The occurrence of hypoxia has become more common in coastal systems worldwide. In many of these systems, we know that there are changes in spatial overlap between zooplankton and planktivorous fish, which may lead to changes in energy flux. Hood Canal, Washington, USA, is an estuarine fjord that is prone to severe to moderate hypoxia in the summers. In contrast to other systems, we know that spatial overlap between zooplankton and planktivorous fish in Hood Canal is maintained in the presence of moderate hypoxia. However, the presence of hypoxia can also cause other ecological changes that affect energy flow, such as changes in community composition, abundance, and/or changes in consumption rates. Therefore, we investigated whether changes in energy flux from zooplankton to planktivorous fish occur in Hood Canal, given that spatial overlap between predators and prey is maintained. We used a paired-site sampling design to quantify the diets of fish predators, total per-capita consumption, total population consumption, and total consumption relative to prey availability, before, during, and after the development of hypoxia.
In Hood Canal, there was no change in energy flux due to hypoxia. Predator diets, per-capita consumption, and total population consumption did not change over the course of the season. While prey availability did vary at some sites over the course of the season, there were no differences between sites with and without the presence of hypoxia. These surprising results highlight the importance of in situ observations, rather than solely depending on extending lab experiments and observations from different systems.