The influence of environmental characteristics on the distribution of ciliates in two coastal areas in the Eastern Mediterranean Sea (Gulf of Gabes, Tunisia)

Spatial variability of ciliates in relation to chlorophyll a, nutrients, and environmental variables were studied in lagoon (Boughrara lagoon) and marine ecosystems (Ghannouch coast) in the surface layer and at the water–sediment interface during autumn. Results revealed a striking difference between the two ecosystems, with higher nutrient concentrations at the Ghannouch coast, except for ammonium. Ciliate abundance was five to six times higher in the lagoon (123.43–126.80×102cellsl−1) than at the marine coast (21.80–24.50×102cellsl−1) (ANOVA, P < 0.001). Loricate ciliate attained high abundance in Boughrara lagoon (88%–90% of total ciliates abundance) and decreased at Ghannouch coast (44%–51% of total ciliate abundance). The species richness of the loricate ciliates was higher in Boughrara lagoon (68 taxa at both surface layer and water–sediment interface) than at Ghannouch coast (32 taxa at the surface layer and 23 taxa at the water–sediment interface). Naked ciliates were poorly represented (10%–12% of the total ciliate abundance) in the lagoon stations but had substantial percentages (49%–56% of the total ciliate abundance) at Ghannouch coast. The spatial distribution patterns of ciliate communities at both sites were strongly influenced by nutrient heterogeneity. Small species (naked ciliates) are favoured by higher nutrient concentrations at Ghannouch coast, whereas the contribution of large species (loricate ciliates) increased with the lower nutrient concentrations in Bouhgrara lagoon. It has been suggested that nutrient changes could drive shifts in ciliate community structure and abundance. The deteriorated water quality of the Ghannouch coast likely induces the low loricate ciliate abundance and diversity and the higher relative abundance of naked ciliates compared to the less polluted Boughrara lagoon. •Ciliate diversity and abundance were higher in Boughrara than in Ghannouch.•Ciliate community was dominated by large species in Boughrara and by small ones in Ghannouch.•The spatial distribution of ciliates was strongly influenced by nutrients heterogeneity.