In general, reefs create a physical obstruction to existing circulation patterns as they develop and grow vertically; thus the growth of a reef can promote changes in the local physical-sedimentological conditions. As a reef grows, it develops a distinctive biological zonation that appears to be correlated with changes in the associated sedimentological regime of the reef. Thus some change should also be produced in the interreef organism-sediment associations. These changes should be more pronounced near the reef, where the perturbation in the circulation pattern is higher, and diminish gradually away from the reef. Such physical changes may be small and yet have a significant effect on the local biological habitat.

Archaeocyathid patch reefs of the lower Forteau Formation, southern Labrador, developed in the relatively quiet water environment of the Lower Cambrian epeiric sea. Even though the

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patch reefs only stood 1 to 3 m (3 to 10 ft) above the sea floor, it is obvious from the external morphology of individual Archaeocyathids and the number of overturned and restored colonies that the patch reefs encountered higher energy conditions as they grew. Thus, these Archaeocyathid patch reefs apparently created many small, discontinuous obstructions to currents and waves.

Two distinctly different faunas associated with the environments around these reef complexes show a similar change in dominance diversity (d). The faunal assemblages on the flanks of each complex illustrate a general decrease in the abundance of the dominant species or a more equitable distribution of individuals per species with distance from the complex. However, the composition of each faunal association remains consistent with distance from the complex. The gradual lowering of dominance diversity with distance from the complex could be related to a subtle but gradual change in the marine environment.

In each assemblage, the high degree of dominance close to the reef appears to approach a geometric distribution of the individuals per species. Faunal assemblages in which the distribution of the individuals per species approaches a geometric distribution are indicative of physically disturbed habitats. The decrease in dominance farther from the complex suggests that these areas could have been under less physical stress.

Currents diverted by a small patch reef would flow over and around this obstruction, thereby creating a zone of higher physical energy around the reef. In each of the lower Forteau reefs examined, the interreef faunal distribution could be related to a zone of higher physical disturbance directly around the reef. Thus, changes in the local physical-sedimentological environment produced by the reefs appear to have affected the ecological structure of the interreef faunas.

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