Seagrass meadows provide numerous ecosystem services and their rapid global loss may reduce human welfare as well as ecological integrity. In common with the other ‘blue carbon’ habitats (mangroves and tidal marshes) seagrasses are thought to provide coastal defence and encourage sediment stabilisation and surface elevation. A sophisticated understanding of sediment elevation dynamics in mangroves and tidal marshes has been gained by monitoring a wide range of different sites, located in varying hydrogeomorphological conditions over long periods. In contrast, similar evidence for seagrasses is sparse; the present study is a contribution towards filling this gap. Surface elevation change pins were deployed in four locations, Scotland, Kenya, Tanzania and Saudi Arabia, in both seagrass and unvegetated control plots in the low intertidal and shallow subtidal zone. The presence of seagrass had a highly significant, positive impact on surface elevation at all sites. Combined data from the current work and the literature show an average difference of 31 mm per year in elevation rates between vegetated and unvegetated areas, which emphasizes the important contribution of seagrass in facilitating sediment surface elevation and reducing erosion. This paper presents the first multi-site study for sediment surface elevation in seagrasses in different settings and species.

Vegetated marine habitats are globally important carbon sinks, making a significant contribution towards mitigating climate change, and they provide a wide range of other ecosystem services. However, large gaps in knowledge remain, particularly for seagrass meadows in Africa. The present study estimated biomass and sediment organic carbon (Corg) stocks of four dominant seagrass species in Gazi Bay, Kenya. It compared sediment Corg between seagrass areas in vegetated and un-vegetated ‘controls’, using the naturally patchy occurence of seagrass at this site to test the impacts of seagrass growth on sediment Corg. It also explored relationships between the sediment and above-ground Corg, as well as between the total biomass and above-ground parameters. Sediment Corg was significantly different between species, range: 160.7–233.8 Mg C ha-1 (compared to the global range of 115.3 to 829.2 Mg C ha-1). Vegetated areas in all species had significantly higher sediment Corg compared with un-vegetated controls; the presence of seagrass increased Corg by 4–6 times. Biomass carbon differed significantly between species with means ranging between 4.8–7.1 Mg C ha-1 compared to the global range of 2.5–7.3 Mg C ha-1. To our knowledge, these are among the first results on seagrass sediment Corg to be reported from African seagrass beds; and contribute towards our understanding of the role of seagrass in global carbon dynamics.

Purpose of Review We use the 'seascape' concept to explore how interactions between mangrove forests, tidal marshes and seagrass influence the storage of carbon in these ecosystems. Mangrove forests, with the other two 'blue carbon' habitats, are exceptionally powerful carbon sinks. Maintaining and enhancing these sinks is an emerging priority in climate change mitigation. However, managing any one ecosystem on its own risks is ignoring important contextual drivers of carbon storage emerging from its place in the seascape. We consider how interactions between these coastal habitats directly or indirectly affect the amounts of carbon they can store. Recent Findings The export of carbon from seagrasses may occur over hundreds or thousands of kilometres, much further than reported for mangroves or tidal marshes. Seagrasses may buffer mangroves from wave impacts, assisting forest regeneration. Trophic cascades supported by contiguous blue carbon habitat may limit excessive herbivory and bioturbation in them but evidence is limited. Summary Direct transfers of carbon between blue carbon habitats are common and are likely to enhance total carbon storage, but our understanding of their contribution to carbon stocks at the seascape level is elementary. There is evidence for indirect enhancement of carbon storage at the seascape by close association of habitats, mostly through the creation and maintenance of propitious conditions by one ecosystem for another. Protection from waves of mangroves by seagrass and protection from excess nutrients and sediment of seagrass by mangroves and tidal marsh are key mechanisms. There is little evidence or theory suggesting negative effects on carbon storage of one blue carbon habitat on another. Keywords Mangrove. Seagrass. Tidal marsh. Carbon. Seascape. Sequestration 'When we try to pick out anything by itself, we find it hitched to everything else in the Universe' (John Muir) My first summer in the Sierra,