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.
Highlights
Bacteria are ubiquitous in the ocean (Flombaum et al, 2013; Sunagawa et al, 2015) and play a are critical to ecosystem functions such as biogeochemical cycling of elements (Falkowski et al, 1998)
During the strongly stratified periods, water stratification was due to the large temperature gradient caused by high sea surface temperature (Supplementary Figure S1)
We found that the pure effect of vertical distance ([Dist|Env+BF]) is marginally significant (Table 1), whereas the pure effect of stratification ([BF|Env+Dist]) is not significant (Table 1). These results suggested that vertical beta diversity can be influenced by dispersal limitation contributed by the pure effects of vertical distance ([Dist|Env+BF]) but to a much lesser extent by the pure effects of stratification strength ([BF|Env+Dist])
Summary
Bacteria are ubiquitous in the ocean (Flombaum et al, 2013; Sunagawa et al, 2015) and play a are critical to ecosystem functions such as biogeochemical cycling of elements (Falkowski et al, 1998). In addition to physicochemical dissimilarity and geographic (vertical) distance, these recent studies suggest that that vertical beta diversity may be influenced by physical attributes such as water column stratification (Treusch et al, 2009; Qian et al, 2011; Giovannoni and Vergin, 2012; Chow et al, 2013; Li et al, 2018). At the Hawaii Ocean Time-series (HOT) site where water stratification is prolonged throughout the whole year, discrete surface and DCM communities were persisted the euphotic zone (Giovannoni and Vergin, 2012). The strength of stratification should be considered as part of a clearer understanding of the mechanisms determining vertical beta diversity
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