The enhanced neutral process with decreasing cell size: a study on phytoplankton metacommunities from the glacier-fed river of Qinghai-Xizang Plateau.

The cell size of phytoplankton is an important defining functional trait that can serve as a driver and sentinel of phytoplankton community structure and function. However, the study of the assembly patterns and drivers of phytoplankton metacommunities with different cell sizes has not been widely carried out. In this study, we systematically investigated the biodiversity patterns, drivers, and assembly processes of the three phytoplankton cell sizes (micro: 20-200 μm; nano: 2-20 μm; pico: 0.2-2 μm) in the Za'gya Zangbo River from the source to the estuary using 18S rDNA amplicon sequencing. The results demonstrated that the alpha diversity and co-occurrence network complexity for all three sizes of phytoplankton increased to a peak downstream of the glacier sources and then decreased to the estuary. The nanophytoplankton subcommunity consistently had the highest alpha diversity and co-occurrence network complexity. On the other hand, total beta diversity followed a unimodal trend of decreasing and then increasing from source to estuary, and was dominated by species replacement components. In addition, deterministic processes driven mainly by physiochemical indices (PCIs) and biogenic elements (BGEs) dominated the assembly of micro- and nanophytoplankton subcommunities, whereas stochastic processes driven by geographical factors (GGFs) dominated the assembly of picophytoplankton subcommunities. The results explained the contradictions in previous studies of phytoplankton community assembly processes in highland aquatic ecosystems, elucidating the different contributions of deterministic and stochastic processes, and the complexity of compositional mechanisms in shaping the assembly of micro-, nano-, and picophytoplankton in this highland glacial river.

Importance: The cell size of phytoplankton is a key life-history trait and key determinant, and phytoplankton of different cell sizes are differentially affected by ecological processes. However, the study of the assembly patterns and drivers of phytoplankton metacommunities with different cell sizes has not been widely carried out. We provide an in-depth analysis of phytoplankton community diversity across three cell sizes in the glacier-fed river, describing how the pattern of phytoplankton communities differs across cell sizes in response to geochemical gradients. The results show that the smaller phytoplankton (picophytoplankton) are relatively more influenced by dispersal-based stochastic processes, whereas larger ones (microphytoplankton and nanophytoplankton) are more structured by selection-based deterministic processes.