Conclusion
The eukaryotic phytoplankton and bacterial networks constructed in our research conformed to the properties of non-randomly connections, modular structure, and ”small world”. The complexity of the network was susceptible to temperature, high and low temperatures were not conducive to the stability of communities. For example, eukaryotic phytoplankton networks in spring and autumn were more complex than in summer and winter. In co-occurrence patterns, the positive interactions between bacteria and eukaryotic phytoplankton OTUs may be evidence of mutual influence (cooperation), while the negative correlations may indicate predation or competition. Through its within-module connectivity (Zi ) and among-module connectivity (Pi ), we identified the keystone species that can maintain the stability of microbial networks. The keystone species in bacterial networks were mainly OTUs of Proteobacteria, Planctomycetes, Bacteroidetes, Verrucomicrobia, Acidobacteria, Chloroflexi, and Firmicutes, which were defined as rare taxa; and in eukaryotic phytoplankton networks, most OTUs of Chlorophyta, Bacillariophyta, and Ochrophyta belonging to rare taxa, and only 2 OTUs of Chlorophyta were abundant taxa. With the disappearance of the ”keystone species” in the network center, the aggregation structure will suffer major damage, which may bring huge biogeochemical consequences. The abundance of these keystone species was significantly related to many ecosystem functions and environmental changes, highlighting the key ecological role of rare species, and providing a new ecological significance for the seasonal succession patterns of bacteria and eukaryotic phytoplankton in urban aquatic ecosystems.