Seasonal and spatial distribution of two Bosmina species (B-longirostris and B-fatalis) in Lake Suwa, Japan:: its relation to the predator Leptodora

In Lake Suwa, Japan, two Bosmina species, B. fatalis and B. longirostris, coexist and show different seasonal succession patterns. To determine the relative importance of factors affecting their succession, the seasonal and spatial distribution of the two Bosmina species and their relation to environmental variables were studied at six near-shore sites on Lake Suwa from May to October 2000, with measurements at each site made at 20 and 80m from the shore. We analyzed the seasonal and spatial distribution patterns of Bosmina, rotifers, and the major invertebrate predators, cyclopoid copepods and Leptodora kindti. A range of variables (water temperature, dissolved oxygen concentration, and chlorophyll a concentration) was also measured. Multiple stepwise regression analysis was performed between Bosmina and the measured biotic and abiotic variables. Two species of Bosmina showed a marked pattern of seasonal succession, B. longirostris being dominant in May and October and B. fatalis exceeding B. longirostris in summer. However, the densities of both species and their density ratio differed between sites. The ratio of B. fatalis density to total Bosmina density, which indicates the dominance of B. fatalis, was correlated positively with water temperature and Leptodora density and negatively correlated with dissolved oxygen and cyclopoid cope-pod density. The results suggest that invertebrate predators play an important role in regulating the succession of Bosmina species in Lake Suwa.