Alexandrium catenella and Alexandrium tamarense in the North Lake of Tunis: bloom characteristics and the occurrence of paralytic shellfish toxin

Abstract

The dinoflagellate Alexandrium is thought to be the main species responsible for paralytic shellfish poisoning (PSP) throughout the world. The genus Alexandrium is present in unusually high diversity in the North Lake of Tunis, a productive Mediterranean fishing and shellfish collecting area. High abundances of Alexandrium tamarense (7 × 103 cells l–1) were recorded here in August 2007, and of A. catenella (5.5 × 04 cells l–1) in January 2008 for the first time. High densities of A. tamarense were associated with high surface temperature (29.3 °C) and high salinity (40.6), while A. catenella blooms occurred at lower temperature (15.9 °C) and salinity (36.7). Alexandrium tamarense cell densities correlated positively with NH4 + (47.10 μmol l–1), but negatively with NO3 − (0.55 μmol l–1). In contrast, A. catenella proliferated when NO3 − was high (2.15 μmol l–1) and NH4 + was relatively low (8.38 μmol l–1). To study the toxicity of A. catenella and A. tamarense, extracts from clams Ruditapes decussatus were analysed by mouse bioassay and Jellett rapid PSP test from July 2007 to April 2008. The December 2007 and January 2008 samples were positive, corresponding to blooms of Alexandrium catenella, but the PSP concentrations were below 80 μg STX equivalent 100 g–1 of shellfish meat.