Calcium Carbonate Deposition By Coralline Algae And Corals In Relation To Their Roles As Reef-Builders
Thomas F. Goreau
Department of Physiology, University of the West Indies, Mona, Jamaica, W. I.
and Department of Marine Biochemistry and Ecology, New York Zoological Society, New York, N.Y
Annals New York Academy of Sciences
BACKGROUND
The scleractinian corals and the lithothamnioid algae which secrete large massive skeletons of aragonitic or calcitic CaC03 have for a long time been regarded as the main reef frame-builders and cementers (Gardiner, 1931; Ladd and Tracey, 1949; Ladd, 1961), the algal component being dominant in the building up of atoll reefs (Gardiner, 1903; Tracey et al., 1948). Much less well understood in the formation of reefs and their associated unconsolidated deposits is the part contributed by the smaller calcareous algae since these are not, except for some of the Halimedas, very conspicuous components of the reef biota. Nevertheless, a study of fine reef sediments indicates that these algae are much more important in this respect than has been realized.
Field investigations under way in Jamaica since 1960 show that over 70 percent of the total CaCO3 contained in the large reef systems is in the form of fine, unconsolidated sand deposited in thick beds over large areas adjacent to the living reef frame. The chief sand builders appear to be the small delicate calcareous algae, with Foraminifera, mollusks, and echinoderms also making important contributions; the corals and massive lithothamnioid algae are relatively insignificant as a source of fine detritus in most environments. Our studies have also pointed out that the majority of the sand building flora actually occurs in the reef itself, so that the framework zones (see below for definitions) must be the principal site of sand production, not the lagoons.
It is often very difficult to evaluate quantitatively the role played by a species in a reef community in terms of the rate with which it contributes CaCO3 to the system. This is due to the lack of correlation between the abundance, or standing crop of a species, and the volume of skeletal detritus it produces. Neither underwater population surveys nor sedimentary analyses give accurate information in this respect: biological surveys tend to overemphasize the importance of the standing crop, whereas sedimentological analyses deal with the end product of a complex chain of biological, chemical and hydrological events and ignore losses due to mass transportation and solution. The unifying principle that is missing is a rate parameter, i.e., the velocity with which calcium carbonate is formed by the different lime producing organisms under natural conditions, probably a far more important quantity than either standing crop or sedimentary mass.
In addition to their ability to form CaCO3, impregnated exoskeletons, reef-corals, and calcareous algae possess in common the capacity to photosynthesize. Indeed calcification and photosynthesis unfailingly occur together in all Recent reef-building organisms regardless of whether they are plant or animal; but, whereas the algae are true photoautotrophes, the reef-corals are carnivorous plankton feeders (Yonge, 1940) which have only secondarily acquired photosynthesis by their commensal association with zooxanthellae. The effect of xanthellar photosynthesis in corals is to stimulate CaCO3 deposition into the skeleton so that the rate of calcification increases in bright light, and falls by an average of ninefold when the corals are put into total darkness (Goreau, 1959b). In species which lack zooxanthellae, or in cases where the algae have been removed by growing the corals in darkness for a time, the calcification rates are low and unresponsive to changes in the light intensity (Goreau, 1961a; 1961b). Preliminary experiments on three calcareous algae showed that these calcify more rapidly than the corals and that skeletogenesis is less effectively controlled by the light intensity (Goreau, 1961a). In view of the need for more information on the calcification rates of reef-building algae, a comprehensive physiological and environmental study on CaCO3 production and photosynthesis was undertaken on 36 of the principal coralline algae inhabiting the reefs of Jamaica; only the first part of the work is presented in this paper, the ecological investigations are to be published elsewhere in greater detail.