Community structure of epiphytic macroalgae on mussel culture rafts in Gouqi Island

Macroalgae that grow naturally on the artificial rafts of shellfish culture, have attracted much attention due to their high coverage, high biodiversity, and large biomass, as well as the following ecologica side effects of the formation of amphipods such as Gammaridea and Caprellidae, and the microbial on the leaves communities. In order to understand the characteristics of the species composition, distribution pattern and seasonal variation of macroalgae in the mussel raft culture area, the epiphytic macroalgae adhere to the mussel rafts around Gouqi Island in spring (May), summer (August), and autumn (November) of 2018, and winter (February) of 2019 were sampled and analyzed. The results showed that 49 species of epiphytic macroalgae belonged to 11 orders, 20 families, and 33 genera. In terms of seasonal variation, the number of species was the highest in autumn (29 species), the lowest in spring (20 species), with an average of 23 species; the seasonal variation of average biomass was the highest in summer (5529.40 g/m²), and the lowest in autumn (2340.45 g/m²). The annual average value was 3455.06 ± 1480.40 g/m²; the horizontal distribution of species number and biomass were gradually decreasing from the inside to outside of the culture zones; the highest index of epiphytic macroalgae diversity (H') was in autumn (2.30). The lowest was in summer (1.87), the annual average was 2.07 ± 0.18; there were 13 dominant species in the epiphytic macroalgae community, the highest in autumn (8 species) and the lowest in summer (5 species), with an average of 6 species. The dominant species that existed throughout the year were Jania adhaerens and Hypnea boergesenii; among the dominant epiphytic macroalgae, the maximum and minimum niche breadth occurred in autumn, they were Hypnea boergesenii (2.214) and Sargassum horneri (0.739); the maximum and minimum values of niche overlap occurred in autumn, and they were Sargassum horneri-Gracilaria textorii (0.861) and Grateloupia turuturu-Sargassum horneri (0) respectively. The present study has shown that the existence of large-scale raft culture can prolong the growth period of red algae such as Grateloupia livida and Hypnea boergesenii, and broaden the habitat range of brown algae such as Sargassum horneri, thus changing the community structure of macroalgae in the culture area, which is conducive to maintaining a higher level of annual primary productivity and the production of macroalgae debris in the sea area, and providing new ideas and important scientific basis for the construction of seaweed beds in the process of marine ranch construction.