3. Ecosystem health

Reef building, referring to the growth of coral reef framework structures and associated carbonate rubble and sediments, occurs when net accretion of calcium carbonate by calcifying organisms on the reef exceeds net erosion.¹⁸⁶ Coral reefs are structures of bound and loose calcium carbonate (limestone) with relatively thin veneers of corals and other reef organisms, a variety of which contribute to reef growth.⁸⁸⁴ The organisms that contribute to the positive net carbonate budget of a coral reef are primarily the hard corals themselves,⁸⁸⁵ which build skeletons of layer upon layer of calcium carbonate crystals.⁸⁸⁶ Coralline red algae, certain types of green algae (such as Halimeda,⁸⁸⁷ Section 2.3.8), benthic foraminifera, sponges, echinoderms, and molluscs are also important contributors to the reef carbonate budget.^100,885 Crustose coralline algae are of particular importance for their role contributing to cementation of the complex and interwoven calcareous framework that makes up the structural foundation of coral reefs, and in hardening the reefs against mechanical breakdown.⁸⁸⁵ Microbes are also vital to the cementation process and contribute significant amounts of calcium carbonate to coral reef structures.^776,888,889 This microbial calcification may be highly sensitive to ocean acidification (Sections 3.3.2 and 6.3.2).⁸⁹⁰

Calcifiers in coral reef lagoons and other interreefal areas, such as benthic foraminifera and the green alga, Halimeda, produce carbonate material that can be transported and deposited onto nearby coral reefs ^885,887 by waves and currents or by the activity of species such as parrotfish.⁸⁹¹ In this way, lagoon calcifiers also contribute to the growth of consolidated reef structures and to the carbonate sediments of coral reefs and reef islands (Section 2.3.1).¹⁰⁰ Generation and accumulation of rubble beds is a core element of reef building. However, increasing coral rubble production, one of the expected outcomes of climate change on coral reefs,^34,186 may lead to persistence of unstable rubble beds that hamper coral recruitment⁸⁹² (Section 6.3.2).

Erosion of coral reef structures is both a constant process and an acute function of exposure to severe storms.⁵⁴⁴ It is driven by a combination of mechanical breakage due to waves and currents, chemical dissolution if carbonate chemistry is sufficiently disrupted, and bioerosion.^746,893 Both dissolution and bioerosion are expected to increase with ocean acidification.^894,895 Sea temperature rise and ocean acidification may already be impairing coral reef growth in the Region, and there are significant concerns for the future.^{739,748,760,895}

Calcification and reef growth is particularly important against a backdrop of rising sea levels (Section 3.2.5). Rates of calcification are influenced by the abundance of calcifying organisms, light ^885,896 and nutrient availability, sea temperature, water flow and the concentration of carbonate ions in sea water.^739,897 Calcification rates at Lizard Island exhibited declines ⁸⁹⁸ and subsequent slight recovery ⁷⁵¹ following cyclones in 2014 and 2015 and coral bleaching in 2016 and 2017. At One Tree Island, which was not affected by the 2016–17 coral bleaching event, ecosystem calcification rates in 2017 were found to be similar to their rates in the 1970s.⁸⁹⁹

Estimates of declining calcification rates differ considerably and are hampered by a lack of continuous long-term datasets that could resolve drivers of change over widely varying timescales.⁸⁹⁹ Since 2019, reported increases in cover of fast-growing corals (Section 2.3.5 and 2.4.4) may have had a positive impact on reef-building processes. However, the extent to which coral growth influences overall reef growth and how variable this is between reefs remains uncertain.^900,901

A general decline in calcification rates is likely to have occurred, due to expected ongoing impacts from ocean warming and acidification. However, in the past 5 years, documented recovery of some key reef-building corals may have moderated this projected long-term trend. Changes in some other reef-building processes, such as dissolution and bioerosion, may also have occurred, but they are only likely to become evident over longer time frames. Increasing coral rubble may be becoming an important driver of reef-building processes on some reefs.