Variability of blue carbon stocks in restored saltmarsh wetlands: Bay of Plenty, Aotearoa New Zealand

As anthropogenic pressures including urbanisation and climate change continue to pressure the natural environment, there is a great need to focus on solutions that both increase ecosystem resilience and mitigate climate change. Coastal wetlands are a “powerhouse” environment that have the potential to achieve this and much more. Among the variety of coastal wetlands, saltmarsh habitats are one of the most degraded environments globally, with a mere <10% estimated to remain in Aotearoa New Zealand. Many saltmarshes have been lost due to factors such as, conversion to pasture, and/or environmental degradation. As part of the conversation around saltmarsh restoration, nature-based solutions are a tool that is being increasingly considered when it comes to climate change mitigation and adaptation, due to the large potential saltmarshes possess to sequester CO2 alongside adapting to sea level rise such as by providing a space for flood water and for vegetation to slow down storm surges, alongside other environmental and societal benefits. This study investigates the blue carbon dynamics in restored and control saltmarsh habitats in the Bay of Plenty Aotearoa New Zealand, focusing on bulk density, concentration of carbon (%), Total Organic Carbon (TOC), Sediment Accumulation Rates (SAR), and Carbon Accumulation Rates (CAR). We focus on changes in blue carbon with time since saltmarsh restoration. Bulk density analysis revealed minimal variability among sites, with Matua Saltmarsh exhibiting the largest standard error (0.765 ± 0.057g cm3) and Wainui River Saltmarsh the lowest mean bulk density (0.633 ± 0.032g cm3). Carbon concentration was generally highly variable across sites, with Matua Saltmarsh exhibiting the highest mean concentration (5.83 ± 1.151%) and Wainui Repo Whenua the lowest (2.91 ± 0.26%). Total organic carbon stock decreased with depth across sites, with Matua Saltmarsh having the highest total organic carbon stock (1.21 ± 0.005g cm3 or 121.42 ± 0.534t ha-1) and Te Awa o Ngātoroirangi the lowest (0.69 ± 0.004g cm3 or 68.66 ± 0.401t ha-1). Carbon accumulation rates generally increased with restoration age, with Wainui River Saltmarsh exhibiting the highest sediment accumulation rate (11.57mm yr-1) and Matua Saltmarsh the highest carbon accumulation rate (161.87g m-2 yr-1 over the last 20 years). While no distinct trends in blue carbon stocks and concentration emerged between the restored and control environments, it is notable that restored saltmarshes consistently exhibited substantial carbon stocks compared to controls and global standards. Moreover, there were higher mean TOC figures within the first ~30 years post-restoration. These results align with expectations from previous studies, emphasizing the efficacy of saltmarsh restoration efforts in enhancing future carbon storage potential in Aotearoa. These findings underscore the need for comprehensive investigations into the roles of vegetation, sedimentation rates, microbial processes, alongside catchment hydrological conditions history. This thesis also highlights that long-term monitoring beyond the initial 20- 30 years post-restoration is crucial to comprehensively understand the trajectory of blue carbon dynamics. In addition, comparative studies accounting for diverse restoration techniques and assessments of broader ecosystem services delivered by saltmarsh restoration are essential. Addressing these factors will deepen our understanding of the specific mechanisms influencing blue carbon dynamics in Aotearoa's restored saltmarshes.
Type of thesis
The University of Waikato
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