Abstract
Abstract. Vegetated coastal habitats, including seagrass and macroalgal beds, mangrove forests and salt marshes, form highly productive ecosystems, but their contribution to the global carbon budget remains overlooked, and these forests remain hidden in representations of the global carbon budget. Despite being confined to a narrow belt around the shoreline of the world's oceans, where they cover less than 7 million km2, vegetated coastal habitats support about 1 to 10 % of the global marine net primary production and generate a large organic carbon surplus of about 40 % of their net primary production (NPP), which is either buried in sediments within these habitats or exported away. Large, 10-fold uncertainties in the area covered by vegetated coastal habitats, along with variability about carbon flux estimates, result in a 10-fold bracket around the estimates of their contribution to organic carbon sequestration in sediments and the deep sea from 73 to 866 Tg C yr−1, representing between 3 % and 1∕3 of oceanic CO2 uptake. Up to 1∕2 of this carbon sequestration occurs in sink reservoirs (sediments or the deep sea) beyond these habitats. The organic carbon exported that does not reach depositional sites subsidizes the metabolism of heterotrophic organisms. In addition to a significant contribution to organic carbon production and sequestration, vegetated coastal habitats contribute as much to carbonate accumulation as coral reefs do. While globally relevant, the magnitude of global carbon fluxes supported by salt-marsh, mangrove, seagrass and macroalgal habitats is declining due to rapid habitat loss, contributing to loss of CO2 sequestration, storage capacity and carbon subsidies. Incorporating the carbon fluxes' vegetated coastal habitats' support into depictions of the carbon budget of the global ocean and its perturbations will improve current representations of the carbon budget of the global ocean.
Highlights
Accounts of the role of primary producers in the global oceanic carbon cycle traditionally focus on the role of planktonic photosynthetic organisms and ignore, altogether, the potential contribution of marine vegetated coastal habitats (e.g. Falkowski et al, 2000; Fig. 6.1 in Ciais et al, 2013)
Despite current uncertainties it is clear that future representations of the carbon budget of the coastal ocean should cease to ignore vegetated coastal habitats or assume that this component is lumped within the term “marine biota” present in current representations (e.g. Ciais et al, 2013), which is not the case, as the associated fluxes and pools are those corresponding to marine plankton
The important role of vegetated coastal habitats in the carbon budget, contributing 1 to 10 % of oceanic net primary production (Smith, 1981), 0.3 to 1/3 of the oceans’ biological pump and > 0.6 % to 2/3 of carbon burial in sediments is evident to scientists and policy makers and seems to be ignored only by global carbon budget modellers (e.g. Ciais et al, 2013), for whom these habitats continue to be hidden forests
Summary
Accounts of the role of primary producers in the global oceanic carbon cycle traditionally focus on the role of planktonic photosynthetic organisms and ignore, altogether, the potential contribution of marine vegetated coastal habitats (e.g. Falkowski et al, 2000; Fig. 6.1 in Ciais et al, 2013). The focus on Blue Carbon has driven attention to other aspects of the contribution of marine vegetated coastal habitats to the oceanic carbon budget beyond carbon burial in sediments, including export of organic carbon from the coastal to the open ocean (Dittmar et al, 2006; Barrón and Duarte, 2015; Barrón et al, 2014; Krause-Jensen and Duarte, 2016). I provide an overview of the extent, biomass and production of vegetated coastal habitats and the evidence for their role in the global carbon cycle and discuss how integrating their role in the context of the global ocean leads one to reconsider some of the elements of the status quo of the global ocean carbon budget (e.g. as represented in Fig. 6.1 in Ciais et al, 2013). I discuss how changes to marine vegetated coastal habitats derived from local impacts and direct human intervention and from the consequences of climate change would affect the contribution of vegetated coastal habitats to carbon budgets regionally and globally, and identify future research challenges
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