass wrack into biochar as a ‘climate-friendly’ disposal option for resource managers. Wrack samples from 11 seagrass species around Australia were initially screened for their biochar potential using pyrolysis techniques, and then two species – Posidonia australis and Zostera muelleri – underwent detailed analyses. Both species had high levels of refractory materials and high conversion efficiency (48–57%) of plant carbon into biochar carbon, which is comparable to high-quality terrestrial biochar products. P. australis wrack gave higher biochar yields than Z. muelleri consistent with its higher initial carbon content. According to 13C NMR, wrack predominantly comprised carbohydrates, protein, and lignin. Aryl carbon typical of pyrogenic materials dominated the spectrum of the thermally-altered organic materials. Overall, this study provides the first data on the feasibility of generating biochar from seagrass wrack, showing that biocharring offers a promising climate-friendly alternative to disposal of beach wrack in landfill by avoiding a portion of the greenhouse gas emissions that would otherwise occur if wrack was left to decompose

Sediment and carbon deposition vary among vegetation assemblages in a coastal salt marsh

Coastal salt marshes are dynamic, intertidal ecosystems that are increasingly being recognised for their contributions to ecosystem services, including carbon (C) accumulation and storage. The survival of salt marshes and their capacity to store C under rising sea levels, however, is partially reliant upon sedimentation rates and influenced by a combination of physical and biological factors. In this study, we use several complementary methods to assess short-term (days) deposition and medium-term (months) accretion dynamics within a single marsh that contains three salt marsh vegetation types common throughout southeastern (SE) Australia. br br We found that surface accretion varies among vegetation assemblages, with medium-term (19 months) bulk accretion rates in the upper marsh rush (Juncus) assemblage (1.74g ±g 0.13g mmg yrg '1) consistently in excess of estimated local sea-level rise (1.15g mmg yrg '1). Accretion rates were lower and less consistent in both the succulent (Sarcocornia, 0.78g ±g 0.18g mmg yrg '1) and grass (Sporobolus, 0.88g ±g 0.22g mmg yrg '1) assemblages located lower in the tidal frame. Short-term (6 days) experiments showed deposition within Juncus plots to be dominated by autochthonous organic inputs with C deposition rates ranging from 1.14g ±g 0.41g mgg Cg cmg '2g dg '1 (neap tidal period) to 2.37g ±g 0.44g mgg Cg cmg '2g dg '1 (spring tidal period), while minerogenic inputs and lower C deposition dominated Sarcocornia (0.10g ±g 0.02 to 0.62g ±g 0.08g mgg Cg cmg '2g dg '1) and Sporobolus (0.17g ±g 0.04 to 0.40g ±g 0.07g mgg Cg cmg '2g dg '1) assemblages. br br Elemental (Cg :g N), isotopic (?13C), mid-infrared (MIR) and 13C nuclear magnetic resonance (NMR) analyses revealed little difference in either the source or character of materials being deposited among neap versus spring tidal periods. Instead, these analyses point to substantial redistribution of materials within the Sarcocornia and Sporobolus assemblages, compared to high retention and preservation of organic inputs in the Juncus assemblage. By combining medium-term accretion quantification with short-term deposition measurements and chemical analyses, we have gained novel insights into above-ground biophysical processes that may explain previously observed regional differences in surface dynamics among key salt marsh vegetation assemblages. Our results suggest that Sarcocornia and Sporobolus assemblages may be particularly susceptible to changes in sea level, though quantification of below-ground processes (e.g. root production, compaction) is needed to confirm this

Seventy years of continuous encroachment substantially increases ‘blue carbon’ capacity as mangroves replace intertidal salt marshes.

Shifts in ecosystem structure have been observed over recent decades as woody plants encroach upon grasslands and wetlands globally. The migration of mangrove forests into salt marsh ecosystems is one such shift which could have important implications for global ‘blue carbon’ stocks. To date, attempts to quantify changes in ecosystem function are essentially constrained to climate-mediated pulses (30 years or less) of encroachment occurring at the thermal limits of mangroves. In this study, we track the continuous, lateral encroachment of mangroves into two south-eastern Australian salt marshes over a period of 70 years and quantify corresponding changes in biomass and belowground C stores. Substantial increases in biomass and belowground C stores have resulted as mangroves replaced salt marsh at both marine and estuarine sites. After 30 years, aboveground biomass was significantly higher than salt marsh, with biomass continuing to increase with mangrove age. Biomass increased at the mesohaline river site by 130 ± 18 Mg biomass km−2 yr−1 (mean ± SE), a 2.5 times higher rate than the marine embayment site (52 ± 10 Mg biomass km−2 yr−1), suggesting local constraints on biomass production. At both sites, and across all vegetation categories, belowground C considerably outweighed aboveground biomass stocks, with belowground C stocks increasing at up to 230 ± 62 Mg C km−2 yr−1 (± SE) as mangrove forests developed. Over the past 70 years, we estimate mangrove encroachment may have already enhanced intertidal biomass by up to 283 097 Mg and belowground C stocks by over 500 000 Mg in the state of New South Wales alone. Under changing climatic conditions and rising sea levels, global blue carbon storage may be enhanced as mangrove encroachment becomes more widespread, thereby countering global warming. © 2015, John Wiley & Sons Ltd

Geochemical analyses reveal the importance of environmental history for blue carbon sequestration

©2017. American Geophysical Union. All Rights Reserved. Coastal habitats including saltmarshes and mangrove forests can accumulate and store significant blue carbon stocks, which may persist for millennia. Despite this implied stability, the distribution and structure of intertidal-supratidal wetlands are known to respond to changes imposed by geomorphic evolution, climatic, sea level, and anthropogenic influences. In this study, we reconstruct environmental histories and biogeochemical conditions in four wetlands of similar contemporary vegetation in SE Australia. The objective is to assess the importance of historic factors to contemporary organic carbon (C) stocks and accumulation rates. Results from the four cores—two collected from marine-influenced saltmarshes (Wapengo marine site (WAP-M) and Port Stephens marine site (POR-M)) and two from fluvial influenced saltmarshes (Wapengo fluvial site (WAP-F) and Port Stephens fluvial site (POR-F))—highlight different environmental histories and preservation conditions. High C stocks are associated with the presence of a mangrove phase below the contemporary saltmarsh sediments in the POR-M and POR-F cores. 13C nuclear magnetic resonance analyses show this historic mangrove root C to be remarkably stable in its molecular composition despite its age, consistent with its position in deep sediments. WAP-M and WAP-F cores did not contain mangrove root C; however, significant preservation of char C (up to 46% of C in some depths) in WAP-F reveals the importance of historic catchment processes to this site. Together, these results highlight the importance of integrating historic ecosystem and catchment factors into attempts to upscale C accounting to broader spatial scales