Stochastic Processes Drive the Assembly of Planktonic Ciliates in A Trellised River Network

The topology and connectivity of rivers can be modified by the water project operation for agriculture and civilization, as exemplified by the trellised river system in Chongming Island. Ciliates, as an exceptional indicator, are often employed for monitoring the health of aquatic ecosystems. However, the assembly and structure of planktonic microeukaryote (e.g., ciliates) communities, especially in complex river networks, are rarely investigated. Here, we investigated the ciliate community structure and assembly mechanism among seasons in Chongming Island's trellised river network using both metabarcoding and quantitative protargol stain (QPS) approaches. Both methods showed that ciliates exhibited similar community composition at the class level and distinct seasonal succession, which are attributed to variations in nutrients, dissolved oxygen and water temperature. The stochastic process was the main factor in explaining the assembly of the ciliate community in all seasons, which is probably attributed to the high connectivity of the river network and the frequent opening of sluices for the water project operation. Collectively, unique ciliate community structure, seasonal variation, and specific community assembly mechanism driven by hydrology were reported in this study. Furthermore, methodological differences also should be fully considered when conducting an ecological study in complex hydrographic waters.