Effect of algal phenology on seasonal dynamics of gammarid assemblages: differences between canopy and understory strata in a Sargassum yezoense bed

Macroalgal beds can be vertically differentiated into canopy and understory strata, and each can exhibit different phenology in algal biomass and morphology. Thus, for epibenthic invertebrates, macroalgal beds provide both vertically and seasonally heterogenous habitats, and epibenthic assemblages can be predicted to show different patterns of seasonal dynamics reflecting the different effects of algal phenology between the canopy and understory. We described and compared the seasonal dynamics of gammarid assemblages and algal phenology in the canopy and understory of a Sargassum bed in Otsuchi Bay, Japan. Gammarid assemblages showed distinctly different seasonal dynamics in abundance and species composition between the 2 habitats. In the canopy, gammarid abundance peaked in July, which was mainly explained by fluctuations in abundance of the dominant tube-dwelling filter-feeder Jassa morinoi. In the understory, gammarid abundance peaked in August−September, clearly later than in the canopy, and was also dominated by tube-dwelling filter-feeders, but different species such as Monocorophium uenoi and Gammaropsis japonicus. These differences in assemblage dynamics of gammarids can be ex plained by the Sargassum defoliation in late summer, resulting in decreased habitat space and complexity for gammarids in the canopy. In the understory, however, Sargassum defoliation in creased the amount of detrital deposition, providing food sources and additional habitat for gammarids. This study demonstrates that algal phenology can have strong but different effects on epibenthic invertebrates inhabiting canopy and understory strata.