[Composition and spatiotemporal variation of macroalgae in large-scale raft culture area of Gouqi Island, Zhejiang, China].

To comprehensively understand the impacts of large-scale raft culture activities on the diversity and biomass of macroalgae, we collected samples monthly from July 2021 to June 2022 in the Gouqi Island mussel farming area (raft culture area) and the natural rocky intertidal zone (control area). By using indices such as Shannon diversity, Margalef richness, Pielou evenness and relative importance index, as well as redundancy analysis, we compared the responses of alpha diversity, growth cycle, biomass spatiotemporal patterns of macroalgae to environmental factors in both raft culture area and control area. The results showed that a total of 61 macroalgae species were collected in the raft culture area, belonging to 13 orders, 25 families, and 31 genera. There were 24 dominant macroalgae species annually, among which Jania decussato-ichotoma, Pachymenia carnosa, and Chaetomorpha aerea dominated in most of months. 47 species of macroalgae, belonging to 31 genera and 24 families in 13 orders, were collected in control area. There were 13 dominant species, but only Sargasum thunbergii dominated throughout the year. The vigorous growth period of macroalgae represented by Hypnea cervicis, Hypnea boergesenii, P. carnosa, Grateloupia sp., J. decussato-dichotoma, Ulva lactuca, Ulva fasciata and Sargasum horneri in raft culture area significantly increased. Shannon index and Margalef index of macroalgae in raft culture area (2.36±0.25 and 1.45±0.23, respectively) were higher than those in control area (1.85±0.32 and 0.96±0.39, respectively). The macroalgae biomass in raft culture area exhibited a notable monthly variation, with a mean biomass of (10374.53±4688.17) g·m^-2, which was considerably higher than that in the control area (3090.40±2230.33 g·m^-2). In terms of spatial distribution, the mean monthly biomass was higher in nearshore region of raft culture area (11466.12±227.65 g·m^-2) than that to the outer region (10498.29±1266.77 g·m^-2) and the middle region (9229.95±252.75 g·m^-2). The redundancy analysis indicated that transparency, salinity, dissolved oxygen, water temperature, turbidity, and ammonium nitrogen were significantly correlated to community structure. Those results indicated that large-scale raft culture area supported both higher diversity and biomass of macroalgae than intertidal zones. Meanwhile, dominant macroalgae such as J. decussato-ichotoma, P. carnosa and S. horneri presented extended periods of vigorous growth in artificial habitat. The conclusions could be taken into consideration on construction of raft macroalgal bed in marine ranching programs.