Habitat selection drives diatom community assembly and network complexity in sediment-laden riverine environments.

Microbial communities assemble stochastically and deterministically, but how different assembly processes shape diatom community structure across riverine habitats is unclear, especially in sediment-laden environments. In this study, we deciphered the mechanisms of riverine diatom community assembly in the water column and riverbed substrate with varying sediment concentrations. Water and sediment samples were collected from 44 sampling sites along the Yellow River mainstream during two seasons. Diatom communities were characterized based on high-throughput sequencing of the 18S ribosomal RNA genes coupled with multivariate statistical analyses. A total of 198 diatom species were taxonomically assigned, including 182 free-living and particle-attached species and 184 surface-sediment species. Planktonic communities were structurally different from benthic communities, with Cyclotella being dominant mainly in the middle and lower reaches of the river with higher sediment concentrations. Both stochastic and deterministic processes affected diatom community assembly in different habitats. Species dispersal was more important in the water than in the substrate, and this process was strengthened by increased sediment concentration across habitats. Diatom communities exhibited lower network complexity and enhanced antagonistic or competitive interactions between species in response to higher sediment concentrations compared with lower sediment concentrations mainly in the source region of the river. Differences in the species composition and community diversity of planktonic diatoms were closely correlated with the proportion of bare land area, nitrogen nutrients, precipitation, and sediment concentration. In particular, particle-attached diatoms responded sensitively to environmental factors. These findings provide strong evidence for sediment-mediated assembly and interactions of riverine diatom communities.

Characteristics of phytoplankton community and its influencing factors in the Yanhe River Basin, Northwest China.

Yanhe River is one of the important tributaries of the Yellow River, with a vital role in the maintenance of biodiversity and ecological conservation in the middle reaches of the Yellow River. In this study, we conducted a systematic aquatic ecological survey of the Yanhe River Basin in spring (April-May) and autumn (September-October) of 2021, with phytoplankton as indicator organism. A total of 33 sampling sections were selected in the mainstem, five first-class tributaries, and impounded water bodies (reservoir and check dam water bodies) of the Yanhe River Basin. The results showed that a total of 253 phytoplankton species, belonging to 7 phyla and 91 genera, were detected in the two surveys. Diatoms and green algae prevailed in spring (168 species), while diatoms and cyanobacteria dominated in autumn (179 species). The mean phytoplankton density and biomass were 316.07×104 cells·L-1 and 6.41 mg·L-1 in spring, and 69.56×104 cells·L-1 and 1.59 mg·L-1 in autumn, respectively. At the temporal scale, phytoplankton abundance in spring was higher than that in autumn. At the spatial scale, the phytoplankton abundance in the middle and lower reaches of the mainstream was higher than that in the upper reaches. Phytoplankton biomass in the impounded water bodies formed by dam interception was maintained at a high level, which was significantly higher than that in the mainstem and tributary water bodies in autumn. The phytoplankton diversity, as indicated by Shannon diversity index, Margalef richness index, and Pielou evenness index, in spring was greater than that in autumn. Phytoplankton diversity was greater in the trunk and tributary waters than that in impounded waters. The results of redundancy analysis showed that the key factors driving the phytoplankton community structure in spring were flow velocity, dissolved oxygen, nitrite nitrogen, and water depth. In contrast, the key driving factors in autumn were nitrate nitrogen, water depth, and dissolved oxygen.