Living in the dead zone: Patterns of bacterial communities in Hood Canal, WA, USA

Background/Question/Methods

Hood Canal, a long, narrow glacial fjord located 80 miles West of Seattle, WA, USA, has been impacted by human activities primarily through eutrophication, or high terrestrial input of nutrients leading to periodic and sustained low dissolved oxygen concentrations. The hypoxic waters in these so-called “dead zones” or oxygen minimum zones exclude fish and benthic organisms and eliminate habitat that is important both for commercial fishing and recreation. However, the hypoxic dead zones of Hood Canal are far from dead; rather, they support active and abundant bacterial and archaeal populations. Here, we examined (1) how environmental parameters change over space and time in Hood Canal, (2) how bacterial communities vary with abiotic factors (including depleted dissolved oxygen concentration) over space and time in Hood Canal, and (3) how these patterns varied for the dominant bacterial groups observed. We employed 454-pyrosequencing to obtain 165,022 reads of the V6 hyper-variable region of the 16S rRNAgene from 16 water samples.

Results/Conclusions

Using univariate and multivariate statistical approaches, we find that bacterial richness, as defined by number of operational taxonomic units (OTUs) at 97% identity, is significantly higher in deep waters than surface waters, and OTU richness is strongly correlated with dissolved oxygen, ammonium, and nitrite concentrations. Using an abundance-based measure of bacterial composition, surface water communities differed significantly from deep-water communities and, within each depth, a strong seasonal change in community composition was evident. Using a constrained analysis of principal coordinates, dissolved oxygen was shown to be most highly correlated with variation in both abundance-based and occurrence-based measures of bacterial community composition across all samples. Patterns of bacterial richness and composition differed among the dominant bacterial groups. For example, while overall bacterial diversity increased significantly as dissolved oxygen concentration decreased, diversity of Gammaproteobacteria remained constant across the dissolved oxygen gradient. The results of this study suggest that abiotic environmental variables are dominant factors structuring bacterial communities in Hood Canal and changes in the bacterial community are highly associated with levels of dissolved oxygen. As ocean temperatures continue to gradually increase, it is predicted that hypoxic waters will become exacerbated and impact bacterial communities and potentially their role in nutrient cycling. Continued research efforts of bacterial community dynamics in oxygen minimum zones, such as Hood Canal, will thus be essential for understanding and predicting the future of marine resources.