Physicochemical Factors Influencing Blooms of the non-toxic HABs species Chaetoceros spp. and Neoceratium lineatum in the Bizerte Lagoon (SW Mediterranean)

An intense phytoplankton bloom occurred in the Bizerte Lagoon in early spring, following heavy rainfall and pronounced freshwater supply that was accompanied by nutrient enrichment. This algal outbreak was associated with spectacular discolored water event. A weekly sampling was conducted from March to April 2012 at each depth from the surface down to 9 m in order to investigate the dynamics of this bloom in relation to physicochemical factors. During the sampling period, the development of a clear halocline and the high nutrient availability were favorable for the phytoplankton proliferation, reaching very high biomass (13 - 20 µg Chl a L-1) and abundance (> 107 cells L-1). The phytoplankton bloom was mainly caused by three species that succeeded one another over time. Chain-forming diatoms Chaetoceros (C. holsaticus and C. constrictus) showed firstly a dominance (6 – 10 ´ 106 cells L-1) coinciding with low salinity (14-20 PSU) and high nutrient concentrations, particularly of N and Si. Subsequently, the decrease in Si availability and the increase in salinity (30oPSU) resulted in a decline in diatom species in favor of the dinoflagellate Neoceratium lineatum, which gained the advantage (7 106 cells L-1) when N and P levels increased. Although the blooming species were non-toxic, they had harmful effects, as they were involved in fish and cuttlefish mortality. The study indicated that the Bizerte Lagoon is highly vulnerable to hydrological variability and nutrient enrichment, conditions that can trigger the proliferation of harmful phytoplankton species. This underscores the importance of implementing continuous monitoring programs to detect and manage harmful algal blooms at an early stage.