|The growth of coral colonies and the importance of crustose coralline algae and burrowing sponges in relation with carbonate accumulation|
Bak, R.P.M. (1976). The growth of coral colonies and the importance of crustose coralline algae and burrowing sponges in relation with carbonate accumulation. Neth. J. Sea Res. 10(3): 285-337
In: Netherlands Journal of Sea Research. Netherlands Institute for Sea Research (NIOZ): Groningen; Den Burg. ISSN 0077-7579, more
Carbonate accumulation of hermatypic coral colonies is a function of at least four parameters: rate of calcification per unit of living coral surface area, growth and maintenance of the living coral surfaces, epigrowth of carbonate producers and occurrence and rate of excavation by burrowing organisms. Rates of calcification can differ not only between various species but also between different parts of a colony. The growth of coral colonies, expressed as increase in skeletal mass, of 5 species (Madracis mirabilis, Acropora palmala, Agaricia agariciles, Montastrea annularis and Meandrina meandriles) has been measured by means of an underwater weighing apparatus. The growth of series of colonies, which have been surveyed monthly for periods up to 26 months, can be mathematically described by exponential functions. Growth in mass of coral colonies is usually checked by either damage or by decrease in growth rate of the living tissue. The first is noticed when the growth of series of chosen, undamaged corals is compared with growth of unsorted colonies. The second is observed in species in which the calcification rates per unit of living surface area remain constant but the angle of the slope of the growth curve decreases over long term periods. The water depth where optimum calcification occurs differs in coral species. In the present study it was found that Meandrina meandrites reflects the common notion that corals grow more slowly in deep (25 m) as compared with shallower water (6 m). Agaricia agaricites displayed the opposite phenomenon as calcification rates per unit of living tissue are greater on the deep (24 m) than on the shallower reef (13 m). The most abundant calcifying epigrowing organisms in Curaçao are crustose coralline algae. Although these algae grow more vigorously in shallow water (3 m), in the deeper reef (25 m) the weight increment, due to the accretion of the calcareous thalli in situ, is greater. This is caused by the much higher grazing pressure by herbivorous organisms in the shallow reef as compared with that in the deep reef. Maximum accretion rates are 44 g/m² month. In shallow water the activities of herbivores favour the continuous presence of opportunistic species. Sections through coral colonies from depths down to 25 m showed excavation of skeletons to be limited. Sections through Montastrea annularis colonies (15 m) showed 1.4% ofthe section area to be burrowed. Sections through colonies of Meandrina meandrites (25 m) displayed 1.7% excavation. Clionid sponges appeared to be the most abundant burrowing organisms. The excavating powers of Cliona peponaca were measured as the decrease in weight of infested coral slabs. These slabs were exposed to the sponges at different depths on the reef for a one year period. Weight decrease was not depth dependent and amounted up to maximum values of about 3 kg per year per m². This is a lower figure than previously estimated which, at least partly, will be caused by the long duration of the experiment minimizing the impact of initial sponge colonization. Actual rates of excavation as well as growth of calcareous epigrowth will be strongly restrained by competition with other benthic organisms. In the coral species employed in this study, the substraction and addition of carbonate material both do not exceed a small percentage (1 to 2%) of the carbonate contribution generated by the living coral tissue. It appears that often the increase in size, resulting from the continuous growth of corals, must be considered as a relative weakening of the base of coral colonies. As such, growth can be as limiting to the survival of individual colonies as the destructive activities of burrowing organisms.