The waters are teeming with plankton!

Written by Amber Wilkinson

This week, September 8, we looked at plankton in the water column 0.5 um and larger. Before conducting two plankton tows we discussed diel vertical migration and larval transport. Diel vertical migration is a behavioral pattern in which organisms come up to the surface at night and down during the day.

The collection method was new to me. Plankton tows I have done in the past were off the side of docks and boats that are in neutral. This method used a bongo net which consisted of two plankton nets side by side resembling bongo drums. The bongo layout allows for replicate samples for each tow. Tows were conducted for 5 minutes. Tow 1 sampled the top meter of the water, and tow 2 sampled the entire water column (an oblique tow). The boat speed for our plankton tow was 2 knots. We sampled against the current and in the wake of the boat. If you want to measure the volume of water sampled, it is not recommended to sample in the wake of the boat because the prop creates turbulence, disrupting the sample area.

After a 5 minute tow, the net was rinsed in the water by holding the net’s ring and dipping the net in the water vertically 2-3 times to get most of the sample down in the cod end. The cod end is the end of the net where the sample collects. Our nets have mesh openings in the cod end to allow for more water flow. Before conducting another tow, the net was rinsed for 30 seconds in the water without the cod end to minimize contamination between samples.

To keep a lot of water out of our sample jars (more condensed samples) the contents from the cod end can be rinsed into a sieve than from there into the sample jars. For the purpose of our class, we looked at our samples in Pyrex dishes (9x13in).

Back in the lab, we observed the samples under dissecting microscopes. The diversity amazed us, there were fewer species near the top. The surface tow contained crab larvae and a skeleton shrimp. An interesting observation was that the crab larvae followed the light from the microscope when we were looking at the different species in the petri dishes. This may suggest why we found so many crab larvae near the surface. Also, there was greater diversity throughout the water column. Juvenile shrimp, mature smaller shrimp, larval fishes, parasitic isopods, crab larvae, and a lot of plant matter were in the oblique tow.