Along with the CTD action you saw in Ring around the Rosette, many of our stations also involve lowering zooplankton nets into the water. Zooplankton are an incredibly diverse group of animals that float freely with the ocean’s currents; they range from tiny, microscopic larvae to giant, 50-ft long jellyfish (this video captures some of their amazing diversity).
To catch these creatures, we’re using three types of nets. The first is called a vertical net, which is lowered 100 meters into the water, and then pulled straight back up. This net captures the entire community of zooplankton living in that first 100 meters, and can be used in a larger data set to examine long-term changes in the California Current plankton. To catch some of our target animals, like pteropods, krill, and copepods, we use a Bongo net, which has two side-by set nets that resemble bongo drums. We tow this net behind the ship from deep waters up to the surface, allowing us to collect more animals in one tow. This net is also important for catching krill, which can swim fast enough to escape the slower-moving nets. Finally, we use a Neuston net, which is towed alongside the ship right at the surface of the water. The goal of this net is to catch young Dungeness crabs (Metacarcinus magister) that come to the surface at night. Sometimes, we do have “slow” days—many zooplankton species are not equally distributed, so we won’t catch any for a few days, and then suddenly we’ll catch a ton!
So what are we looking for? All sorts of zooplankton come up in our nets: larval fish, crabs, sea stars, clams, oysters, and barnacles, which will all stop being plankton once they’re mature enough to fight the currents; along with animals that will always be plankton, like pteropods, copepods, and krill. Besides the pteropods that we featured in this post, we’re particularly interested in young Dungeness crabs, krill, and copepods. It’s during early development that many zooplankton species appear to be most susceptible to environmental stresses like ocean acidification.
In the lab, we’ve seen that lower pH can have have a negative effect on baby Dungeness, and our goal on this cruise is to see if changing ocean pH is having an effect on Dungeness larvae in the field. Dungeness are a large part of the California Current fisheries, and so have considerable economic importance. Additionally, like every animal, they’re vital to the ecosystem, so a loss of Dungeness could have many negative impacts.
We’re also studying krill (specifically, Euphausia pacifica) because, like Dungeness crabs, laboratory experiments have shown that their early stages are vulnerable to low pH. Krill are vital to marine ecosystems because they are consumed by lots of fish, seabirds, and whales. Finally, we’re looking at copepods because they’re an important source of food for larval fish. As food sources, both animals are hugely ecologically important to the health and stability of our oceans.
Every time we bring a net up, there’s an air of excitement: what will we have caught this time? There’s always something new and unusual—favorites have included small fish, baby squids, and large gelatinous salps. It certainly never gets boring!
Author: Anna McLaskey