Pelagic and epiphytic summer distributions of Prorocentrum lima and P. mexicanum at two mussel farms in the Gulf of St. Lawrence, Canada

Abstract

The variations in abundance of Prorocentrum lima (Ehrenberg) Stein, an epiphythic species implicated in diarrhetic shellfish poisoning (DSP), were studied during the summer of 1999 in the Magdalen Islands, an archipelago located in the southern Gulf of St. Lawrence, eastern Canada. This investigation was initiated as a result of an incident in 1998, when 20 people became ill and exhibited DSP-like symptoms following the consumption of blue mussels collected from one of the Magdalen Islands lagoons. P. lima was regularly found as an epiphyte on the epibionts growing on the mussel socks at the 2 aquaculture sites investigated between June and October 1999. P. lima abundance on the epibionts varied from 100 to 9600 cells g-1 dry wt epibiont, with maximum abundances observed in July and August. P. lima was found in low abundance in the water column as well as in the digestive glands of the mussels. There was no correlation between cell abundances in the digestive glands, the water column and the cells attached to the epibionts. At the same sites, P. mexicanum Tafall was identified for the first time in eastern Canada. P. mexicanum abundances were low throughout the summer and began to increase both in the water column and in mussel digestive glands by early September. A significant correlation was found between the P. mexicanum abundances in the water column and in the mussel digestive glands at both sampling sites. The identification of these 2 potentially toxic species in the St. Lawrence highlights the importance of Prorocentrum spp. as toxin producers in coastal waters, as well as the need for adequate monitoring of non-pelagic harmful algal bloom species (HABs).