Spatial and temporal variation in nearshore community structure in a seasonally hypoxic estuary

Background/Question/Methods

Hypoxia [dissolved oxygen (DO) < 2 mg L^-1] has emerged as a key threat to marine and estuarine ecosystems around the world. While organisms may suffer direct mortality, mobile species can display patterns of avoidance away from the deep, offshore hypoxia-impacted waters. However, the non-lethal impact of low DO conditions on the nearshore community is not well understood, despite the importance of nearshore habitats for marine and estuarine species. We chose to investigate the sub-lethal influence of hypoxia on nearshore community structure in Hood Canal, Washington, a seasonally hypoxic basin of Puget Sound. The specific aim of the study was to evaluate whether hypothesized community differences - consistent with predictions based on physiological tolerances of organisms - would exist between hypoxia-impacted and less impacted areas of Hood Canal. Comparing a southern low DO region and a northern reference region, we used underwater video monitoring techniques to record weekly video of the benthos via transects at three depths (10, 20, 30m), four sampling sites per region, in order to measure mobile species density and composition. A combination of multivariate statistics, qualitative assessment of tolerance levels, and non-parametric modelling were used to determine the potential influence of hypoxia on the nearshore community.

Results/Conclusions

Although we did not detect acute shifts of macrofauna into nearshore habitat, the community composition was significantly different between the two study regions. More specifically, the south was primarily composed of hypoxia tolerant invertebrates and fewer fish species compared to the north. Relative to other potential predictors, DO performed moderately well in describing the occurrence of the most abundant species. In addition, tolerant species displayed almost a three-fold increase in presence below a specific DO-tolerance threshold (mean threshold ± SE = 3.77mg L^-1 ± 0.27), while the more sensitive species declined. The magnitude in compositional change towards more tolerant species was also significantly greater in the southern region. Ultimately, comparisons of our findings to long-term DO trends in Hood Canal reveal the potential for a more persistent low DO state in the southern reaches. As a result, this study provides further insight into the complex regional differences in community structure and potential sensitivity of the nearshore community to other perturbations in estuarine systems.