We examine biodiversity in Tarut Bay, Saudi Arabia, around the effluent of a refinery that has operated since 1945. The spatial study involved quarterly assessment of diatom biodiversity in 1976/77 at 24 sites. Taxonomic distinctness decreased significantly towards the refinery outfall, in contrast to the pattern for number of species, Simpson'sindex and Margalef'sspecies richness index. Ammonia-nitrogen concentrations and primary productivity (mg chlorophyll-a cm−1 d−1) also increased significantly towards the effluent, probably due to nearby sewage/nutrient enrichment.

The spatio-temporal assessment involved comparison of echinoderm biodiversity at eight sites in 1945 through 1947 and 1977. No significant temporal differences were observed in number of species, taxonomic distinctness or beta-diversity, nor was significant correlation detected between biodiversity and proximity to the refinery outfall. The absence of obvious impact is confirmed by statistical comparison of taxonomic distinctness for echinoderms in Tarut Bay with values for the whole Gulf.

Periphyton in Tarut Bay may be perturbed towards the refinery outfall, ‘positively’ when measured by Simpson's and other ‘absolute’ biodiversity measures, but ‘negatively’ according to taxonomic distinctness, a measure of ‘average’ properties. Hence, diatom persistence stability appears moderate to low. In contrast, the echinoderm fauna may exhibit high stability, explained by either high persistence (withstanding perturbation), or high resilience (perturbation experienced, but system reverted or acclimated to ‘normal’ Gulf state) over a time scale of 30 y.

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