Things got a bit interesting last night when we found our first ‘blob’ of hypoxic water. You can see it in the red colors on the top, middle panel which shows the dissolved oxygen concentration. There were some interesting patterns in the zooplankton and the fish as well, suggesting fewer fish in the low oxygen water (as we might expect) and some zooplankton and fish piling up right at the edge of the low oxygen water (also something we might expect). It’s important to keep in mind this is all preliminary and still must undergo rigorous QA/QC after the cruise is over, but regardless, there was definitely a region with dissolved oxygen concentrations lower than we had seen on the previous transects, and the fish and zooplankton patterns appeared to be different as well.
Now, I’m off to bed. More later…
Question from a non-scientist: I was wondering this even on your last cruise. What is the lowest level of dissolved oxygen that can support life in the water? I’m guessing it’s different for different creatures but maybe a general answer (if there is one). Also, and I’m guessing again, the creatures know when they have ventured into an area too low in oxygen to live?
Ed, you are absolutely right – there are different thresholds for different organisms, and it’s not even as simple as generalizing among broad groups. Within fish there is varied tolerance, and some other work suggests that within one species of copepod there is variable tolerance depending on where the source of the population is – animals from areas with persistent low oxygen are more tolerant than animals from an area where this is less frequent low oxygen.
And yes again, most animals seem to change their behavior a bit when they encounter low oxygen, but in various ways. That is, in fact, one of the things we’re studying out here!