Skeletal extension, density and calcification of the reef coral, Montastrea annularis: St. Croix, U.S. Virgin Islands

Parameters of the annual and subannual skeletal growth of 61 Montastrea annularis corals, collected at a variety of shallow depth sites on reefs of St. Croix, U.S. Virgin Islands, are determined by X-radiography and scanning densitometry for each year in the IO-year period, 1970-1979. Extension (linear growth) of the coral skeleton is correlated negatively with bulk density (mass per unit volume) and positively with calcification (mass addition). Density and mass are slightly positively correlated. No one parameter, however, is a perfect predictor of another. At least two parameters, from which the third can be calculated, are required for complete description of coral growth. The variations of parameters of subannual density bands in this study do not convey information additional to those of annual bands. When compared to a pristine site in St. Croix, coral from Round Reefwithin Christiansted Harbor (a location of past dredging and sewage pollution) have equivalent (and high) extension but significantly lower density and calcification. On the south coast, corals from a location of major dredging activity in the past have relatively low extension and calcification. These growth anomalies are probably pollution related. Collections of corals from one south coast site may have been biased by hurricane effects to nonrepresentative samples. The island of St. Croix, U.S. Virgin Islands, is located in the northeastern Caribbean sea at 17°45'N and 64°52'W. Surrounding the island on most sides are coral reefs, some of which have been described (Ogden, 1972; Adey, 1975; Adey et aI., 1977). This study evaluates annual and subannual skeletal growth ofa major coral species, Montastrea annularis. from various reef areas. Relationships of the parameters-extension, density, and calcification-are investigated. Growth is typically considered an index of viability of an organism and for corals can be an important indicator oftheir state of health. 51. Croix is not highly developed, but sources of pollution do exist. We have found that coral growth in certain areas appears to reflect the presence of environmental degradation. A number of methods are available to measure skeletal growth of scleractinian corals and a variety of studies have been done (Buddemeier and Kinzie, 1976). We wished to evaluate natural in situ growth over a decade and used the X-radiography method (Buddemeier, 1978; Dodge and Vaisnys, 1980) to reveal annual and subannual skeletal density variations or bands (Knutson et al., 1972; Dodge et al., 1974; Macintyre and Smith, 1974; Hudson et aI., 1976). Width of growth increments measures extension of the coral skeleton. Bulk density measures the distribution of CaC03 within the skeleton. The mass of CaC03 deposited (over time) is the calcification (rate) of the coral. If any two of the parameters are known, the third can be calculated. A variety of studies have been done on the individual variability of at least two of these parameters (e.g., for extension: Weber and White, 1977; Weber et al. 1975; Glynn and Stewart, 1973; Isdale, 1977; Dodge et aI., 1974; Hudson, 1981; and for calcification: Goreau and Goreau, 1959; Jokiel and Coles, 1977; Coles and Jokiel, 1978; Bak, 1974; 1978). The inter-relationships and simultaneous variability of each of the parameters are only beginning to be understood (Dodge and Thomson, 1974; Buddemeier et aI., 1974). Barnes and Taylor (1973), Dustan (1975), and Baker and Weber (1975) have