Effects of water stratification on bacterial communities in scallop farming waters of the North Yellow Sea in China

Abstract

Water stratification is a prominent oceanographic phenomenon in the North Yellow Sea, a vital region for mollusk aquaculture in northern China. Understanding the influences of water stratification on planktonic bacterial communities in those waters is crucial for the sustainable development of mollusk aquaculture in this region. In the present study, the bacterial community profiles across different water layers in a representative Yesso scallop farming area of Zhangzi Island in the North Yellow Sea were surveyed from March 2021 to January 2022. Water stratification was present in June, July, and August (the stratification period, abbreviated as S period) and absent in March, October, and January (the mixing period, abbreviated as M period), based on the vertical distribution of water temperatures. Results of the 16S rRNA gene sequencing showed that the alpha diversity of bacterial communities in the S period (Chao1 index: 500.78, Shannon index: 6.85) was significantly lower than in the M period (Chao1 index: 575.00, Shannon index: 7.24). The relative abundances of Synechococcus CC9902 and Vibrio were significantly higher in the S period compared to the M period, while the abundance of Clade Ia showed the opposite trend. Water temperature, salinity, and dissolved inorganic nitrogen were identified as the main environmental drivers of the bacterial communities, which accounted for 22.4%, 20.0%, and 17.1%, respectively, of the total variation in bacterial community composition. Functional prediction suggested that the pathways associated with cytochrome biosynthesis and nutrient utilization of the bacterial communities were significantly enriched in the S period, while those involved in energy metabolism were enhanced in the middle and bottom layers compared to the surface layer during the S period. The connectivity of the molecular ecological network increased in the S period, with the number of edges, average degree, and average clustering coefficient being 574, 11.36, and 0.57 in the S period and 524, 9.44, and 0.51 in the M period, respectively. Collectively, these results indicated that water stratification led to a decrease in bacterial community diversity and an increase in the relative abundance of Synechococcus CC9902 and Vibrio, which were potentially detrimental to mollusk health and aquaculture. Our results contribute to clarifying the dynamics of planktonic bacterial communities and their impacts on mollusk aquaculture in the context of intensifying ocean stratification.