Geographical variation in the growth rate of the hermatypic coral Porites in the Great Barrier Reef Province, Australia

Abstract

This thesis examines the growth rate of 452 colonies of the massive reef-building coral Porites from the Great Barrier Reef Province, Australia. The corals were collected from thirty-four different reefs, and from different zones within those reefs.

Five species of the genus were used in the project, each being a massive, hemispherical form, with a relatively smooth surface, small corallites, and a readily-identifiable point of origin of colonial growth.

The mean annual growth rate for each colony was measured using X-radiography of coral sections, and densitometry on the X-ray plates. The density variations found within the coral skeleton were used to identify periods of lunar and seasonal extension. Growth was measured radially, along maximum colony diameter.

The time of deposition of maximum density skeleton was found to differ with latitude, occurring in October/November in the south of the Province, but later, in April, north of Cooktown.

Colony growth rate was found to vary by a factor of two on a year to year basis, and that of similar colonies, varied, even under “uniform” conditions. Despite this, broader patterns of growth rate variation were found within the Province. Colony growth varied according to location on reef platforms, with maximum rates on the leeward reef flat and slowest growth on the more exposed central reef flat and windward crest areas. Growth was faster and more variable on the fringing reefs of the inner continental shelf than at the outer barrier, where rates were depressed and stable. Annual growth also varied with depth and according to species.

Examination of the spatial patterns of growth variability at each of the scales employed provides a background to the discussion of the relationships between scleractinian skeletogenesis and environmental and genetic factors. These findings are discussed in the context of modern reef investigations, in the light of their contribution to the study of reef development within the Province