Seasonal variations of plankton food web structure in the coastal water off Usujiri Southwestern Hokkaido, Japan

The planktonic food web structure in the subarctic coastal water off Usujiri southwestern Hokkaido, Japan was investigated from June 1997 to June 1999, based on seasonal biomass data of pico- (< 2 mu m), nano- (2-10 mu m), micro- (10-200 mu m) and mesoplankton (> 200 mu m), and path analysis using the structural equation model (SEM). In spring, microphytoplankton predominated due to diatom bloom, while pico-and nanophytoplankton predominated in the other seasons, except November and December 1997. The seasonal change in size distribution of heterotrophic plankton was almost similar to that of phytoplankton, and mesozooplankton biomass was high in spring. The path analyses suggest that the main channel in the microbial food web could vary according to phytoplankton size composition, indicating not only the classical food chain (microphytoplankton - copepods) but also the indirect route (microphytoplankton - naked dinoflagellates - copepods).