Intensive macroalgal blooms, a source of labile organic carbon (LOC) induced by coastal nutrient loading in some seagrass ecosystems, create ideal conditions for enhanced recalcitrant organic carbon (ROC) loss via the cometabolism effect. Here, we carried out a 62-day laboratory experiment to see if density-dependent addition of macroalgal biomass can influence the seagrass decomposition process, including seagrass detritus carbon chemistry, greenhouse emissions, and bacterial communities. We found that higher density macroalgal addition stimulated microbes to decompose ∼20% more of the seagrass biomass compared to other treatments, which was also reflected in enhanced (∼twofold) greenhouse gas emissions. Although the composition of the seagrass-associated microbiome communities was unaffected by the addition of macroalgae, we showed that high macroalgal addition caused a relative depletion in the ROC as lignin and lipid compounds, as well as $δ$13C depletion and $δ$15N enrichment of the seagrass detritus. These results suggest that macroalgal blooms may stimulate the remineralization of recalcitrant components of seagrass detritus via cometabolism, possibly through providing available energy or resources for the synthesis of ROC-degrading enzymes within the resident microbial population. This study provides evidence that cometabolism can be a mechanism for leading to reduced seagrass blue carbon sequestration and preservation.

Abstract. Tidal marshes, mangrove forests, and seagrass meadows are important global carbon (C) sinks, commonly referred to as coastal blue carbon. However, these ecosystems are rapidly declining with little understanding of what drives the magnitude and variability of C associated with them, making strategic and effective management of blue C stocks challenging. In this study, our aims were threefold: (1) identify ecological, geomorphological, and anthropogenic variables associated with C stock variability in blue C ecosystems; (2) create a predictive model of blue C stocks; and, (3) map regional blue C stock magnitude and variability. We had the unique opportunity of using a high-spatial-density C stock dataset from 96 blue C ecosystems across the state of Victoria, Australia, integrated with spatially explicit environmental data to reach these aims. We used an information theoretic approach to create, average, validate, and select the best general linear mixed effects model for predicting C stocks across the state. Ecological drivers (i.e. ecosystem type or dominant species/ecological vegetation class) best explained variability in C stocks, relative to geomorphological and anthropogenic drivers. Of the geomorphological variables, distance to coast, distance to freshwater, and slope best explained C stock variability. Anthropogenic variables were of least importance. We estimated over 2.31 million Mg C stored in the top 30 cm of sediment in coastal blue C ecosystems in Victoria, 88 % of which was contained within four major coastal areas due to the extent of blue C ecosystems ($\sim$ 87 % of total blue C ecosystem area). Regionally, these data can inform conservation management, paired with assessment of other ecosystem services, by enabling identification of hotspots for protection and key locations for restoration efforts. Globally, these methods can be applied to identify relationships between environmental drivers and C stocks to produce predictive C stock models at scales relevant for resource management.

This thesis generated new knowledge related to the role of seagrass meadows, salt marshes, and mangrove forests as dense carbon (C) sinks (‘blue carbon’), including the distribution and magnitude of blue C stocks, identification of environmental drivers of sediment C stocks, and the impacts of anthropogenic disturbance to sediment C.

Tidal marsh ecosystems are among earth s most efficient natural organic carbon (C) sinks and provide myriad ecosystem services. However, approximately half have been ‘reclaimed – i.e. converted to other land uses – potentially turning them into sources of greenhouse gas emissions. In this study, we applied C stock measurements and paleoanalytical techniques to sediments from reclaimed and intact tidal marshes in southeast Australia. We aimed to assess the impacts of reclamation on: 1) the magnitude of existing sediment C stocks; 2) ongoing C sequestration and storage; and 3) C quality. Differences in sediment horizon depths (indicated by Itrax-XRF scanning) and ages (indicated by lead-210 and radiocarbon dating) suggest a physical loss of sediments following reclamation, as well as slowing of sediment accumulation rates. Sediments at one meter depth were between $\sim$2000 and $\sim$5300 years older in reclaimed cores compared to intact marsh cores. We estimate a 70% loss of sediment C in reclaimed sites (equal to 73 Mg C ha −1 ), relative to stocks in intact tidal marshes during a comparable time period. Following reclamation, sediment C was characterized by coarse particulate organic matter with lower alkyl-o-alkyl ratios and higher amounts of aromatic C, suggesting a lower extent of decomposition and therefore lower likelihood of being incorporated into long-term C stocks compared to that of intact tidal marshes. We conclude that reclamation of tidal marshes can diminish C stocks that have accumulated over millennial time scales, and these losses may go undetected if additional analyses are not employed in conjunction with C stock estimates.

Seagrass beds have decreased in abundance and areal coverage over the past several decades. Although previous studies have examined the importance of seagrass for benthic community assemblages and abundances, the effect of seagrass on deep-dwelling, large (high-biomass) infauna and the importance for benthic secondary production in Chesapeake Bay have not been addressed. Using benthic suctions and push cores, we compared density, diversity, and secondary productivity of benthic communities in seagrass to those in other shallow-water habitats and estimated benthic secondary productivity lost in the York River due to loss of seagrass from 1971 to 2016. We examined four habitat types in the York River: unvegetated, Gracilaria spp., mixed seagrass (multiple seagrass species), and Zostera marina L. seagrass. Physical characteristics of habitat types and biomass of organisms were assessed, and annual secondary productivity was calculated using biomass and production-to-biomass ratios. Benthic density, diversity, secondary productivity, sedimentary chlorophyll a, and percent sand were all highest in seagrass beds with Z. marina alone. Approximately 35% of benthic secondary productivity, or 1.51 × 108 g C yr-1, was lost in the York River in 1971-2016 due to the loss of seagrass beds to unvegetated substrate. The loss of seagrass in the York River over time and the associated decrease in benthic secondary productivity that we estimated could have negative consequences for the productivity of epibenthic predators. Our data emphasize the importance of conservation and restoration of seagrass.

With the growing recognition that effective action on climate change will require a combination of emissions reductions and carbon sequestration, protecting, enhancing and restoring natural carbon sinks have become political priorities. Mangrove forests are considered some of the most carbon-dense ecosystems in the world with most of the carbon stored in the soil. In order for mangrove forests to be included in climate mitigation efforts, knowledge of the spatial distribution of mangrove soil carbon stocks are critical. Current global estimates do not capture enough of the finer scale variability that would be required to inform local decisions on siting protection and restoration projects. To close this knowledge gap, we have compiled a large georeferenced database of mangrove soil carbon measurements and developed a novel machine-learning based statistical model of the distribution of carbon density using spatially comprehensive data at a 30 m resolution. This model, which included a prior estimate of soil carbon from the global SoilGrids 250 m model, was able to capture 63% of the vertical and horizontal variability in soil organic carbon density (RMSE of 10.9 kg m −3 ). Of the local variables, total suspended sediment load and Landsat imagery were the most important variable explaining soil carbon density. Projecting this model across the global mangrove forest distribution for the year 2000 yielded an estimate of 6.4 Pg C for the top meter of soil with an 86–729 Mg C ha −1 range across all pixels. By utilizing remotely-sensed mangrove forest cover change data, loss of soil carbon due to mangrove habitat loss between 2000 and 2015 was 30–122 Tg C with >75% of this loss attributable to Indonesia, Malaysia and Myanmar. The resulting map products from this work are intended to serve nations seeking to include mangrove habitats in payment-for- ecosystem services projects and in designing effective mangrove conservation strategies.

In their recent review, Geoengineering with seagrasses: is credit due where credit is given?, Johannessen and Macdonald (2016) invoke the prospect of carbon offset-credit over-allocation by the Verified Carbon Standard as a pretense for their concerns about published seagrass carbon burial rate and global stock estimates. Johannessen and Macdonald (2016) suggest that projects seeking offset-credits under the Verified Carbon Standard methodology VM0033: Methodology for Tidal Wetland and Seagrass Restoration will overestimate long-term (100 yr) sediment organic carbon (SOC) storage because issues affecting carbon burial rates bias storage estimates. These issues warrant serious consideration by the seagrass research community; however, VM0033 does not refer to seagrass SOC burial rates or storage. Projects seeking credits under VM0033 must document greenhouse gas emission reductions over time, relative to a baseline scenario, in order to receive credits. Projects must also monitor changes in carbon pools, including SOC, to confirm that observed benefits are maintained over time. However, VM0033 allows projects to conservatively underestimate project benefits by citing default values for specific accounting parameters, including CO2 emissions reductions. We therefore acknowledge that carbon crediting methodologies such as VM0033 are sensitive to the quality of the seagrass literature, particularly when permitted default factors are based in part on seagrass burial rates. Literature-derived values should be evaluated based on the concerns raised by Johannessen and Macdonald (2016), but these issues should not lead to credit over-allocation in practice, provided VM0033 is rigorously followed. These issues may, however, affect the feasibility of particular seagrass offset projects.

Blue carbon ecosystems-seagrasses, tidal marshes , and mangroves-serve as dense carbon sinks important for reducing atmospheric greenhouse gas concentrations, yet only recently have stock estimates emerged. We sampled 96 blue carbon ecosystems across the Victorian coastline (southeast Australia) to quantify total sediment stocks, variability across spatial scales, and estimate emissions associated with historical ecosystem loss. Mean sediment organic carbon (C org) stock (±SE) to a depth of 30 cm was not significantly different between tidal marshes (87.1 ± 4.90 Mg C org ha-1) and mangroves (65.6 ± 4.17 Mg C org ha-1), but was significantly lower in seagrasses (24.3 ± 1.82 Mg C org ha-1). Location (defined as an individual meadow , marsh, or forest) had a stronger relationship with C org stock than catchment region, suggesting local-scale conditions drive variability of stocks more than regional-scale processes. We estimate over 2.90 million ± 199,000 Mg C org in the top 30 cm of blue carbon sediments in Victoria (53% in tidal marshes, 36% in seagrasses, and 11% in mangroves) and sequestration rates of 22,700 ± 5510 Mg C org year-1 (valued at over $AUD1 million ± 245,000 year-1 based on the average price of $AUD12.14 Mg CO 2 eq-1 at Australian Emissions Reduction Fund auctions). We estimate ecosystem loss since European settlement may equate to emissions as high as 4.83 million ± 358,000 Mg CO 2 equivalents (assuming 90% remineralization of stocks), 98% of which was associated with tidal marsh loss, and what would have been sequestering 9360 ± 2500 Mg C org year-1. This study is among the first to present a comprehensive comparison of sediment stocks across and within coastal blue carbon ecosystems. We estimate substantial and valuable carbon stocks associated with these ecosystems that have suffered considerable losses in the past and need protection into the future to maintain their role as carbon sinks.