Species composition and spatial distribution of abundances and biomass of phytoplankton and ciliates during summer stratification in the Gulf of Hammamet (Tunisia)

Abstract

We studied the distribution of phytoplankton and ciliate communities in relation to environmental factors at 6 stations sampled between 28 and 31 July 2006 during the summer water stratification in the Gulf of Hammamet (Tunisia, eastern Mediterranean Sea). A strong thermocline was established at 30 m, and, on average, the N/P ratio was lower than the Redfield ratio (16), suggesting a potential N limitation. The inshore location was numerically dominated by dinoflagellates (55%) represented essentially by members of the generaProtoperidinium, Gymnodinium, and crypticScrippsiella trochoideaand on the offshore by diatoms (68%). The phytoplankton assemblage was largely dominated by the diatomsThalassionema nitzshioidesandRhizosolenia styliformis, while the ciliate community was numerically dominated by small taxa such asLohmanniella oviformis(6 × 102cells l−1) andUronema marinum(5.50 × 102cells l−1). The total phytoplankton abundance increased from the coastal area (5.26 × 102 ± 4.48 × 102) to the open sea (10.33 ×102 ± 28.06 × 102) and decreased from the surface to the bottom, inversely to the ciliate abundance. Total phytoplankton and abundances showed similar patterns. Total ciliate biomass decreased from the inshore (0.25 ± 0.58) to the offshore (0.06 ± 0.10) areas but increased from the surface to the bottom. The diversity index of both phytoplankton and ciliate communities showed a decrease with a coastal–open sea gradient. The relationships between phytoplankton and ciliates suggest planktonic micro-heterotrophs were implicated in the channelling of matter and energy through the microbial loop in the Gulf of Hammamet.