Guest post by Dr. Jackson Chu, Post-doctoral fellow, Fisheries & Oceans, Canada
How do you determine when a system has become oxygen deficient, or hypoxic, for the species in our marine ecosystems? Part of the answer comes from mapping where species are found in their habitat and then repeatedly mapping their habitat under changing oxygen conditions. With enough repetition we can then get a sense of the average oxygen levels where various species occur, the variability at which they occur, and, eventually, identify years where the animal community doesn’t align with the long term baseline.
October 2016 was the 13th time that we have repeated our benthic hypoxia transect in Saanich Inlet, British Columbia Canada. In a transect from earlier this year, I noted that 2016 was already a severe year where the expansion of oxygen deficient waters had forced animals into shallower depths at a much earlier time of year than we had come to expect from past surveys. My trailing thoughts at the time were whether the severity of this ‘habitat compression’ would increase as the summer progressed and how this year would compare to our baseline records developed from a decade of surveys at this site. From my quick look at the data, it does appear that the severity of habitat compression increased over the past three months.
An indicator species of the severe hypoxic zone, the slender sole, maintained their shallow depth of occurrence relative to our May survey. What was surprising was the change in occurrence (or absence) of the other usual suspects of the benthic community. In every complete survey dating back to 2006, we have always observed hundreds of commercial shrimp species hiding in the shallow, ‘sea whip zone’ of the study site.
October 2016 was the first survey in ten years that we observed no shrimp. Instead, we counted dozens of striped nudibranchs which is surprising as only a combined handful were documented in all our past surveys.
Another outlier to the general community pattern was the occurrence of dozens of white sea cucumbers – I had never observed this class of echinoderms in our surveys until this dive. Only a complete synthesis of the surveys from this year and follow-up monitoring will tell us if the animal community has permanently shifted or if 2016 was just an outlier year.
These preliminary observations highlight the importance of long-term ecological monitoring. Through repeated observations, we get a sense of the year to year variability that feeds into our ability to predict the long-term consequences of oxygen loss on the ecosystems on our Pacific coast.