Two groups of modern patch reefs exist off Veracruz, Mexico. Terrigenous sediments of the Rio Jamapa are being deposited between the two complexes. Longshore drift of these sediments is causing declining coral growth on the southern group (Anton Lizardo), whereas corals on the northern group (Veracruz) are thriving. These differences in coral vitality should be reflected in the water chemistry and microfauna. Environmental data (depth, salinity, pH, temperature, Eh, and dissolved oxygen) were collected and treated as random samples from populations whose normality was established by chi-square goodness-of-fit tests. Chi-square testing at the 0.01 level demonstrated that, out of the six sample populations, only depth, pH, and dissolved oxygen were distributed normally. Dep h, pH, and dissolved oxygen data were assembled into two populations representing the Veracruz and Anton Lizardo groups. This permitted us to test the null hypothesis that the variances of the respective populations were equal (F = 0.005 level). The variances were found to be equal. T tests (0.01 level) on the population-means of depth, pH, and dissolved oxygen of both Veracruz and Anton Lizardo groups disclosed that there is no significant difference between the two groups of reefs in terms of these characteristics. However, the mean-percent relative abundance of the most common ostracod species (Loxocorniculum tricornatum) on both reef complexes is significantly different. Ostracod species dominance also differs radically between the two groups. Therefore, ostracod species abundance an dominance are more sensitive indicators of reefal equilibrium than the environmental parameters themselves.

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