Annual changes in the microeukaryotic community in intertidal sediments, Qingdao, China

Microeukaryotic communities in intertidal sediments play an important role in maintaining ecosystem health and resilience, as they are involved in ecosystem services such as nutrient cycling, organic matter breakdown, and the support of diverse marine food webs. However, the seasonal variations in these communities and the differences between communities in muddy and sandy sediments are poorly understood. To address these gaps, surface sediments from both muddy and sandy sediments were collected monthly over a year (4 seasons, 10 months). Microeukaryotic community structure was analyzed using 18S rRNA sequencing, focusing on alpha-diversity, operational taxonomic unit composition, community composition, network structure, and trophic functional group structure. Results show that there were significant monthly changes in community composition, which were closely linked to changes in environmental factors such as temperature, dissolved oxygen (DO), and salinity. Species co-occurrence network analysis showed that interaction numbers and network density peaked in summer and declined in autumn and winter. This study also highlights the different characteristics of eukaryotic communities of muddy versus sandy sediments. Mantel tests show that temperature, DO, and chlorophyll a (chl a) have a significant influence on eukaryotic communities in muddy environments, while DO, chl a, and particulate organic carbon are important in sandy environments. It was found that autotrophs, heterotrophs, and mixotrophs react differently to seasonal climate shifts and different sediment types, with notable peaks in relative abundance for autotrophs in June, heterotrophs in December, and mixotrophs from September to November, as inferred from 18S rRNA amplicon sequencing data. These results will provide an important baseline on the microeukaryotic communities in intertidal sediments, essential for forecasting changes in community structure and ecosystem functions resulting from environmental changes.IMPORTANCEThis study is the first to report on dynamic changes in microeukaryotic operational taxonomic unit (OTU) composition in intertidal sediments, focusing on differences between muddy and sandy sediments over 10 months and across two different stations. A total of 4,452 OTUs were identified. Our findings offer new insights into how these communities are influenced by environmental factors, seasons, stations (Licun and Zhanqiao), and sediment types (sandy and muddy). Our results show significant monthly changes in community composition, closely linked to environmental factors such as temperature, dissolved oxygen, and salinity. The species co-occurrence network analysis revealed that interaction numbers and network density peaked in summer and declined in autumn and winter.