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Abstract

Abstract Patch reefs are a significant component of the geologic record; thus an understanding of their modern analogs is important. Coral and foraminiferal communities of 32 randomly selected patch reefs from the lagoon of Glovers Reef Atoll, Belize (formerly British Honduras), were studied. Samples of patch-reef tops, flanks, and adjacent lagoon floor were also examined for quantitative differences in sediment composition and texture.

The grain-size distribution and sorting reflect the influences of wave energy and depth on the patch reefs. The uniformity of the sedimentologic patterns from one patch reef to another reflects the uniformity of these environmental parameters in the lagoon.

The foraminiferal assemblages, including a Quinquefocu/ina-D\scorb\dae-Archaias-Homotrema reef-top assemblage and a Quinqueloculina-Cribroelphidium-Archaias lagoon-floor assemblage, occur on patch reefs throughout the lagoon.

Although the foraminiferal assemblages and the sediment-distribution patterns did not vary significantly from one patch reef to another, there was considerable variation in the coral-algal assemblages. Three “end-member” coral-algal assemblages were recognized: (1) the Montastrea assemblage which tends to have a high degree of faunal zonation and is dominated by the species Montastrea annularis, Acropora cervicornis, A. palmata, and Diploria strigosa; (2) the Acropora assemblage which is less well zoned and is dominated by Acropora cervicornis, A. palmata, and Porites porites; and (3) the A\gae-Porites assemblage which is poorly zoned and is dominated by sparse colonies of Porites porites and Acropora cervicornis. The three assemblages are gradational with each other across the atoll. They change from Montastrea assemblages in the southern portion to Acropora assemblages in the central portion to Algae-Porites assemblages in the northwestern portion of the lagoon. These coral-algal assemblages mimic, in a general way, assemblages recognized on seaward reef slopes, but depth and wave energy, used to explain seaward zonations, cannot explain the distribution of these patch-reef assemblages. No other environmental differences were recognized within the lagoon that could explain these patterns. In fact, the uniform distribution of the foraminiferal assemblages and the similarity of the sedimentologic patterns from one patch reef to another argue against any such differences.

An alternative possibility is that the assemblages may reflect different stages of recolonization of a patch reef following some infrequent environmental change that caused the demise of the coral populations. The assemblages may reflect stages in coral-population successions: the Algae-Porites assemblage representing an early colonizing stage, the Acropora assemblage an intermediate stage, and the Montastrea assemblage a later mature stage. If this hypothesis is correct, the environmental change probably occurred most recently, or had its greatest effect, in the northwestern portion of the lagoon which is most distant from channels through the main reef tract. The southeastern portion of the lagoon is closer to channels through the reef; thus this region might be less affected or would return to normal more quickly during a period of environmental stress. However, care must be taken when interpreting fossil coral assemblages. These assemblages may be indicators of different environmental regimes or they may be indicators of environmental predictability within regions of otherwise similar environmental parameters.

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