The capacity of the world’s coastal ecosystems to sequester carbon dioxide (CO2), termed “Blue Carbon,” has been a major focus of research in recent decades due to its potential to mitigate climate change. Vegetated coastal ecosystems such as mangroves, seagrass beds, and salt marshes represent a global area that is one to two orders of magnitude smaller than that of terrestrial forests, yet their contribution to long-term carbon sequestration is much greater per unit-area, in part because of their high productivity and efficiency in trapping suspended matter and associated organic carbon. Despite the value that Blue Carbon (BC) systems offer in sequestering carbon, as well as providing numerous other goods and services, these habitats are being lost at critical rates and require urgent action in order to prevent further degradation and loss. Recognition of the carbon sequestration value of vegetated coastal ecosystems provides a strong argument for their protection and restoration, and global efforts are now underway to include BC ecosystems into global carbon offset budgets, focusing on their optimal management to optimize CO2 sequestration and minimize CO2 emissions. Here, BC was investigated in the Cowichan Estuary in relation to habitat type (salt marsh, eelgrass meadow, non-vegetated mudflats, and oyster shell beds), and habitat degradation. Stored organic carbon (OC) and inorganic carbon (IC) were quantified in the top 20 cm depth of sediment cores, as well as in eelgrass and salt marsh vegetation, and then extrapolated to the areal extent of each habitat type based on a high resolution 1:12,000 scale base map of the estuary. Rates of sedimentation and carbon sequestration were quantified in each habitat type using 210Pb radiometric dating, and organic matter (OM) sources and quality were assessed in each habitat type using δ13C, C/N ratios and photopigment content in the sediments. A particular focus on the lower intertidal zone allowed us to examine the potential impact of industrial activity (log transport and storage) on the estuary’s capacity for carbon storage, as a result of a reduction of suitable habitat for eelgrass and microphytobenthos (MPB). Additionally, IC was quantified in aboveground oyster shell beds and buried oyster shell to assess inorganic storage. Finally, potential valuation of Blue Carbon in the Cowichan Estuary was investigated by comparing carbon sequestration to provincial greenhouse gas (GHG) emission equivalents as well as carbon sequestration in B.C. forests. We found that the salt marsh was the most important carbon reservoir, with a mean per-hectare sediment organic carbon (SOC) stock of 49.1 ± 19.9 Mg C ha-1, total ecosystem carbon stock (TECS) of 5443.75 Mg C, and carbon accumulation rate of 74 ± 23 g C m-2 yr-1. In the other habitats, we found SOC stocks and TECS respectively 19.1 ± 3.78 Mg C ha-1 and 3651.6 ± 72.3 Mg C in the upper mudflats, 16.9 ± 4.36 Mg C ha-1 and 1058.85 Mg C in the lower mudflats, 17.9 ± 1.21 Mg C ha−1 and 324.57 Mg C in the eelgrass meadow, and 9.43 ± 1.50 Mg C ha-1 and 59.4 Mg C in the oyster beds. The eelgrass meadow had a carbon accumulation rate of 38 ± 26 g C m-2 yr-1, while the mudflats could not successfully be dated due to erosion and/or mixing. Furthermore, the salt marsh contained the highest proportion of recalcitrant, terrestrial-derived root material which was more protected from hydrodynamic forces compared to other habitats. No pattern differences were observed between the carbon reservoirs or bulk properties of the log boom area (lower mudflat) compared to the upper mudflat, and thus there was no evidence that the log booms significantly decrease carbon sequestration in the areas where they make frequent contact with the seafloor. However, decreased chlorophyll a (chl a) concentrations in the lower mudflat sediment suggests a possible detrimental impact on microphytobenthos in addition to preventing the recolonization of the seagrass Zostera marina (Z. marina). Carbon stocks in the eelgrass meadow were similar to those of the mudflats. These carbon stocks were lower than global averages but consistent with those recently reported in low Z. marina meadows in the Pacific Northwest. Evidence of significant eelgrass vegetation outwelling necessitates further investigation to elucidate the degree to which these primary products are being decomposed or buried elsewhere in the estuary or open ocean. Since approximately half of the historical salt marsh habitat is currently reclaimed for agricultural and industrial use, consideration should be given to the role of the marsh system as a carbon reservoir in future land-use policy in the Cowichan Estuary. / Graduate / 2022-04-30

Coastal ecosystems comprise a relatively small area of the ocean, yet they play a disproportionate role in greenhouse gas (carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O)) and nutrient cycling. Vegetated coastal ecosystems (e.g., mangroves, salt marshes, and seagrasses) are key drivers of coastal greenhouse gas and nutrient cycling because of their environmental characteristics (e.g., shallow depths, organic matter rich sediments, etc.). My dissertation addresses the role of vegetated coastal ecosystems in greenhouse gas budgets and biogeochemical cycling. In Chapter 1, I conducted a meta-analysis to quantify the global emissions of CH4 from mangrove, salt marsh, and seagrass ecosystems. Here I show that mangrove ecosystems contribute the most CH4 out of these vegetated areas to the global marine CH4 budget. Further, while a well-known negative relationship between salinity and CH4 fluxes exists for salt marshes globally, this relationship does not hold for mangrove or seagrass meadows, suggesting that other environmental drivers are more important for predicting CH4 fluxes in these ecosystems. In Chapter 2, I present in situ fluxes of CH4 and N2O across the sediment-water interface as well as air-sea fluxes in seagrass meadows and adjacent non-vegetated sediments in two temperate coastal lagoons. Here I demonstrate that seagrass meadows can be sources or sinks of CH4 and that N2O uptake can enhance carbon sequestration in seagrass meadows by ~10%. In Chapter 3, I quantify fluxes of dissolved inorganic carbon, nitrogen, and phosphorous across the sediment-water interface in seagrass meadows and adjacent non-vegetated sediments in the same two coastal lagoons. I found that both seagrass and non-vegetated sediments exhibited dissolved inorganic carbon emission and denitrification, and that dissolved inorganic phosphorous fluxes varied by site and not with vegetation presence. This dissertation highlights the dynamic role coastal ecosystems play in biogeochemical cycling and the importance of vegetated coastal ecosystems in coastal greenhouse gas budgets. / 2024-10-03T00:00:00Z

As the anthropogenic emissions of greenhouse gases (GHGs) have increased and received moreattention, the need for climate mitigation solutions has become more urgent. Today, several methodologies for carbon accounting exists, as well as the possibility to offset emissions by buying carbon offsets. All these methodologies directly or indirectly spring from LCA but have developed to metrics considering only climate and with a dissatisfying sustainability performance in other aspects. Four carbon accounting methodologies (VCS, Gold Standard, GHG Protocol, XPRIZE Carbon Removal) have been examined and compared, showing differences in the approach to quantification and to more holistic sustainability aspects. They have also been analysed in comparison to the QU.A.L.ITY framework proposed by the European Commission, which aims to ensure the quality of carbon removals and prevent greenwashing. The results showed a lack of coherence between the carbon offset standards as well as a low coherence with the criteria in the proposed framework and lack of holistic sustainability perspectives. Algae are photosynthesizing organisms fixating CO2 in the same process as terrestrial plants. They are fast growing and does not require land or much maintenance, which is why seaweed aquaculture has been suggested as a method for carbon removals. This have given birth to a plethora of start-ups aiming to sell carbon offsets from systems based on macroalgae, here called “seaweed offsets”. Nine companies/projects were identified and examined. The results showed that most of the companies used systems involving cultivation, while one company use wild seaweed. The two largest techniques for sequestration of the carbon fixated in the biomass were 1) sinking the seaweed into the deep sea and 2) making biochar from the biomass. However, there are not yet any standards covering this type of carbon offset methodologies and these offsets are thus not certified and the results also show that the efficiency of these activities is highly unreliable. Large uncertainties remain regarding the net carbon removal, sequestration, and potential ecological impacts. These uncertainties and knowledge gaps also cause the suggested methods for seaweed offsetting to clash with the QU.A.L.ITY framework. / I takt med att de antropogena utsläppen av växthusgaser har ökat och fått mer uppmärksamhet har behovet av lösningar för att begränsa klimatförändringarna blivit mer brådskande. Idag finns flera metoder för koldioxidredovisning och -beräkning, liksom möjligheten att klimatkompensera genom att köpa klimatkompensation. Alla dessa metoder här rör direkt ellerindirekt från LCA men har utvecklats till mått som endast tar hänsyn till klimat, och med en otillfredsställande hållbarhetsprestanda i andra aspekter. Fyra metoder förkoldioxidredovisning (VCS, Gold Standard, GHG Protocol, XPRIZE Carbon Removal) har undersökts och jämförts, vilket visar skillnader i tillvägagångssättet för kvantifiering och förmer holistiska hållbarhetsaspekter. De har också analyserats i jämförelse med ”QU.A.L.ITY”-ramverket som föreslagits av Europeiska kommissionen, vilket syftar till att säkerställa kvaliteten på lösningar som skall ta bort CO2 ur atmosfären och förhindra greenwashing. Resultaten visar på bristande samstämmighet mellan klimatkompensationsstandarderna samt låg överensstämmelse med kriterierna i det föreslagna ramverket och brist på holistiskahållbarhetsperspektiv. Alger är fotosyntetiserande organismer som fixerar CO2 i samma process som markväxter. De växer snabbt och kräver inte mark eller mycket underhåll, vilket är varför algodling har föreslagits som en metod för att minska halten CO2 i atmosfären. Detta har gett upphov till en uppsjö av nystartade företag som syftar till att sälja koldioxidkompensation från system baserade på makroalger, här kallade "tångkompensationer" (seaweed offsets). Nio företag/projekt identifierades och granskades. Resultaten visar att de flesta av företagen använder system som involverar odling, medan ett företag använder vild tång. De två största teknikerna för att binda kolet fixerat i biomassan var 1) sänka tången i djuphavet och 2) göra biokol av biomassan. Det finns dock ännu inga standarder som täcker denna typ av klimatkompenseringsmetoder och dessa kompensationer är därför inte certifierade. Resultaten visar också att effektiviteten i dessa system är mycket otillförlitlig. Storaosäkerheter kvarstår när det gäller netto upptag av CO2, lagring och potentiella ekologiska effekter. Dessa osäkerheter och kunskapsluckor gör också att de föreslagna metoderna för tångkompensation kolliderar med QU.A.L.ITY-ramverket.