Substantial role of macroalgae in marine carbon sequestration

Journal name:
Nature Geoscience
Volume:
9,
Pages:
737–742
Year published:
DOI:
doi:10.1038/ngeo2790
Received
Accepted
Published online

Abstract

Vegetated coastal habitats have been identified as important carbon sinks. In contrast to angiosperm-based habitats such as seagrass meadows, salt marshes and mangroves, marine macroalgae have largely been excluded from discussions of marine carbon sinks. Macroalgae are the dominant primary producers in the coastal zone, but they typically do not grow in habitats that are considered to accumulate large stocks of organic carbon. However, the presence of macroalgal carbon in the deep sea and sediments, where it is effectively sequestered from the atmosphere, has been reported. A synthesis of these data suggests that macroalgae could represent an important source of the carbon sequestered in marine sediments and the deep ocean. We propose two main modes for the transport of macroalgae to the deep ocean and sediments: macroalgal material drifting through submarine canyons, and the sinking of negatively buoyant macroalgal detritus. A rough estimate suggests that macroalgae could sequester about 173 TgC yr−1 (with a range of 61–268 TgC yr−1) globally. About 90% of this sequestration occurs through export to the deep sea, and the rest through burial in coastal sediments. This estimate exceeds that for carbon sequestered in angiosperm-based coastal habitats.

At a glance

Figures

  1. Map of the locations where macroalgal carbon storage has been reported.
    Figure 1: Map of the locations where macroalgal carbon storage has been reported.

    The types of macroalgae are indicated for observations from sediment traps that are in the water column, on the sediment surface and buried in sediments. Inset, the frequency distribution of the water depths of macroalgae observations, with the majority representing the deep sea (<1,000 m). All references of observations are available in Supplementary Table 1.

  2. Conceptual diagram of the pathways for export and sequestration of macroalgal carbon.
    Figure 2: Conceptual diagram of the pathways for export and sequestration of macroalgal carbon.

    Air bladders are common among brown algal taxa and facilitate their long-range transport (i). Langmuir circulation forms windrows of macroalgae (ii) and can force the algae to depths where water pressure makes the air bladders burst and the algae then sink. Macroalgal carbon can be sequestered either via burial in the habitat or by transport to the deep sea where it is sequestered whether buried or not (iii).

  3. Pathways for the sequestration of macroalgal carbon in the ocean.
    Figure 3: Pathways for the sequestration of macroalgal carbon in the ocean.

    Each step of the carbon flow from global macroalgal net primary production (NPP) to carbon sequestration (in blue) is supported by the literature or inferred by a difference between a total and subcomponents supported by literature (Table 1). The means (with 25 to 75% quartile ranges in parentheses) shown are derived from an uncertainty propagation analysis (Methods), except for those fluxes not conducive to carbon sequestration (all values are in TgC yr−1). As the estimates have been derived independently, their total does not necessarily match to the mean global NPP estimate. Grazing (33.6% of the NPP) and remineralization (37.3% of the NPP) in the algal bed are adopted from a previous budget7.

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Affiliations

  1. Department of Bioscience, Aarhus University, Vejlsøvej 25, DK-8600 Silkeborg, Denmark

    • Dorte Krause-Jensen
  2. Arctic Research Centre, Department of Bioscience, Aarhus University, Ny Munkegade 114, bldg. 1540, 8000 Århus C, Denmark

    • Dorte Krause-Jensen
  3. King Abdullah University of Science and Technology (KAUST), Red Sea Research Center, Thuwal 23955-6900, Saudi Arabia

    • Carlos M. Duarte

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