Calcium carbonate production in coastal environments

Biological production of calcium carbonate in the oceans is an important process. Although carbonate is produced in the open ocean (pelagic, see Chapter 5), this chapter concentrates on production in coastal waters (neritic) because this contributes sediment to the coast through skeletal breakdown producing sand and gravel deposits on beaches, across continental shelves, and within reefs. Marine organisms with hard body parts precipitate calcium carbonate as the minerals calcite or aragonite. Corals, molluscs, foraminifera, bryozoans, red algae (for example the algal rims that characterize reef crests on Indo-Pacific reefs) are particularly productive, as well as some species of green algae (especially Halimeda). Upon death, these calcareous organisms break down by physical, chemical, and biological erosion processes through a series of discrete sediment sizes (Perry et al., 2011). Neritic carbonate production has been estimated to be approximately 2.5 Gt year-1(Milliman and Droxler, 1995; Heap et al., 2009). The greatest contributors are coral reefs that form complex structures covering a total area of more than 250,000 km2 (Spalding and Grenfell, 1997; Vecsei, 2004), but other organisms, such as oysters, may also form smaller reef structures.

Global climate change will affect carbonate production and breakdown in the ocean, which will have implications for coastal sediment budgets. Rising sea level will displace many beaches landwards (Nicholls et al., 2007). Low-lying reef islands called sand cays, formed over the past few millennia on the rim of atolls, are particularly vulnerable, together with the communities that live on them. Rising sea level can also result in further reef growth and sediment production where there are healthy coral reefs (Buddemeier and Hopley, 1988). In areas where corals have already been killed or damaged by human activities, however, reefs may not be able to keep pace with the rising sea level in which case wave energy will be able to propagate more freely across the reef crest thereby exposing shorelines to higher levels of wave energy (Storlazzi et al., 2011; see also Chapter 43).

Reefs have experienced episodes of coral bleaching and mortality in recent years caused by unusually warm waters. Increased carbon dioxide concentrations are also causing ocean waters to become more acidic, which may affect the biological production and supply of carbonate sand. Bleaching and acidification can reduce coral growth and limit the ability of reef-building corals and other organisms to produce calcium carbonate (Kroeker et al., 2010).