Seasonal changes on planktonic diatom communities along an inshore-offshore gradient in the Gulf of Gabes (Tunisia)

We investigated the composition, biomass and cell size dynamics of the marine planktonic diatom species along a coastal-open sea gradient in relation to the hydrological characteristics, during four oceanographic cruises in July 2005, May-June 2006, September 2006 and March 2007 in the Gulf of Gabes. The study of the marine planktonic diatoms throughout the sampling period showed the presence of 40 different species belonging to 22 pennate and 18 centric diatom species. Centric diatoms were more abundant than the pennate ones; 56% and 44% of total diatom abundance, respectively. Diatoms were very abundant, representing about 60% of the total phytoplankton abundance, with an exception in July 2005 (19%) during which Dictyochophyceae were the most dominant group (41% of the total phytoplankton abundance). Diatoms, which were dominant in the coastal samples, mainly proliferate in the semi-mixing conditions (May-June 2006), whereas they declined in the offshore area, most likely due to silicate shortage. In this period, in spite of the high abundance of diatom planktonic cells, only diatom biomass was correlated with silica amount, proving that biomass was a better ecological bio-indicator than abundance. The results suggest that the marine planktonic diatoms taxa were generally adapted to specific hydrological structure. In fact, the dominance index of diatoms showed that the biodiversity of diatoms increased gradually along the coastal-open sea gradient except in May-June 2006 during which a slight decrease along the inshore towards offshore areas was observed. This index increase depended on a coastal-open sea distance during thermal stratification (July 2005 and September 2006) and mixing periods (March 2007).