Community structure of floating raft macroalgae in Gouqi Island, Zhoushan Archipelago and its influence on the epiphytic preference of algae-dwelling amphipods.

We collected samples in the August (summer) and October (autumn) of 2022, as well as in the February (winter) and May (spring) of 2023 in the raft-style mussel aquaculture area on Gouqi Island in the Zhoushan Archipelago, aiming to analyze the structure of the raft-style algal community on Gouqi Island and its impact on the epibenthic preference of algae-dwelling amphipods (Gammaridea and Caprellidae). The results showed that: 1) The number of algal species and biomass on the nearshore side of the aquaculture area were higher than those on the offshore side, with seasonal fluctuations. The number of algal species was highest in summer (19 species) and lowest in winter (15 species). The species richness index was higher in summer (2.65) and lower in winter (2.38). 2) There were significant differences in the epibenthic density of Gammaridea and Caprellidae across seasons, with higher densities in summer and lower densities in winter. Gammaridea was predominantly distributed on the nearshore side of the aquaculture area, while Caprellidae was more abundant on the offshore side. 3) Both Gammaridea and Caprellidae preferred branching and foliar algae, with the foliar algae <i>Chondrus ocellatus</i> exhibiting the highest epibenthic density. Among the branching algae, Gammaridea tended to attach to the structurally complex and color-similar <i>Grateloupia okamura</i>, while Caprellidae preferred to attach to the rough-surfaced, hard-textured, and structurally stable <i>Corallina officinalis</i>. In addition to season, morphological characteristics such as branching number, surface roughness, and structural complexity were also major factors affecting the epibenthic preference of Gammaridea and Caprellidae. The study could contribute to understanding the ecological adaptation strategies of amphipods and provide theoretical basis and data support for ecological management and algal diversity conservation in raft-style aquaculture areas.