Carbonate secreted by benthic algae from Bermuda (5 genera, 3 phyla) can be grouped into four types according to the delta 18 O and delta 13 C composition: a "heavy" and a "light" set for oxygen, and a "large-variability" and a "small-variability" set for carbon. The sets are not congruent. In one alga, different parts of the same skeleton belong to different oxygen sets. Vital effects, which are tied to developmental stages as well as to environmental response, are predominant in producing the observed isotopic variations.

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