The study area was located in a typical plain river network in the Yangtze River Delta region, China. Industry and economy are developed, and the value of GDP in this area is the highest in China. Exogenous pollution was one of the most significant sources of pollution in this area. Most pollutants were discharged into the lake wetland and large amounts were deposited on the lake wetland sediments. The microbial communities in the sediment has a great influence on the pollutant content, and are important in the transformation and immigration of the pollutants (Huang et al., 2019, Huang and Jiang, 2016). There was considerable variation in the bacterial sediment communities, and they differed between seasons and layers.
The main sources of pollution were printing and dyeing industries, agriculture, and livestock waste, especially around sites 14, 18, 19 and 20. In sediment samples from sites 14, 18, 19 and 20 the highest values of TN, TP and OM contents reached 4847 mg kg−1, 1697 mg kg−1 and 17.66%, respectively. There was frequent pollutant entrance and deposition in these areas, and the relative abundance of the bacterial communities in the sediment samples from these four sites was higher than in sediments from sites 3 and 10 (Liu et al., 2009, Deng et al., 2014). The average concentrations of TN, TP and OM did not vary significantly in lake wetland sediments from different seasons. The TN, TP and OM in the different regions were higher in winter than in other seasons. This may have been because the low temperature in winter inhibited the activity of the bacterial communities. Reduced rainfall in winter also reduced the concentrations of TN, TP and OM through dilution and biological interception, which weakened the biological removal of TN, TP and OM (Nie et al., 2012, Zhang et al., 2019a).
Most surface sediments in the plain river network area were from the deposition of the pollutants, and the sediments from some sites (such as 3 or 10) have been removed by dredging in recent years. The vertical diversity distribution results indicated that there was greater diversity in the bacterial community in surface sediment samples (Chao, ACE, Shannon) than in the bottom sediments. In addition, pollutant contents in the bottom sediments were low, and the bottom sediments were clearer than the surface sediments, which is one of the main reasons for greater diversity in the surface sediments. On the basis of the richness estimators (Chao and ACE) and the diversity indices (Shannon and Simpson), there were differences in the richness and diversity of the sediment bacterial communities in different seasons. The bacterial community diversity in summer was generally higher than in other seasons owing to the warm environment that favors the growth of microorganisms during this humid season (Zhang et al., 2019b).
Proteobacteria was commonly found to have the greatest relative abundance in sediments, and sedimentary degradation and metabolism mainly relied on this phylum (Chaudhry et al., 2012). Proteobacteria was also the most abundant phylum in all sediment layers, and at greater abundance in the surface sediments than in the bottom sediments. RDA analysis showed that Proteobacteria was negatively correlated with the contents of other environmental variables (TP, TN, OM, DO, PH). Firmicutes predominated in some sediment sites (19); this can generate spores that endure dehydration and harsh environmental states (Huang and Jiang, 2016).
LEfSe analysis showed that Firmicutes were enriched in autumn, and this may have been related to DO and TN levels. The phylum Firmicutes has the ability to perform denitrification processes under anaerobic conditions (Luo et al., 2013). In the bottom sediments Chloroflexi was abundant, while Bacteroidetes was the dominant phylum in the surface sediments. Members of the Chloroflexi are facultative anaerobes and have been identified from freshwater sediments (Kadnikov et al., 2012). They can fit into the changing redox conditions of an aquifer and participate in the degradation of organics, and this action may lead to the low OM content in the sediment (LA et al., 2013, Huang et al., 2017). Bacteroidetes is widespread in a variety of environments such as surface sediments (Vandieken et al., 2017, Orsi et al., 2013, Inagaki, 2015). It is thought to play a vital role in ecosystems because its members process complex molecules into simpler compounds in predominantly anaerobic habitats (Liu et al., 2009, Rosselló-Mora et al., 1999). They are in relatively high concentrations in surface sediment owing to nearby sewage discharge into the lake wetlands, and some of the pollutants from the towns and cities have been deposited into the surface sediments (Liu et al., 2009). Nitrospirae is also an important phylum in the lake wetland sediments and was more abundant on the surface sediments than on the bottom sediments because it is largely dependent on higher oxygen availability (Vetterli et al., 2015). The RDA result indicated that Nitrospirae was negatively correlated with depth, confirming that the phylum prefers aerobic environments.
The 120 surface sediment samples collected in all four seasons and the 30 sediment samples from different layers were obtained in a plain river network in lake wetlands. Large amounts of pollutants were mixed, and some were deposited into the surface sediments (Liu et al., 2009). The highest levels of TN (4847 mg kg− 1), TP (1697 mg kg−1) and OM (17.66%), and the relative abundance of the bacterial communities in the sediment samples from sites 14, 18, 19 and 20, were higher than in sediments from sites 3 and 10. The results indicated that the bacterial communities were influenced by environmental factors and that environmental conditions in these areas were complex.