Predicting Future Viable Aerobic Habitat for the Green Sea Urchin (Strongylocentrotus droebachiensis) in the Northeastern Pacific

Kelp forests are valuable marine ecosystems that foster incredible biodiversity by providing food and protection to countless fish, vertebrate, and invertebrate species. In North America, over-hunting drove sea otter populations close to extinction. As top predators in many kelp forest ecosystems, the absence of sea otters allowed sea urchin populations to rise, which can pose a great threat to kelp forests if gone unchecked. Overgrazing of kelp by sea urchins can lead to vast areas of kelp forest degradation, or urchin barrens, and threatens the intrinsic commercial and recreational value of kelp forests. To better understand the ecophysiological impacts of rising sea temperatures on urchin fitness and viable aerobic habitat, we conducted respirometry experiments on green urchins, Strongylocentrotus droebachiensis, collected from Quadra Island, Discovery Islands, Canada. By performing these respirometry experiments at five different temperatures spanning 5-22C, we can determine how temperature actively affects the absolute pO2 tolerance of green urchins (Pcrit). Preliminary data suggest the lowest Pcrit values occur at 16C, just below the average annual maximum sea temperature of 18C in the region. This suggests that these urchin populations--and consequently their effects in kelp forest ecosystems--may be relatively robust to predicted 21st century warming. Further respirometry measurements should be taken between 16C and 22C to fully resolve the effect of temperature change on future viable aerobic habitat. Body size data of the green urchins was also collected, allowing us to estimate the effect of size on Pcrit. These data can ultimately be used in the Metabolic Index framework to understand how rising sea temperatures will impact green urchin populations and size classes in the northeastern Pacific according to different climate change projections.