This is the fourth ADEON cruise, and we’re slowly accumulating little habits and traditions among the science crew. These include things like the PIs bringing out special treats for after we finish up all the tasks at each site (it was Almond Joys and Mounds after the last one), and the epic night shift vs. day shift board game each cruise (to claim gloating rights until the next trip, of course). And of course, at a certain point of dusk or dawn, whoever is near the monitor displaying the echosounder data might shout out to the rest of the science crew, “THEY’RE MIGRATING!!”
The shouter is referring to the diel vertical migration (DVM) of zooplankton in the water column. Ocean currents dominate the horizontal movement of zooplankton, but many of these animals can control their vertical movement. DVM happens when some of these zooplankton rise up from the mesopelagic, the ocean layer from about 100 m to 1000 m, into the epipelagic (surface layer) at night to feed on phytoplankton or epipelagic zooplankton. Once the sun starts to come up, these animals descend tens to hundreds of meters back down to the dark mesopelagic, to stay hidden from predators that could see them if they stayed in the sunlit surface waters. For this reason, DVM is driven entirely by light.
DVM is such a large event that we can often see it on the echosounders! It’s always a cool thing to point out to the newcomers to the ADEON science crew, who can then join in the fun of watching for the migration each morning and night!
By Hannah Blair, Stony Brook University graduate student.