Protoceratium reticulatum in northern Japan: environmental factors associated with seasonal occurrence and related contamination of yessotoxin in scallops

Abstract

In 2003, an occurrence of the yessotoxin (YTX)-producing dinoflagellate, Protoceratium reticulatum, and resultant toxin concentration in scallops was monitored in Okkirai Bay, northern Japan. The occurrence peaked after 1 July and reached a maximum density (400 cells L -1 ) on 16 July. It occurred over a wide range of water temperature and inorganic nutrient concentrations and was tolerant of a broad range of environmental conditions. Hydrological parameters were monitored, and it was obvious that P. reticulatum flourished under low salinity (30.59-32.60) and occurred at highest density in the surface layer (0-5 m depth) where effects from rainfall were greatest. In addition, dinoflagellate density increase and decrease was well correlated with inflows of oceanic water into the bay. Thus, it can be assumed that the oceanic inflows may cause initial population increases (e.g. excystment or input of a 'seed population'), with resultant growth in rain-affected waters and subsequent spread throughout the entire bay via water movement. YTX and 45-OHYTX concentrations in scallops reached maximum levels (0.79 μg g -1 and 0.44 μg g -1 of digestive gland, respectively) 2 weeks after the maximum cell density of P. reticulatum, and high levels of the toxin continued for a month. Low levels of the toxin were detected even during periods when cells were not observed.