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

In 2003, an occurrence of the yessotoxin (YTX)-producing dinoflagellate,Protoceratium reticulatum, and resultant toxin concentrationin scallops was monitored in Okkirai Bay, northern Japan. Theoccurrence peaked after 1 July and reached a maximum density(400 cells L^–1) on 16 July. It occurred over a wide rangeof water temperature and inorganic nutrient concentrations andwas tolerant of a broad range of environmental conditions. Hydrologicalparameters were monitored, and it was obvious that P. reticulatumflourished under low salinity (30.59–32.60) and occurredat highest density in the surface layer (0–5 m depth)where effects from rainfall were greatest. In addition, dinoflagellatedensity increase and decrease was well correlated with inflowsof oceanic water into the bay. Thus, it can be assumed thatthe oceanic inflows may cause initial population increases (e.g.excystment or input of a ‘seed population’), withresultant growth in rain-affected waters and subsequent spreadthroughout the entire bay via water movement. YTX and 45-OHYTXconcentrations in scallops reached maximum levels (0.79 µgg^–1 and 0.44 µg g^–1 of digestive gland, respectively)2 weeks after the maximum cell density of P. reticulatum, andhigh levels of the toxin continued for a month. Low levels ofthe toxin were detected even during periods when cells werenot observed.