Lethal and functional thresholds of hypoxia in two key benthic grazers

Climate drivers can impact ecosystem structure and processes by changing the composition of organisms through lethal impacts, or by altering the ecological roles of organisms through sublethal physiological and behavioral impacts. We examined potential effects of upwelling-driven hypoxia on California Current kelp forests by assessing hypoxia impacts on 2 species having important functional roles in these ecosystems. We quantified dissolved oxygen (DO) thresholds for lethal and functional-role impacts in purple and red sea urchins (Strongylocentrotus purpuratus and Mesocentrotus franciscanus). For both species in 2 size classes, we measured median lethal exposure time to severe hypoxia of 1.0 mg l(-1) of DO, and quantified grazing at 7.5 (ambient), 6.5, 5.5, and 4.5 mg l(-1) DO. Sea urchins of both species and size classes were tolerant of severe hypoxia, but showed 39 to 47% declines in grazing at 5.5 mg l(-1) DO. We compared these thresholds to a decade-long dataset of DO in a Monterey Bay kelp forest and found that sea urchins are unlikely to experience direct mortality under current or near-future exposures to upwelling-driven hypoxia. In contrast, they are likely to experience sublethal effects of low oxygen and decreased grazing rates for substantial periods of time under current conditions. These results represent the first documentation of hypoxia-modulated kelp grazing in sea urchins. They indicate that upwelling-driven hypoxia can have important impacts on these ecologically important benthic grazers and possibly on whole ecosystems through altered trophic interactions, highlighting that sublethal, functional-role impacts of climate drivers can be as or more important than lethal impacts.