Grazing pressure by amphipods on microalgae in Gamo Lagoon, Japan

Abstract

Grazing effects of amphipods were examined for 3 groups of diatoms (loosely attached cells, moderately attached cells and strongly attached cells) in Gamo Lagoon, Miyagi Prefecture, Japan. Diatom species with filamentous forms or low adhesive strength were susceptible to grazing, while those with stronger adhesion were less affected by grazing pressure. Seasonal trends for both amphipods and microalgae found on the macroalgae Gracilaria vermiculophylla showed high densities of amphipods and low densities of microalgae during warmer months (June to September) and vice versa in colder months (December to March). This inverse relationship between densities of amphipods and diatoms suggests a causal relationship and that effects of grazing pressure by amphipods on epiphytic diatom communities seem to be strong in Gamo Lagoon. A manipulative experiment (feeding pressure experiment) revealed that Melosira sp. and Nitzschia sp. were mostly selected by tube-dwelling species (Corophium uenoi and Grandidierella japonica), whilst Gyrosigma sp. and Melosira sp. were mostly selected by free-living species (Eogammarus possjeticus and Melita setiflagella).