BLUE CARBON – THE ROLE OF
OCEANS AS CARBON SINKS
Vegetated coastal habitats – mangrove forests, salt­marshes and seagrass meadows –
have much in common with rain forests: they are hot spots for biodiversity, they provide
important and valuable ecosystem functions, including a large carbon sink capacity, and
they are experiencing a steep global decline (Duarte et al., 2008, Duarte, 2009). Indeed,
the world is losing its coastal habitats four times faster than its rain forests (Duarte et al.,
2008, Duarte, 2009) and the rate of loss is accelerating (Waycott et al., 2009). However,
whereas society is well informed of the benefits and threats associated with rainforests,
there is a comparative lack of awareness on the status and benefits of vegetated coastal
habitats. This is perhaps because of a “charisma” gap, where these often submerged, out
of sight coastal habitats, are not as appealing to the public as their terrestrial counterparts
(Duarte et al., 2008). Yet, because of their similar functions and threats, coastal habitats
can be considered as blue carbon sinks.
BLUE CARBON SINKS excess production of mangrove forests, salt-marshes and sea-
One key function of vegetated coastal habitats is their role as grass meadows is buried in the sediments, where it can remain
carbon sinks. Benefiting from the excellent conditions avail- stored over millenary time scales (Mateo et al., 1997), thereby
able to support plant growth, vegetated coastal habitats rank representing a strong natural carbon sink. This is most evident
amongst the most productive habitats in the world, comparable in the case of seagrass meadows, which accumulate enough
in production to the most productive agricultural crops (Table 1, materials as to significantly raise the seafloor, forming mats
Duarte and Chiscano, 1999). Much of their production is used that can exceed 3 metres in depth.
to support ecosystem functions (Duarte and Cebrián, 1996).
However, blue carbon sinks are strongly autotrophic, which In addition to burying a fraction of their own production, blue
means that these ecosystems fix CO
2
as organic matter photo- carbon sinks reduce flow, alter turbulence and attenuate wave
syntheticaly in excess of the CO
2
respired back by biota (Duarte action (Koch et al., 2006), thereby promoting sedimentation
and Cebrián, 1996; Gattuso et al., 1998; Duarte et al., 2005a), and reducing sediment resuspension (e.g. Gacia and Duarte,
thus removing CO
2
from the atmosphere. Some of this excess 2001). Recent research has shown that the canopies of seagrass
carbon is exported and subsidises adjacent ecosystems, includ- meadows trap particles entrained in the flow, which lose mo-
ing open ocean and beach ecosystems (Duarte and Cebrián, mentum upon impacting on the leaves, thereby promoting the
1996; Heck et al., 2008; Bouillon et al., 2008). The remaining sedimentation of suspended material to the seafloor (Hendriks
33
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