Bacterial alpha-diversity varies by DO.

<p>A significant relationship exists between DO concentration and bacterial taxa richness. Points are coded by water depth as either deep (filled symbols) or surface (open symbols) and by season as April (square), June (circle), or October (triangle). Dashed line represents the best fit from the linear regression model of bacterial richness as a function of dissolved oxygen (adjusted R² = 0.7379, p < 0.01).</p

Continuous abiotic variables affect the diversity and composition of bacterial communities in Hood Canal, WA, USA (α = 0.05 for all tests).

<p>The strength of richness and evenness, measured by Pielou’s J, models was assessed by the adjusted R² value from multiple linear regressions, and the strength of the composition models was assessed by the Spearman’s rho correlation coefficient from BIO-ENV analyses.</p

DO rather than depth affects taxa distributions and twenty-five taxa were identified as indicators of different DO conditions.

<p>(A) Plot of each OTU’s contribution to the dissimilarity between high and low DO groupings of samples versus surface and deep grouping of samples. Red line shows one-to-one line representing equal contribution to both DO effect and depth effect. Taxa falling to the right of the red one-to-one line contribute more to the dissimilarity between high and low DO groupings than depth groupings suggesting a stronger effect of DO than depth. (B) Plot showing the studentized residuals calculated for each OTU to identify significant deviation from the null hypothesis that there is no difference between the OTU’s contribution to dissimilarity between DO and depth groupings. Each red horizontal line shows the cutoff for significance. Points above the upper red line represent OTUs that contribute significant dissimilarity between DO groupings thus are indicators of high or low DO, whereas points below the lower red line are significant indicators of a depth effect.</p

Number of sequences obtained in each sample, as well as the observed and Chao estimates of taxa richness and Pielou’s J index of taxa evenness for all Hood Canal samples based on an operational taxonomic unit cutoff of 97% sequence identity.

<p>BA = Bangor; SP = Sister’s Point; HH = Hama Hama; LC = Lynch Cove; APR = April; JUN = June; OCT = October; S = Surface; D = Deep.</p><p>Number of sequences obtained in each sample, as well as the observed and Chao estimates of taxa richness and Pielou’s J index of taxa evenness for all Hood Canal samples based on an operational taxonomic unit cutoff of 97% sequence identity.</p

Both (A) parametric and (B) non-parametric analyses suggest that the bacterial community composition shifts between 5.18 and 7.12 mg O₂ L^-1.

<p>(A) Dendrogram of parametric hierarchical clustering approach using Sorensen-abundance based community composition dissimilarity. Numbers following Sample ID show the DO concentration in mg L^-1. (B) Non-metric multidimensional scaling ordination shows bacterial community composition of samples are more similar to those from the same water depth, season, or DO concentration. Position of samples is calculated using the Sorensen abundance-based similarity index of bacterial community composition and the absolute dissimilarities between samples is condensed onto two-dimensions with a resulting stress value of 0.0679. Points are coded by water depth as either deep (filled symbols) or surface (open symbols) and by season as April (square), June (circle), or October (triangle). The size of the sample point is proportional to DO content with the highest DO represented by the largest sample point.</p

Principal components analysis of environmental characteristics of the sixteen samples ordinated based on Euclidean distance calculated from environmental factors.

<p>Points are coded by water depth as either deep (filled symbols) or surface (open symbols) and by season as April (square), June (circle), or October (triangle). Nitrate, phosphate, salinity, and dissolved oxygen best explain the separation of samples along axis one, and ammonium alone explains variation along axis two based on the variable loadings on each principal component.</p

Abiotic factors affect the average proportion of various bacterial functional groups.

<p>(+) indicates a significant positive association (p < 0.01)</p><p>(−) indicates a significant negative association (p < 0.01)</p><p>*Average proportion of the functional group within each sample.</p><p>A forward selection stepwise multiple linear regression modeling approach was employed with a requirement of p < 0.05 to enter the model.</p

Bacterial community threshold analysis for DO in Hood Canal.

<p>Taxa-Z plot from TITAN showing the cumulative change points of taxa abundances across the DO gradient. The peak in change-point is considered an approximate community threshold for DO concentration. Z- are taxa that have a negative association with DO and Z+ are taxa that have a positive association with DO.</p

Relative proportions of the most abundant classes change across a dissolved oxygen gradient and are significantly different when grouped by sampling depth.

<p>The samples are ordered by dissolved oxygen concentration (solid line). Abbreviations: Hama Hama (HH), Sister’s Point (SP), Bangor (BA), Lynch Cove (LC), April (APR), June (JUN), October (OCT), Surface (S), Deep (D). Classes that comprised less than 1% of the sample’s total community were condensed into the “other” category.</p

Geography and biogeochemical conditions in Hood Canal, WA, USA in 2007.

<p>(A) Map of sampling stations within Hood Canal, WA, USA. (B) Contour plots showing range of dissolved oxygen (DO) and chlorophyll <i>a</i> concentrations at Hama Hama and Sister’s Point stations in Hood Canal, WA. Data for high-resolution depth profiles were collected by Oceanic Remote Chemical Analyzer buoys maintained by the Northwest Association of Networked Ocean Observing Systems (<a href="http://www.nanoos.org/" target="_blank">http://www.nanoos.org</a>). Note changes in scale on both the x- and y-axes.</p