Seasonal changes in the planktonic food web off Cape Esan, southwestern Hokkaido, Japan

Abstract

The planktonic food web structure in the water off Cape Esan, southwestern Hokkaido, Japan was investi- gated at approximately one or two-month intervals from May 1997 through June 1999, based on biomass data of pico- (0.2-2 mm), nano- (2-10 mm), micro- (10-200 mm) and mesoplankton (200-20,000 mm). In spring (March-April), the water column was stratified and a spring diatom bloom occurred. In addition, microzooplankton and copepod biomass was high among the heterotrophic plankton in spring. In other seasons (summer, fall and winter), pico- and nanoplank- ton assemblages were dominant within the plankton community. These results indicate that the grazing food chain (micro-size phytoplankton - copepods) might be important and the indirect route (micro-size phytoplankton - microzoo- plankton - copepods) probably prevails in spring, while the microbial food web might be the predominant carbon flow route in other seasons.