Vertical Beta-Diversity of Bacterial Communities Depending on Water Stratification

Abstract

Many studies indicate that variation of marine bacterial beta diversity in the horizontal dimension is mainly attributable to environmental and spatial effects. However, whether and how these two effects drive bacterial beta diversity in the vertical dimension remains unclear, especially when considering seasonal variation in the strength of water stratification. Here, we used 78 paired bacterioplankton community samples from surface and deep chlorophyll maximum (DCM) layers along a transect in the Kuroshio region east of Taiwan across multiple seasons. Variance partitioning was used to evaluate the mechanisms driving the vertical beta diversity between surface-DCM bacterioplankton communities during weak stratification periods (i.e., spring and fall) versus strong stratification periods (i.e., summer). During strong periods of stratification, vertical beta diversity was shaped by both environmental and spatial effects; notably, the strength of stratification played an important role in enhancing environmental dissimilarity and creating a barrier to dispersal. In contrast, during periods of weak stratification, environmental effects dominate, with a non-significant spatial effect due to mixing. Variation of vertical beta diversity for bacterioplankton communities in the Kuroshio region east of Taiwan was structured by different mechanisms across seasons, and was further dependent on stratification strength of the water column.

Keywords: 16S rRNA gene; environmental gradients; stratification; vertical beta diversity; water mixing.

FIGURE 1

Map illustrating sampling stations along a transect perpendicular to the Kuroshio east of Taiwan. The bold dashed line indicates the mean position of the maximum current; the dotted line indicates the average width of the Kuroshio.

FIGURE 2

Hydrographical properties along the sampling transect at different sampling periods. (A) Contour plots of density (sigma T) within 200 m water depth. Circles indicate sampling locations (surface and deep chlorophyll maximum layers). Circles combined with crosses indicate the samples with contamination (see Supplementary Figure S2); therefore, these contaminated samplings and their corresponding surface samples were both excluded from the remainder of the analysis. (B) Boxplots illustrating the strength of stratification (quantified by maximum buoyancy frequency) and vertical environmental dissimilarity. The strong stratification periods exhibited significantly greater maximum buoyancy frequency and greater physicochemical dissimilarity than the weak stratification periods (Wilcoxon signed-rank test, ***P < 0.001).

FIGURE 3

(A) Ordination biplot illustrating the association among samples based on detrended Correspondence Analysis (DCA) of the bacterioplankton communities for different cruises and depth layers. The numbers indicate sampling stations (c.f. Figure 1). Each gray line represents vertical beta diversity as measured using Bray-Curtis distance between the paired surface and deep chlorophyll maximum (DCM) communities of a sampling. (B) Boxplots indicating that no significant difference in vertical beta diversity existed between the weakly and strongly stratified periods (Wilcoxon signed-rank test, P = 0.23).

FIGURE 4

Bray–Curtis dissimilarity of paired surface and deep chlorophyll maximum communities in relation to (A) physicochemical dissimilarity, (B) vertical distance, and (C) maximum buoyancy frequency during the weakly stratified periods (blue) and strongly stratified periods (orange). The circled symbol indicates the potential outlier. The relationship without the potential outlier was presented by the gray line. The solid line represents a significant relationship (P < 0.05) while the dashed line represents a marginally significant relationship (P < 0.1).