Fish and macrobenthic diversity and community assembly patterns in the Jiuduansha Wetland based on eDNA metabarcoding

Estuarine wetlands are critical zones of land-sea interaction that play vital roles in maintaining biodiversity and ecosystem functioning. As the only remaining natural estuarine wetland in the Yangtze River Estuary, Jiuduansha's fish and macrobenthic diversity and community assembly mechanisms remain poorly understood due to limitations of traditional survey methods in protected areas. We conducted eDNA metabarcoding surveys at 11 sampling sites (6 intertidal zone and 5 riverine sites) in the Jiuduansha Wetland Nature Reserve during May and July of 2023-2024, combined with measurement of environmental variables, to systematically analyse the mechanisms of species composition, diversity patterns and community assembly of the Jiuduansha Wetland. A total of 64 fish species and 35 macrobenthos were detected. The eDNA approach identified approximately 40% more species than historical traditional surveys. Spatially, fish diversity was higher in riverine sites (average 18.6 ± 2.7 species/site), while macrobenthos were more concentrated in the intertidal zone (average 14.2 ± 2.5 species/site). Seasonal patterns differed: fish species richness was higher in May, whereas macrobenthos peaked in July. Generalized linear models (GLMs) indicated that total nitrogen (TN) and chemical oxygen demand (COD) were key environmental factors that have negative influence on both communities' richness. Beta diversity decomposition showed that species turnover was the dominant component of community variation within the study scale (70.7%-70.9% for fish, 60.0%-69.3% for macrobenthos). Neutral community model (NCM) fitting revealed increased migration rates, with Nm values increased from 4 in 2023 to 17 in 2024 for fish and from 9 to 18 for macrobenthos, based on samples collected in May and July of each year. This indicates enhanced dispersal between communities. Both βNTI analysis (|βNTI| < 2) and the RCbray index (with >90% contribution of ecological drift) demonstrated that stochastic processes, particularly ecological drift, dominated community assembly.