HABs at it again!

Harmful algal blooms (HABs) are ever increasing and are attributed to a number of physical and chemical determining factors such as nutrient input, global warming and the physical environment, amongst many others. It is however difficult to identify individual environmental factors that are responsible for HABs. Toxin-producing dinoflagellates commonly found in HABs are responsible for diarrheic shellfish poisoning (DSP), paralytic shellfish poisoning (PSP) and amnesic shellfish poisoning (ASP) and so when HABs develop in seafood farms it is of particular importance to humans. Toxins can be found inside or outside of dinoflagellate cells and so using dinoflagellate abundance as a warning sign for HABs is not a particularly effective method for prediction. Turki et al. (2014) undertook a 5 year survey study looking at 3 possibly toxic dinoflagellate genera (Alexandrium spp., Prorocentrum spp., Dinophysis spp) and 1 diatom genus (Pseudo-nitzscia spp) in the Bizerte lagoon (Tunisia), an extremely important shellfish producing site for Tunisia. By analysing the three bio toxin groups (DSP, PSP, and ASP) in mussels and oysters produced at the site, the authors aim was to compare dinoflagellate abundance with their associated toxins. The site was chosen because it had experienced HABs repeatedly.

Phytoplankton and then mussel and oyster samples were collected from 4 stations (2 north (STL and CTM) and 2 south (FMB and MAT)) at 2 depths; surface and 4m depth. The [phytoplankton samples were preserved in iodine solution and the shellfish samples were frozen and stored for analysis. PSP and DSP analysis was carried out using mouse bioassay and the ASP was analysed using the Quilliam method.

The study showed that HABs were species specific with Alexandrium showing blooming during the winter months particularly in 2007-08. The other genera displayed very little temporal or spatial distribution. They found no correlation between HABs and toxicity in either mussels or oysters in relation to PSP, ASP and DSP. Some toxins were present in the mussel bioassay, but are not of significant oral toxicity. There was little correlation between toxic species and shellfish toxicity in either the mouse bioassay for PSP and DSP or in the detection of domoic acid for ASP. In terms of shellfish safety monitoring they propose that toxigenic species determination along with the use of shellfish as bio-indicators is the best method in testing for possible outbreaks of shellfish toxicity. They also conclude that mussels are likely better than oysters as bio indicators as oysters such as C. gigas can react to contamination by closing its valve to reduce tissue contamination.

This was an ambitious project set out over 5 years and although it maybe did not give them the answer they particularly wanted, it provide a good long term study with very real and useable data.  It provides a basis for further work and possible alternative hypotheses than have been proposed in other papers.

Reviewed Paper

Turki, S., Dhib, A., Fertouna-Bellakhal, M., Frossard, V., Balti, N., Kharrat, R. and Aleya, L., 2014. Harmful algal blooms (HABs) associated with phycotoxins in shellfish: What can be learned from five years of monitoring in Bizerte Lagoon (Southern Mediterranean Sea)?. Ecological Engineering, 67, pp.39-47.