Population, community and food web impacts of hypoxia : a synthesis of findings from Hood Canal

Hypoxia is a regular, yet increasingly prevalent feature of southern regions of Hood Canal, WA. While occasional fish kill events garner much public attention, these events are rare and may therefore may not be as important as effects from non-lethal levels of dissolved oxygen. Low levels (near 2 mg / l ) are common and species have a range of responses Here I use results from multiple investigations to illustrate the nature and magnitude of effects. On a population level, long lived sessile species like geoduck clams show clear evidence of substantial impacts from hypoxia. In addition, several sessile invertebrate species are locally impacted. In contrast, mobile demersal fish and invertebrates show a behavioral response to periods of low dissolved oxygen but these are temporary. We also find that valuable species such as Dungeness crab shoal in nearshore habitats where they are vulnerable to recreational fishing gear. We find that effects at the food web level are unclear. Specifically, euphausiids play a key role in the food web, yet show little evidence of density or distributional response. However, detailed tracking of planktivorous fish feeding and distribution suggests that low dissolved oxygen might tighten trophic connectivity between these two species. Notably, this occurs despite levels of dissolved oxygen that are physiologically stressful. Future work should focus more on ecological consequences of localized depletion and behavioral responses to low dissolved oxyge