Carbon sequestration processes in tropical seagrass beds

Abstract: Seagrass meadows may play a substantial role in climate change mitigation as they are capable to sequester and store substantial amounts of anthropogenic carbon in plant biomass and, more importantly, in their underlying sediments. In this PhD thesis, the carbon-burial potential was assessed by quantifying the amount of organic carbon stored in different seagrass meadows, each dominated by one of the four major seagrass species in the Western Indian Ocean region. Impacts of anthropogenic disturbances on biomass carbon allocation, greenhouse gas emission (methane and nitrous oxide) and production of sulphide were investigated in Chwaka Bay, Zanzibar. The findings showed that east African seagrass meadows generally have high carbon sink capacity. The storage of sedimentary organic carbon, however, varied among seagrass habitats and across sites, and was up to five-fold higher in seagrass sediment to those of nearby unvegetated sediments. Seagrass meadows in eutrophicated sites had higher sedimentary organic carbon content, and substantially higher emission rates of nitrous oxides and methane, compared to more pristine meadows. Disturbances in terms of shading and simulated grazing of seagrass affected several processes, with major decreases in seagrass primary productivity, net community production and biomass carbon, in turn influencing seagrass carbon sequestration as well as stimulating anaerobic microbial processes. In addition, production of sulphide in the sediment and methane emissions from the sediment surface increased significantly when disturbed. At present, seagrass meadows in the Western Indian Ocean have high carbon sink capacity. This important ecosystem service is, however, highly threatened due to regional anthropogenic pressure, which may change the role of blue carbon rich habitats, such as seagrass meadows, from being a sink to a source of greenhouse gases.