Relationships between environmental conditions and phytoplankton in the Mellah lagoon (South Western Mediterranean, Algeria), with an emphasis on HABs species

For the first time, a bi-monthly monitoring of environmental parameters (temperature, salinity, nutrients) and phytoplankton assemblages was conducted in the well-preserved Mellah lagoon ecosystem (South Western Mediterranean, Algeria). Sampling was performed at 3 stations in 2016. The objective was to study the evolution of the microphytoplankton community with a focus on HABs species in relation with the environmental characteristics.
In total, 227 microphytoplankton species belonging mainly to Diatomophyceae (160 species) and Dinophyceae (53 species), were inventoried in the Mellah lagoon. There was a clear dominance of diatoms (63%), compared to dinoflagellates (37%). Proliferations of some potentially toxic species such as Prorocentrum micans (7 200 cells.l-1) and Gymnodinium sanguineum (6 000 cells.l-1), were observed in summer and autumn respectively. Other HABs species as Alexandrium tamarense/catenella (1 350 cells.l-1) and Alexandrium minutum (1 420 cells.l-1), were present in winter and spring respectively.
Temporal differences in the qualitative composition and quantitative phytoplankton communities in the three stations are discussed in relation with environmental variables. Microphytoplankton populations in the Mellah show diversity and equitability indices of 3.04 bits.cell-1 and 0.94 respectively, showing a structured community during winter and spring. The rest of the year is characterized by unstable phytoplankton populations, where only opportunistic species develop.