In October and November 2003 a bloom of the toxic dinoflagellate
Alexandrium catenella was observed in the North-east zone of Thau lagoon (French Mediterranean
coast). Sea water samples were collected every hours to evaluate
time-related variations of phytoplankton concentrations and to compare the
relative ratio of A. catenella versus other phytoplankton species during the outbreak.
From these observations, trials using recirculated sea water systems were
performed to: i) evaluate the physiological effects on oyster of increasing
proportions of A. catenella within a mixed microalgal diet where the non-toxic diatom
Thalassiosira weissflogii was present at concentration: 1500 cells ml−1, ii) compare the effect
of two temperatures (12 °C and 18 °C) on paralytic toxin accumulation
rates in oyster flesh by ion-pairing high performance liquid chromatography
(IP-HPLC) detection, iii) analyse toxin biotransformation during the
contamination process, iv) evaluate the role of the different types of
oyster tissue on the bioaccumulation mechanism.
The results showed: i) a significant effect of temperature increase on
clearance rate and toxin uptake, ii) no detectable time-related effects of
toxic algal food on pre-ingestion sorting or on toxin profiles in oyster
flesh, iii) either negative or positive effects of A. catenella concentrations on toxin
uptake (“threshold” effect), iv) high amounts of toxins in the digestive
gland, accounting for more than 80% of overall shellfish toxicity. The
daily amount of toxins (Qtox) taken up by each oyster was evaluated by
means of a global one-compartment model. However a two-compartment model
finally gave the best match with real contamination kinetics, since it
integrated both toxins sequestered in oyster tissues and toxins moving in
through the digestive tract.