Spatial variations of planktonic ciliates, predator-prey interactions and their environmental drivers in the Gulf of Gabes-Boughrara lagoon system

Spatial variability of ciliates in relation to environmental variables and abundance of heterotrophic prokaryotes, picoeukaryotes, nanoeukaryotes, microphytoplankton and mesozooplankton were studied in the Gulf of Gabes- Boughrara lagoon system in the surface layer and at the water-sediment interface between 9 and April 15, 2017. Results revealed a striking difference between the two ecosystems, with higher temperature and salinity and high availability of nitrite and total nitrogen concentrations in the Boughrara lagoon, probably generated by limited exchanges of water with the Gulf of Gabes associated with disturbed areas in the lagoon. Ciliates abundance was slightly higher in the Gulf of Gabes (16–19 × 102 cells l−1) than in the Boughrara lagoon (13–16 × 102 cells l−1), but the species richness was significantly higher in the Boughrara lagoon (40 taxa in the surface layer and 35 taxa at the water-sediment interface) than in the Gulf of Gabes (35 taxa in the surface layer and 33 taxa at the water-sediment interface). The ciliate community was characterized by the predominance of Spirotrichea. Tintinnids were more abundant than naked ciliates in both sampled ecosystems, except in a few stations. Some tintinnid species, such as Tintinnopsis radix, T. nana and T. parvula, showed high abundance, indicating their tolerance to environmental pollution. The dominant species T. beroidea, T. nana and Strombidium compressum showed significant relationships with nutrient concentrations, illustrating their affinity for highly eutrophic sites. Regression and correlation analyses suggest food dependence of Spirotrichea and tintinnids up on picoeukaryotes, whereas naked ciliates appeared more controlled by the availability of nanoeukaryotes and microphytoplankton. Metazooplankton abundance showed no significant relationships with ciliate abundance.