Microbial driven greenhouse gas flux of the intertidal seagrass Zostera noltei, across a seasonal cycle

Seagrass meadows are important blue carbon habitats, due to their high productivity and large sedimentary carbon stocks. However, this can be biased when data from larger species, such as Posidonia oceanica, is extrapolated to global coverage. Carbon budgets of smaller seagrass species, such as Zostera noltei, have been seldom studied, despite these being one of the most common species found on North-West European coasts. In seagrass blue carbon studies, the inclusion of greenhouse gas (GHG) emissions is rare. This study addresses key knowledge gaps by analysing GHG (CO₂ and CH₄) exchange of the intertidal seagrass Z. noltei, in the southern North Sea, UK, across a full seasonal cycle. GHG exchange of Z. noltei meadows and adjacent non-vegetated mudflats (NVMF) were measured using novel in-situ closed-chambers, and the sedimentary microbial communities (notably, methanotrophs and methanogens) were characterised.

 

Overall, CO₂ fluxes of Z. noltei seagrass were not different to that of NVMFs. Seagrass respiration offsets a large proportion of the plant’s carbon sink capacity, and this was particularly noticeable in spring. When methane emissions were included in the carbon budget, both habitats have net zero carbon emissions. The inclusion of respiration and methane emissions, over multiple seasons, are highly important considerations that are often missed in GHG exchange studies, potentially causing overestimations of seagrass blue carbon, globally. This presentation will also discuss seasonal changes in carbon-cycling microbial communities and how they underpin the measured GHG flux. This pioneering research is of international importance with implications for blue carbon science and natural capital markets.