Does uptake of Alexandrium fundyense cysts contribute to the levels of PSP toxin found in the sea scallop, Placopecten magellanicus?

Atlantic sea scallops, Placopecten magellanicus, in most areas of the Bay of Fundy, New Brunswick, Canada, have year-round concentrations of paralytic shellfish posioning (PSP) toxins greater than the regulatory concentration of 80 μg STX eq. 100 g−1 wet weight. Scallops (mean shell height of 10.7 cm, age 3–5 years) were collected by SCUBA and individually tagged near Parker Island, Bay of Fundy. Half were hung 2 m below the low tide water level and the remainder were placed on the bottom (11 m depth at low tide) under the scallops held at 2 m. Scallop, water and sediment samples were collected monthly for determination of concentrations of PSP toxins and Alexandrium fundyense. In October, 1993, mean concentrations of PSP toxins in digestive gland, and mantle were 3205 and 1018 μg STX eq. 100 g−1 wet weight, respectively. Eight months later (June 1994), PSP concentrations in digestive glands from the surface and bottom had declined to 504 and 682 μg STX eq. 100 g−1 wet weight, respectively, whereas those in the mantle had declined to 802 and 681 μg STX eq. 100 g−1 wet weight. During July 1994, A. fundyense concentrations observed at Parker Island and offshore were 320 cells l−1 and 14,200 cells l−1, respectively. Subsequently, toxin concentrations in surface and bottom scallop digestive glands increased to 12,720 and 11,408 μg STX eq. 100 g−1 wet weight, whereas concentrations in mantles increased to 2126 and 1748 μg STX eq. 100 g−1 wet weight, respectively. Concentrations of PSP toxins in these tissues in October 1994 were similar to those measured in October 1993. Concentrations of PSP toxin were less than the regulatory concentration in the gonads and non-detectable in adductor muscles of all scallops sampled. There were no statistically significant differences in profiles for uptake and depuration of PSP toxins in scallops held at the surface compared to those from bottom, suggesting that A. fundyense cysts at the concentrations found in the sediment (45 cysts cm−3) did not contribute significantly to the year-round presence of PSP toxins within scallop tissues. The year-round occurrence of PSP toxin is probably due to accumulation during summer blooms followed by a very slow rate of depuration.