Community structure and biodiversity of benthic macroalgae in the Shengsi Archipelago of the East China Sea

IntroductionThis study focused on benthic macroalgal communities in the Shengsi Archipelago, East China Sea, a region jointly influenced by pronounced environmental heterogeneity driven by the interplay of the Yangtze River diluted water and the Taiwan Warm Current, and intensive mussel aquaculture. The aim was to characterize spatial patterns in community structure across environmentally distinct zones and to compare contemporary macroalgal assemblages with historical records to identify potential long-term shifts in community composition.MethodsA field survey was conducted in May 2025 at 30 intertidal stations, spanning from estuary-influenced nearshore areas to offshore island environments. The composition, biomass, and abundance of macroalgal species were quantified, and α-diversity indices and taxonomic distinctness (Δ+) were calculated. Contemporary spatial patterns were compared with historical data from the same region to assess long-term shifts in community structure and functional-group dominance.ResultsA total of 43 benthic macroalgal species were recorded, with Rhodophyta representing the dominant taxonomic group. Macroalgal communities exhibited a clear “west–low, east–high” spatial pattern, with extremely low biomass and phylogenetic clustering in estuary-proximal areas, and higher biomass, species richness, and taxonomic uniqueness in offshore waters. Comparisons with historical data indicated an over 80% decline in average biomass on natural reefs since 2010, accompanied by a shift in dominance from canopy-forming Sargassum to opportunistic Ulva australis. Mussel aquaculture zones were associated with relatively high diversity and structural complexity.DiscussionThe results highlight the dominant role of water-mass-driven environmental differentiation in structuring benthic macroalgal communities in the Shengsi Archipelago while also revealing a compensatory role of mussel aquaculture under conditions of natural reef degradation. The study provides important baseline information for evaluating the ecological status and conservation of macroalgae in highly modified estuarine–island systems.