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

Masafumi Kodama*, Tomohiko Kawamura, Kenta Nakamoto, Naoya Ohtsuchi, Jun Hayakawa, Takayuki Kanki, Takashi Kitagawa, Yoshiro Watanabe

*Corresponding author:

ABSTRACT: Macroalgal beds can be vertically differentiated into canopy and understory strata, and each can show a different phenology in algal biomass and morphology. Therefore, for epibenthic invertebrates, macroalgal beds provide both vertically and seasonally heterogenous habitats. Subsequently epibenthic assemblages can be predicted to show different patterns in seasonal dynamics reflecting the different effects of algal phenology between the canopy and understory. In the present study, we describe the seasonal dynamics of gammarid assemblages and algal phenology in the canopy and understory of a Sargassum bed in Otsuchi Bay, Japan, and compare the seasonal patterns between the two habitats. Gammarid assemblages showed distinctly different seasonal dynamics in abundance and species composition between the canopy and understory. In the canopy, gammarid abundance peaked in July, which was mainly explained by the fluctuation in abundance of the dominant tube-dwelling filter-feeder Jassa morinoi. In the understory, gammarid abundance peaked in August to September, clearly later than in the canopy, and was also dominated by tube-dwelling filter-feeders but different species such as Monocorophium uenoi or Gammaropsis japonicus. These differences in assemblage dynamics of gammarids can be explained by the Sargassum defoliation in late summer, with Sargassum defoliation decreasing the habitat space and complexity for gammarids in the canopy. In the understory, however the Sargassum defoliation can be considered to have increased 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 to epibenthic invertebrates inhabiting canopy and understory strata.