No abstract

Seasonal changes in microcystin concentrations in a resident snail (Sinotaia histrica) and an edible clam (Corbicula sandai) in Lake Biwa were surveyed. To clarify both the accumulation and depuration of microcystins, experimental studies with microcystin were also carried out on the snail. In the field investigation, microcystin was detected from the hepatopancreas and intestine of S. histrica (up to 3.2 µg g Ϫ1 dry weight and 19.5 µgg Ϫ1 dry weight, respectively); however, no microcystin was detected in the hepatopancreas of C. sandai. In the laboratory experiment, the microcystin-LR concentration in the hepatopancreas of S. histrica reached a value of 436 µg g Ϫ1 dry weight on day 10 of 15 days of uptake, and a high value persisted despite a depuration period of 15 days. The depuration rate constant of microcystin and its biological half-life were 0.0828 day Ϫ1 and 8.4 days, respectively. These results indicate that S. histrica has a high ability to accumulate microcystin in its tissue. Because S. histrica is predated by fish and water fowl, it is likely to play an important role as a vector for microcystin in lakes with dense blooms of toxic cyanobacteria.

Spatial and temporal variation in Microcystis species composition and microcystin concentration, quantified by enzyme-linked immunosorbent assay and high-performance liquid chromatography, were investigated during a 3-year period (1998-2000) in the Northern Basin of Lake Biwa. The Northern Basin generally had a concentration of 5 microg L(-1) or less, except at station 1 (Nagahama Bay) from July to October during the study period. The maximum concentration at station 1 was 22.7, 35.9, and 22.0 microg L(-1) in October of 1998, 1999, and 2000, respectively. Eleven species of cyanobacteria were observed: Microcystis aeruginosa, M. ichthyoblabe, M. novacekii, M. wesenbergii, Oscillatoria raciborskii, Anabaena oumiana, A. affinis, A. flos-aquae, A. ucrainica, A. smithii, and A. crassa. Of these, M. aeruginosa and M. wesenbergii were the main components observed. A high concentration of microcystin in the lake water was mostly a result of variation in the relative amount of toxic M. aeruginosa rather than of the total Microcystis cell number. This was supported by the analytical results for isolated strains. Microcystis spp. cell density in the Northern Basin appeared to increase gradually over the course of the study. This is the first study to have surveyed the Northern Basin of Lake Biwa, which supplies drinking water to 14 million people and is the largest lake in Japan.