BLOOMS OF THE DIATOM GENUS PSEUDO-NITZSCHIA H. PERAGALLO IN BIZERTE LAGOON (TUNISIA, SW MEDITERRANEAN)

Abstract

The phytoplankton composition, including the potentially toxic diatom genus Pseudo-nitzschia, and related physico-chemical water properties were determined at four stations in Bizerte Lagoon (SW Mediterranean Sea) monthly, from March 2004 to March 2005. Total phytoplankton abundance was generally low (1.50–7.12 × 10⁵ cells L⁻¹), but peaked in April 2004 (19.4 ± 2.9 × 10⁵ cells L⁻¹), July 2004 (19.9 ± 11.1 × 10⁵ cells L⁻¹) and March 2005 (12.2 ± 4.8 × 10⁵ cells L⁻¹), and was dominated by Plagioselmis spp., Pseudo-nitzschia spp. and Thalassiosira spp., respectively. The contribution of Pseudo-nitzschia spp. to the total algal community was generally moderate (0.6–8.9%, when detected). The genus Pseudo-nitzschia was detected in 70% of the samples and thus appeared as a regular component of the phytoplankton that develops in the surface waters of Bizerte Lagoon. Distribution patterns of Pseudo-nitzschia species in the “delicatissima” group (< 3 μm in width) showed a strong seasonality and were correlated with summer conditions (when temperature, salinity and silicate concentration increased), although they were present during at least seven months of the year, at all stations. Pseudo-nitzschia species in the “seriata” group (> 3 μm in width), conversely, revealed a narrower spatio-temporal distribution and appeared uncorrelated with the environmental factors measured. The causative species of the July peak was identified by scanning electron microscopy as Pseudo-nitzschia calliantha. Two out of four isolates of P. calliantha from Bizerte Lagoon produced the neurotoxin domoic acid (causative agent of Amnesic Shellfish Poisoning) in batch culture. Our findings suggest the possibility of domoic acid contamination of bivalve molluscs in Bizert Lagoon, one of the main shellfish aquaculture areas in Tunisia, and therefore the need for continued vigilance.