OCEANS AND COASTS
Without the contribution of oceans and coastal ecosystems to global biological carbon
sequestration today’s CO
2
concentration in the atmosphere would be much larger than
it is. But the uptake capacity of oceans and coasts is both finite and vulnerable. Minimisa-
tion of pressures, restoration and sustainable use are management options that can help
these ecosystems maintain their important carbon management function.
The oceans play a hugely important part in both the organic and nutrient availability. Notable exceptions are upwelling zones,
inorganic parts of the carbon cycle. They contain dissolved in where cold nutrient-rich waters are brought to the surface,
them about fifty times as much inorganic carbon as is found in leading to abundant plankton growth. Phytoplankton here can
the atmosphere, as a complex mixture of dissolved carbon diox- form large-scale ‘blooms’ covering hundreds of thousands of
ide, carbonic acid and carbonates (Raven and Falkowski, 1999). square kilometres of the sea surface and influencing impor-
Carbon dioxide is considerably more soluble in cold water than tant ecological and carbon cycle processes. When remnants of
in warm water, and the relationship between the concentration dead plankton sink to the sea floor, organic matter from their
of carbon dioxide in the atmosphere and of dissolved inorganic biomass is buried as sediments exceptionally enriched in or-
carbon in the oceans is therefore heavily dependent on water ganic carbon – this transfer of carbon from surface waters (and
temperature and ocean circulation. Typically, cold surface waters therefore indirectly from the atmosphere) to the deep ocean
at high latitudes absorb large amount of carbon dioxide. As they floor and ultimately through subduction, into the earth’s crust,
do so they become denser, and sink to the sea-floor, carrying is referred to as the biological pump. Only 0.03% to 0.8% of
dissolved inorganic carbon with them and creating the so-called organic matter in the sea forms sediment (Yin et al. 2006), and
solubility pump. As the concentration (or partial pressure) of in order for this to be permanently sequestered, it is necessary
carbon dioxide increases in the atmosphere, so the oceans ab- that it is not recycled back into the trophic exchange system.
sorb more of it. Because of this, the oceans are believed to have
absorbed around 30% of human carbon dioxide emissions since The coastal zone (inshore waters up to 200 metres in depth,
industrialisation (Lee et al. 2003). The ocean is thus the second which includes coral and seagrass ecosystems) also has an im-
largest sink for anthropogenic carbon dioxide after the atmo- portant role in the oceanic carbon cycle. Various estimates in-
sphere itself (Iglesias-Rodriguez et al. 2008). One impact of the dicate that the majority of mineralisation and burial of organic
extra uptake of carbon dioxide has been a small but measurable carbon, as well as carbonate production and accumulation takes
acidification of the ocean over this period (Orr et al., 2005). place in this region, despite the fact that it covers less than 10%
of total oceanic area (Bouillon et al. 2008). Organic carbon burial
Dissolved inorganic carbon is translated into dissolved or par- here is estimated at just over 0.2 Gt C per year (Duarte 2002).
ticulate organic carbon in the open ocean through photosyn-
thesis by phytoplankton. In total, the oceans are estimated to Coastal wetlands have the potential to accumulate carbon at
account for just under half of global biological carbon uptake high rates over long time periods because they continuously
(Field et al. 1998). The majority of this fixed carbon is recycled accrete and bury organic-rich sediments. For example, Chmura
within the photic zone (the depth of the water column that is et al. (2003), calculated that, globally, mangroves accumulate
exposed to sufficient sunlight for photosynthesis to occur), sup- around 0.038 Gt C per year, which, when taking area of cov-
plying microorganisms that form the basis of the marine food erage into account, suggests that they sequester carbon faster
web. Photosynthetic activity in much of the ocean is limited by than terrestrial forests (Suratman 2008). However there is
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