Catching zooplankton

Zooplankton are a highly diverse functional group that range in size from the micro- (0.002m) to the macro (>0.2m) and span multiple phyla. Included among these are ethereal-winged sea butterflies who flit through the water like hummingbirds (Mollusca), darting copepods (Arthropoda), pulsating hydromedusae (Cnidaria), ambush-ready arrow worms (Chaetogantha), and undulating larval fish (Vertebrate).

Zooplankton are a highly diverse functional group that range in size and span multiple phyla. These include copepods (Arthropod), pteropods (Mollusc), arrow worms (Chaetognath), and larval fish (Vertebrate; larval sunfish shown).

Zooplankton play a critical role in ocean food webs. They transfer energy fixed by phytoplankton (microscopic plants and protists) via photosynthesis to larval and juvenile fish and larger plankton-eating fish like gulf menhaden. These fish, in turn, support important fisheries along the U.S. Gulf Coast as well serve as prey for consumers like sharks, dolphins, and sea birds. Because they are environmentally sensitive, changes in zooplankton communities and populations often portend larger-scale shifts across a marine ecosystem.

One ecosystem-level consequence of changes in zooplankton is the altered frequency and intensity of trophic interactions (i.e. who eats who) with fish, a main consumer of plankton. Like humans, fish have food preferences. If a fish has trouble finding food or finding their favored type of zooplankton to eat, then they are at risk of not getting the nutrition they need to avoid a predator, grow, and reproduce. Intense disturbance events like the Hurricane Harvey floodwater plume can disrupt the types and abundances of zooplankton available to fish by changing the ocean’s biophysical characteristics over a range of time and spatial scales, including water depth.

We can determine the extent of these changes by collecting zooplankton and larval fish in the impacted area at different water depths using a MOCNESS (Short for “Multiple Open Closing Net Environmental Sampling System”). The MOCNESS is a large piece of gear—the frame supports 10 1-m2 nets and a suite of sensors that continuously measure the system’s position and velocity and key environmental parameters.

The MOCNESS is deployed and recovered off the ship’s stern using the A-frame and a winch with a conducting cable (see video).

An operator ‘flies’ the system from the ship using a custom software program that relays commands to the system via the conducting cable and shows the depth the system is at. From this program, the operator remotely triggers and closes each net at a precise depth, collecting zooplankton at features of interest. This trigger and close can be seen in the video below during a MOCNESS tow.

After the MOCNESS is recovered, the bottom 1/3 of the net is rinsed into a bucket. Because our lab also wants to quantify the large jellyfish (>5mm) that often found on the continental shelf, a subset of cod-ends are first filtered through a colander to catch these larger plankters. [Priceless data = $100,000 MOCNESS + $3 bucket + $1 colander].

In the wet lab, zooplankton are concentrated a using 150-µm sieve. You can see the difference in color between a sample collected offshore (left) and one collected near the coast (right). The concentrated sample is then preserved in either ethanol (allows molecular & otoliths analysis) or a formalin seawater solution (allows jellyfish identification). Because every single zooplankter is important to answer our ecological questions, the sorter picks individuals off of the sieve with soft-tipped forceps. (Crab megalope are famous hangers-on!)

Back at the lab, zooplankton in each sample will then be either manually sorted and/or automatically identified from a ZooScan image processed with machine-learning-based classifier program at Oregon State University.