Calcium Carbonate Budget of a Fringing Reef on the West Coast of Barbados

Scoffin, T.; Stearn, C.; Boucher, D.; Frydl, P.; Hawkins, C.; Hunter, I.G.; MacGeachy, J.

The rates and effects of the erosion of a discrete fringing reef were determined so that when combined with figures for the rates of CaCO3 production for the same reef (Steam, Scoffin and Martindale, 1977) a budget for the CaCO3 of the reef could be established (Steam and Scoffin, 1977). Mechanical erosion of the reef edifice is negligible other than during heavy storms. Visual observations suggest that significant movement of grains occurs only during winter. Loose sediments on and around the reef were analyzed for grain size and constituent composition to reveal their origins. These analyses showed that the major portion of the sediment is derived from the breakdown of the primary framework and that bioerosion plays an important role in controlling the grain size, shape and texture of the particulate sediments. Excavation by bivalves, barnacles and worms contribute towards reef degradation, but the boring sponges are markedly the most effective of the boring organisms. The quantity of CaCO3 removed by borers was determined either by direct measurement with calipers and rulers for the thinly branched corals, Porites porites, or by totalling the voids seen in X-radiographs of slices of skeletons for the massive corals. Knowing the growth rates of the corals it is possible to calculate the percentage of their skeletons removed per year. It was concluded that approximately 24.9 ± 4.4 x 106 g CaCO3 are excavated from the reef annually by boring animals. The grazing organisms on the reef include worms, gastropods, parrotfish and sea urchins. Erosion by worms and gastropods is insignificant. Of the parrotfish only Sparisoma viride occurs in considerable numbers and feeds dominantly on the rocky substrates. The abundance and size of the fish were recorded, their gut contents analyzed and the rate of turnover of gut contents assessed. From these observations a figure of 0.4 ± 0.05 x 106 g/yr was deduced for annual removal of CaCO3 from the solid reef by parrotfish. Sea urchins, Diadema antillarum, are exceedingly abundant on the reef rocky substrates averaging 23 individuals per m^2. The test size distribution of the population was measured and the rate of CaCO3 fecal production for each size class was determined. The proportion of the reworked carbonate in the fecal waste was determined after petrographic analyses of the pellets. An annual grazing rate of the reef framework by Diadema of 97.5 ± 5.7 x 106 g CaCO3 was obtained. A quantity of reef sediment is incorporated back into the reef rock which later may be reworked by bioerosion. In order to assess the extent of inorganic incorporation of CaCO3 into the reef edifice buried reef rock was excavated and analyzed by SEM to record the extent of diagenetic alteration. These final studies indicated that the CaCO3 reincorporated and inorganically precipitated into the reef amounts to about 17% by weight of the coral skeletal carbonate, i.e. 29.8 ± 3.9 x 106 g/yr. The budget synthesis indicates a CaC03 production on the reef of 206 ± 10 x 106 g/yr and a removal from the reef of 123 ± 7 x 106 g/yr. The internal structure of the preserved reef is fundamentally shaped by the interaction of the growth and bioerosion processes.

Calcium Carbonate Budget of a Fringing Reef on the West Coast of Barbados

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