Understanding intertidal seagrass meadow dynamics and the role of Zostera noltii as a blue carbon habitat

Distinct knowledge gaps concerning the intertidal seagrass Zostera noltii persist on pivotal environmental drivers contributing to variation in seagrass meadow size, location, structure and sedimentary carbon fluxes. This thesis addresses these gaps by characterising Z. noltii meadows in the southeast of England using key meadow descriptors. While sediment type and seagrass tissue nutrient enrichment were key environmental parameters, they did not fully account for the variation in seagrass meadow descriptors. Seasonal greenhouse gas (GHG) flux estimates and values from northern temperate regions were recently identified as priorities in blue carbon (BC) science. Carbon dioxide (CO2) and methane (CH4) flux of Z. noltii meadows were assessed across four seasons, capturing a full annual cycle. Net CO2 uptake of Z. noltii ranged from 0.77 - 2.33 mmolCO2 m-2 hr-1, of which 1-3% was offset by CO2-equivalent CH4 emissions. Z. noltii CO2 uptake was significantly higher than adjacent bare sediments, though in the lowest range of the seagrass global average (1.73 – 10.27 mmol CO2 m-2 hr-1). Z. noltii remained a net CO2 sink annually, however inclusion in carbon credit schemes should focus on multiple ecosystem benefits, beyond BC. Despite their pivotal role in coastal carbon biogeochemical cycling, microbial communities driving GHG fluxes are frequently overlooked, remaining understudied in seagrass ecosystems and seldom included in GHG research. Methanogen and methanotroph communities of Z. noltii and bare sediments were characterised alongside GHG flux measurements. The dominant methanogen in all sediments, Methanomassiliicoccus, had higher relative abundance during summer when CH4 flux was enhanced. Methanogen and methanotroph communities were comparable between seagrass and bare sediments, congruent to similar CH4 fluxes between habitats. Geographical location is thus critical in BC estimates, as regional characteristics may determine GHG-influencing microbial communities. Additionally, seasonal GHG measurements and interspecific differences in seagrass GHG fluxes are fundamental considerations for BC science.