Reviews and syntheses: Hidden forests, the role of vegetated coastal habitats in the ocean carbon budget

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 km 2 , 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 CO 2 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 CO 2 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.

Methane Production by Seagrass Ecosystems in the Red Sea

“…Moreover, taking in account the range of global estimates for seagrass coverage, from 0.15 × 10 6 to 4.32 × 10 6 Km 2 (Duarte, 2017) and using the mean seagrass CH 4 production rate calculated by compounding our estimates with those available in the literature (105.8 ± 34.4 µmol CH 4 m −2 d −1 ), the global CH 4 production and emission by seagrass ecosystems could range from 0.09 to 2.7 Tg yr −1 . Because CH 4 emission by seagrass ecosystems had not been included in previous global estimates, the estimate provided here would increase the contribution of marine global emissions, hitherto estimated at 9.1 Tg yr −1 (EPA, 2010), by about 30%.…”

“…Seagrass ecosystems cover shallow coastal areas from all continents, except Antarctica, with an estimated global coverage ranging from 0.15 × 10 6 to 4.32 × 10 6 Km 2 (Duarte, 2017). Thus, due to their global coverage, high productivity and high organic matter content in their sediments supporting high microbial activity compared to adjacent bare sediments, seagrass ecosystems could represent a potential important source to be included in global CH 4 budgets.…”

Modeling the coastal ecosystem complex: present situation and challenges

“…An important aspect of coastal ecosystems that distinguishes them from open ocean ecosystems is the presence of benthic habitats. Vegetated coastal habitats, including seagrass beds, seaweed meadows, mangroves, and salt marshes, are highly productive (Duarte 2017;Mann 2000). Benthic microalgae are also important primary producers, and in some areas their contributions to total primary production are comparable to those of phytoplankton in the water column (Underwood and Kromkamp 1999).…”

New Tools to Identify the Location of Seagrass Meadows: Marine Grazers as Habitat Indicators

“…Keywords: blue carbon, ecosystem services, climate change mitigation, drone surveys, satellite tracking, animal movement, benthic habitat mapping BACKGROUND Seagrasses are among the most valuable ecosystems on earth, are of fundamental importance to human life and yet the lack of data on their distribution for much of the globe has limited the ability of scientists to truly quantify and understand their roles on global scales (Duarte, 2017). Each year seagrasses provide ecosystem services worth many trillions of dollars (Costanza et al, 2014).…”

“…At present there is high uncertainty around how much seagrass exists globally, especially in sub-tidal environments and particularly within the tropics. Current estimates of global seagrass area range from 150,000 to 600,000 km 2 , with no progress over the past two decades in narrowing this uncertainty band (Duarte, 2017). However, the potential global seagrass area determined by light regime, bathymetry, and seagrass light requirements is estimated to be 4,320,000 km 2 (Gattuso et al, 2006), potentially yielding a 35-fold increase of recent estimates of seagrass ecosystem services value.…”