Take me down to Plankton City, where the algae is green and the copepods are pretty

By Sophie Dolling, The University of Adelaide

Where is Plankton City you ask? In the mixed layer; the top most layer of the ocean surface where the water column is largely uniform. How do I know where the mixed layer is? An amazing instrument called a CTD.

The CTD (conductivity, temperature, depth) rosette is deployed from the starboard side of the RV Investigator as part of at sea marine science training on the 2019 CAPSTAN voyage
The CTD in action: lowering into the water column to bring us back the goods

The CTD is loaded with bottles that fill up with water at certain depths in the water column. Each bottle will fill at an individually nominated depth, allowing us to see water from all levels of the ocean. We get to sample the deep Antarctic water, the old oxygen depleted water and the nutrient rich mixed layer water all during one deployment!

A small orange colored sea star from the sieved bongo net samples is viewed through a microscope on board RV Investigator as part of the hands on training for marine science students on the 2019 CAPSTAN voyage
Small sea star found in the larger fraction of the sieved sample

Whilst sampling at the Bonney upwelling zone we were given the task of (hopefully) finding some plankton to identify, sort into size classes, and indicate biomass abundance. To do this, we used a bongo net to collect plankton from different points in the water column. These points were predetermined by analysing data from the CTD. We decided to always sample at 100m deep and one other depth depending on what we saw on the CTD profiles. We were looking for the point where the water column exhibits a sharp change in temperature and density; this is known as the mixed layer depth (bottom of the mixed layer). In basic terms, the water column goes from being mixed to more stratified the deeper you go. The mixed layer depth causes a barrier-type density difference, trapping nutrients above or below the boundary. If nutrients are brought into the mixed layer because of upwelling, the water above the mixed layer depth should be Plankton City; full of yummy nutrients allowing plankton growth.

Voyage participants watch as the two half meter diameter bongo nets are deployed over the starboard side of RV Investigator to collect plankton as part of the hands on marine science training of the 2019 CAPSTAN voyage
The bongo nets on their way down while the team anxiously waited, hoping for plankton to analyse

The Bonney upwelling zone is theoretically a ‘hotspot’ for plankton growth because of the nutrient rich bottom water moving up the water column to the surface through a range of mechanical processes. As we soon figured out, science and the ocean don’t care how far you’ve come to see them; they just do their own thing. The upwelling was not happening, in fact there was most likely downwelling occurring while we were on site. The expected abundance of plankton was largely unknown. What would we see? Would we see anything? Would we see lots?

A small bioluminescent organism is visible in one of the bongo net samples collected by CAPSTAN students on board RV Investigator as part of their hands on marine science training
Sparkly Boy, winner of Plankton Cities Most Beautiful

The bongo net tows did not disappoint. Whilst we do not have the final results of biomass abundance or size class just yet, we do know that Plankton City is an exciting and diverse place. Each of the tow samples were passed through a sieve, separating the plankton into size classes: larger than 100 microns and smaller than 100 microns. Among the inhabitants of Plankton City were a couple of tiny juvenile squids, hundreds upon hundreds of copepods, the spiky tennis balls of the water column (otherwise known as radiolarians), a squishy sea star, and many more wild and wonderful things*. There were two specimens in particular that were voted ‘Plankton Cities Most Beautiful’; Mr Fabulous and The Sparkly Boy. This pair of bioluminescent pretty boys were the talk of the lab**. Mr Fabulous was voted Most Beautiful for his sparkling eyes; eyes that would make Mrs Fabulous swoon. The Sparkly Boy took this one step further, showing off his sparkles all over his body.

Tiny squid is seen through the microscope on board RV Investigator along with other organisms trapped by the bongo nets during the 2019 CAPSTAN voyage.
A small squid found in the larger fraction of the sieved samples.

While on site we were only able to do a handful of bongo net tows. We were able to see some pretty amazing stuff from such a tiny sample size. Can you imagine what else we could find down there? I don’t know about you, but Plankton City is certainly somewhere I want to visit again.

*No squid, sea stars or sparkly boys were harmed in the making of this blog (we let them go back to Plankton City).

Check out my group’s hydrochemistry & oceanography blog on AGU’s The Field!

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3 thoughts on “Take me down to Plankton City, where the algae is green and the copepods are pretty

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