Observation: Divers noticed plumose anemones (mostly Metridium giganteum, but possibly a few Metridium senile as well) detaching from rocks. There were visible circles of bare rock on the walls and boulders where the anemones were previously attached, and in some cases, detached anemones were lying at the base of the rock slopes. Some dislodged anemones were being eaten by large aggregations of shag-rug nudibranchs (Aeolidia papillosa), and others were lying loose or deteriorating on the bottom. In addition, some of the anemones that were still attached appeared to have 'skinny' columns, and a ring of bare rock was visible around the base.
One potential hypothesis is that the anemones were reacting to low oxygen conditions (see Whal 1985), possibly resulting from an unusual plankton bloom in Howe Sound and the Strait of Georgia in the late summer. -- Jessica Schultz, Vancouver Aquarium.
Exact location: 49.432388°, -123.275070°
Comment by LEO BC coordinator Tom Okey: The potential connection between low oxygen conditions Metridium spp. anemone morbidity and mortality, if driven at least partially by phytoplankton blooms, implies a potential connection between the conditions that drive harmful algal blooms and Metridium spp. anemone morbidity and mortality. These conditions include human stressors such as coastal nutrient enrichment and other organic pollution, and more regional or global changes in ocean temperature and currents, such as upwelling patterns. Some data from Ocean Networks Canada, highlighted by Ryan Gasbarro indicates anomalously low oxygen conditions and high temperatures during the same time that these Metridium were experiencing morbidity, and mortality, including possible predation mortality. - Thomas A. Okey, PhD, Ocean Integrity Research and School of Environmental Studies, University of Victoria.
Comment by Ryan Gasbarro: I believe that extended periods of low-oxygen are related to what Jessica observed with the Metridium. In fact, Jackson Chu at IOS, who did his PhD using the same Saanich Inlet transect that my poster was on, showed great time-lapse footage of a Metridium closing up and eventually detaching from a rock due to prolonged exposure to low-oxygen conditions during a seminar he gave at UVic when he was finishing up his studies. The plot at the bottom of the page linked here, with data from a subsea cabled observatory node at 95m shows the weak oxygen renewal in 2015 carrying over into 2016 in conjunction with high temperature anomalies. This meant extremely prolonged exposure to low-oxygen for non-mobile critters who can't run away to find more oxygen. If the same thing happened in Howe Sound, which is likely due to the similarities Frank Whitney mentioned, detaching Metridium would be a good indicator. -Ryan Gasbarro, MSc Student, Tunnicliffe Lab, Dept. of Earth & Ocean Sciences, University of Victoria, Canada.
Wahl, M., 1985. The recolonization potential of Metridium senile in an area previously depopulated by oxygen deficiency. Oecologia, 67(2): 255-259.
Ocean Networks Canada Saanich Inlet page