Ecological Roles

Contribution to primary production

A large proportion of the primary production (the creation of organic matter by plants from inorganic material like CO2 and sunlight during photosynthesis) in a coral reef comes from the contribution of benthic algae. Net primary production is variable ranging from 148-500 (g C m2 yr1) for algal turfs, 146-1095 (g C m2 yr1) for fleshy macroalgae, and 73-475 (g C m2 yr1) for crustose coralline algae. Planktonic microalgae and algal symbionts of scleractinian corals also contribute to reef productivity but to a much lesser degree. The organic matter (carbon) produced by planktonic micro-algae enters the reef food chain either by: (1) consumption by herbivorous fishes, crabs, sea urchins and zooplankton; (2) release of Dissolved Organic Matter (DOM) by the algae into the water column where it is consumed by bacteria that in turn may be consumed by a variety of filter feeders, or (3) export to adjacent ecosystems such as seagrass meadows, mangroves or to the sea floor by currents and tides. See further details on the energy flow through coral reefs in Chapter 7.

Nitrogen fixation and nutrient cycling

Filamentous blue-green algae like Calothrix living in algal turf communities (Fig. 15.2O) fix significant amounts of atmospheric (inorganic) nitrogen into ammonia, which is then used by the blue-green algae themselves to build organic matter. Because of the rapid growth rates of blue-green algae and intense grazing on turf communities, the organic nitrogen is then distributed throughout the reef, contributing to reef nutrition (Chapter 7). Macroalgae take up, store, and release nutrients, thereby contributing to nutrient cycling in coral reef ecosystems.


Many macroalgae make important contributions to the construction of the reef framework by depositing calcium carbonate (CaCO3). Crustose calcareous algae (CCA, e.g. Porolithon and Peyssonnelia) are important framework builders and framework cementers in coral reefs, whereby they bind adjacent substrata and provide a calcified tissue barrier against erosion. This process is particularly important on the reef crest environments of the GBR. Crustose calcareous algae are also important in deeper areas at the edge of the continental platform in the southern GBR (80-120 m), where they form large algal frameworks of several metres high. Deposition of calcium carbonate within the tissues of CCA (as calcite as well as high magnesium calcite) can be up to 10.3 kg CaCO3 m2 yr"1 in some parts of the GBR (e.g. Lizard Island).

Upright calcareous algae such as Halimeda, Udotea, Amphiroa and Galaxaura contribute to the production of marine sediments that fill in the spaces between corals. The white sand of beaches and reef lagoons is largely the eroded calcium carbonate skeletons of these algae. Calcium carbonate is deposited as aragonite in Halimeda with an estimated production of around 2.2 kg CaCO3 m2 yr"1. Calcification may be an adaptation to inhibit grazing (a defensive mechanism), resist wave shock, and to provide mechanical support.

Facilitation of coral settlement

Crustose coralline algae induce settlement and metamorphosis of coral larvae and a range of other invertebrates in the GBR, thus playing a critical role in reef resilience. Some evidence supports the idea that this interaction seems to be mediated by chemicals released by the alga.

Roles in reef degradation

Macroalgae play important roles in reef degradation, particularly in ecological phase shifts, where abundant reef-building corals are replaced by abundant fleshy macroalgae. Reductions in herbivory due to overfish-ing, and increases in nutrient inputs leading to eu-trophication (e.g. sewage and fertiliser), have been suggested as causes of increased abundance of fleshy macroalgae leading to coral overgrowth and reef degradation. Coral bleaching, crown-of-thorns starfish outbreaks, extreme low tides, coral diseases, cyclones, and so on result in coral mortality, providing an environment that is rapidly colonised by diverse algal communities (Fig. 15.2N). Such disturbances, and particularly those due to climate change (e.g. bleaching), may lead to an overall increase in the total amount of macroalgae (see Chapters 9 and 10 for further details). Dominance by thick mats or larger, fleshy macroalgae may contribute to reef degradation by overgrowing corals, inhibiting coral recruitment, contributing to coral diseases (e.g. Fig. 15.2P), and thereby decreasing the aesthetic value of reefs.


Endolithic algae that live within the skeletons of both healthy and dead corals as well as other calcareous substrates contribute to reef erosion and destruction. These algae are generally filamentous and microscopic but form a thin dark green band visible to the naked eye underneath the coral and crustose algal tissue (Fig. 15.2L). Some examples of carbonate-boring algae include the greens Ostreobium spp., cyanobacteria Mastigocoleus testarum, Plectonema terebrans, and Hyella spp. and some red algae. Endolithic algae penetrate and dissolve the calcium carbonate, weakening the reef framework and thus hasten other erosive activities. Studies at One Tree Island on the GBR have show rates of bioerosion by endolithic algae to range between 20-30 g m2 yr_1. For more information on bioerosion see Chapter 9.

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