The unicellular Labyrinthulomycete protists have long been considered to play a significant role in ocean carbon cycling. However, their distribution and biogeochemical function remain poorly understood. We present a large‐scale study of their spatiotemporal abundance and diversity in the coastal waters of Bohai Sea using flow cytometry and high‐throughput sequencing. These protists display niche preferences and episodic higher biomass than that of bacterioplankton with much phylogenetic diversity (> 4000 OTUs) ever reported. They were ubiquitous with a typical abundance range of 100–1000 cells ml−1 and biomass range of 0.06–574.59 μg C L−1. The observed spatiotemporal abundance variations support the current ‘left‐over scavengers’ nutritional model and highlight these protists as a significant component of the marine microbial loop. The higher average abundance and phylogenetic diversity in the nearshore compared with those in the offshore reveal their predominant role in the terrigenous matter decomposition. Furthermore, the differential relationship of the protist genera to environmental conditions together with their co‐occurrence network suggests their unique substrate preferences and niche partitioning. With few subnetworks and possible keystone species, their network topology indicates community resilience and high connectance level of few operational taxonomic units (OTUs). We demonstrate the significant contribution of these protists to the secondary production and nutrient cycling in the coastal waters. As secondary producers, their role will become more important with increasingly coastal eutrophication.
