On the coasts and the continental shelves (0 to 200 metre water depths) a large portion of the plankton biomass is not broken down in the water column but sinks to the sea floor. There, in part, the biomass is incorporated into the sediments. The shelf sediments are therefore much larger carbon reservoirs than the deep-sea sediments. More than 90 per cent of the permanent carbon burial occurs in the shelf sediments. On geological time scales, oil and natural gas are formed from the biomass in these sediments. A large proportion of human-induced greenhouse gas emissions results from the fact that, by extracting oil and gas, we remove carbon that was sequestered there long ago. We then burn the fuel and release the carbon into the atmosphere as carbon dioxide.
Vegetation-rich coastal ecosystems such as salt marshes, seagrass meadows and mangrove forests also play special roles in the carbon cycle of the sea. Although they cover only less than one per cent of the total marine area, they are responsible for a significant portion of the natural carbon sequestration in the sea floor, and are thus key components in the Earth’s carbon cycle.
These plant communities flourish in tidal and shallow-water areas and take up carbon dioxide from the surface waters as well as from the air. They subsequently store the carbon bound by photosynthesis predominantly in the subsurface – partly in their dense root systems, and partly directly in the coastal sediments as dead plant material (foliage, deadwood, etc.).
Because the marine meadows and forests also filter large amounts of suspended material out of the water and these particles are deposited between their stems and roots, the plant communities grow steadily upwards. Through the deposition of the particles abundant washed-in animal and plant material is incorporated in the sea floor. These two processes lead to an accumulation of large amounts of carbon beneath the salt marshes, mangroves and seagrass meadows. These deposits can be more than ten metres thick and they continue to grow as long as the ecosystems are healthy. In ideal situations they are preserved for hundreds, and sometimes even thousands of years.