Abstract
The Indian Ocean is characterized by its complex physical systems and strong seasonal monsoons. To better understand effects of seasonal monsoon-driven circulation on the bacterioplanktonic community structure in surface waters and the bacterial distribution response to vertical stratification, patterns of seasonal, and vertical distribution of bacterial communities in the Eastern Tropical Indian Ocean were investigated using 16S rRNA gene profiling. Water samples were collected during the Southwest monsoon (from June to August), the fall inter-monsoon (from October and November) and the Northeast monsoon (from December to January), respectively, onboard during three cruises from July 2016 to January 2018. Surface bacterioplankton communities in these three seasons and in the upper water (3–300 m with six depths) during the Northeast monsoon contained a diverse group of taxa, mainly Proteobacteria, Cyanobacteria, Actinobacteria, Bacteroidetes, and Chloroflexi. Redundancy discriminant analysis (RDA) uncovered that temperature, salinity, and dissolved oxygen (DO) were crucial environmental parameters that affected the structure of bacterial community in overall surface samples. However, significant differences in the composition of the bacterial community are likely due to changes in concentrations of salinity during the fall inter-monsoon, while phosphate for both the Southwest monsoon and the Northeast monsoon. Pearson's analysis revealed that the seasonal variation rather than the vertical variation of environmental factors had a more significant impact on the composition of bacterial community. In addition, a clear seasonal pattern of bacterial co-occurrence showed that inter-taxa associations during the fall inter-monsoon were closer than during the Northeast monsoon and the Southwest monsoon. Overall, our results implied clear differences in the composition of bacterial community, with more pronounced seasonal variation compared to the vertical variation in response to environmental changes.
Highlights
Patterns of diversity and the composition of bacteria in seawater are critical for exploring the evolutionary and ecological processes that form current biodiversity, and monitor the response of marine ecosystems to environmental changes (Suh et al, 2014)
This study demonstrated that the bacterioplankton community of the Eastern Tropical Indian Ocean consisted of various taxa that were commonly detected in marine environments, such as Proteobacteria, Cyanobacteria, Actinobacteria, Bacteroidetes, and Chloroflexi
The bacterial community composition significantly correlated with some environmental variables that influenced the bacterial community, such as temperature, salinity, and dissolved oxygen (DO) in surface water
Summary
Patterns of diversity and the composition of bacteria in seawater are critical for exploring the evolutionary and ecological processes that form current biodiversity, and monitor the response of marine ecosystems to environmental changes (Suh et al, 2014). During the Southwest monsoon, the Southwest Monsoon Current south of Sri Lanka and the South Equatorial Current are the two important currents that influence the Eastern Tropical Indian Ocean. The change of the South Equatorial Current may relate to the northward movement of Southeast trade wind regime (Schott and McCreary, 2009). During the Northeast monsoon, the Northeast Monsoon Current and South Equatorial Countercurrent significantly affect the Eastern Tropical Indian Ocean. The Northeast Monsoon Current is enhanced in December and peaked in January with a maximum velocity of 0.5 m·s−1. A particular phenomenon, singular to the Indian Ocean owing to the semiannual eastward winds along the equator, is the occurrence of strong eastward surface jets (Wyrtki, 1973) during the transition seasons between the monsoons, i.e., from April to June and from October to December
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