Tropical Zooplankton by Martha

For my thesis project, I am examining how the zooplankton community changes in areas of depleted oxygen. My project design includes stations along our transect from San Diego to Manzanillo—at each station I have taken several net tows at different depths so I can figure out exactly what organisms are living where in the water column.

All of my sampling is done with my partner-in-crime, Sun-Li, who is examining jellyfish in the oxygen minimum zone. We use the same net tows at the same stations, but he counts and identifies the jellyfish, while I am focusing on the less gelatinous animals like copepods, chaetognaths, euphausiids, and various larvae.

In order to target specific depths (for example, 50-150 meters), we use a special plankton net with an “open-close” system. After the nets are sent down to 150 meters, we send a messenger, or a heavy weight, down the cable that attaches the nets to the boat, releasing the covers off the nets. The nets are then towed obliquely through the water and collect zooplankton until 50 meters when we send another messenger down the cable, releasing the nets off the metal hoops and cinching them tight. The dual-net design, or bongo nets, is supposed to be excellent at catching the critters, and allowing less to escape as we tow the net to the surface. An added bonus to this design is the automatic duplicate we get for each depth. In some of our deeper tows (300-500 meters), we have ended up counting both bongo nets because there is so little living in that depth range. Likewise, today when one of the cod pieces (the collecting piece) broke as it was coming on deck, we could still rely on the other bongo net to give us a reliable sample.

After rinsing down the nets we’re left with a container full of squirming zooplankton. Before they have too much time to start eating each other, we fix them in formalin, and preserve to count and identify later. I’ve already been seeing differences in the zooplankton community in the areas of low oxygen (deeper, and further south than San Diego). However, until I finish all of my calculations and have a chance to look at factors other than oxygen concentration, it’s difficult to say just how much oxygen concentration is influencing the zooplankton here.

Overall we have been seeing some amazing things in our net tows! My samples have been dominated by copepods so far, but these copepods are so different than the ones I see in Puget Sound! Some are bright blue, some fluoresce when poked, some have flashy red-feathery appendages—it’s almost like I’m identifying tropical birds rather than zooplankton.

When I’m not hunched over a microscope, I’ve definitely been enjoying our daily sea animal sightings. Dolphins, whales, and yesterday I saw a sea turtle! It is definitely a unique experience to spend hours examining the marine world under a microscope, and then to run outside after one of the frequent “whale!!!!” calls to see how these tiny critters have propagated up the food chain.

And, in response to Dawson from Mrs. Anderson’s class:

No, I don’t eat my samples. However, that doesn’t mean I couldn’t! In our net tows today we found a few small fish and shrimp. Some zooplankton are used worldwide in various seafood bases, or for creating health supplements. Also, some zooplankton are the larval stages of things that people commonly eat, like fish, crab, and shrimp.