There are sometimes multiple ways of measuring the same phenomenon at sea. Mark Ohman’s MVP (moving vessel profiler) has an LOPC (laser optical particle counter) on it that scans plankton as it dives through the water. It records pixelated shapes of the plankton as they pass through it.

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The blue oblong shapes on the right of the screens showed up on the LOPC on every dive between 30-10 m, right where the highest concentration of doliolids was.

Tristan Biard’s UVP (underwater vision profiler) takes photos of plankton as the CTD (conductivity, temperature, depth sensor) descends on each dive. It does this by illuminating a flat plane of water that the camera focuses on as the CTD descends, and it takes pictures of all plankton in that plane of light. The photos are then sorted by a computer algorithm in to categories of animals that Tristan then re-sorts manually.

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Left: The UVP attached to the CTD rosette. The red bars emit the light that create the flat plane of light. Right: Dr. Biard hard at work analyzing images.

As we have been going through the doliolid plankton bloom, we wanted to see if the UVP and MVP were both detecting the bloom at the same depth and in similar amounts. We drove the ship in a circle to sample the same water parcel, and we first did five CTD dives to sample the UVP, then five MVP casts, then five more UVP tests. [Also, sorry scientists love acronyms!] Then we preserved the water from the CTD bottles of the last UVP cast, to see if there were doliolids in the actual CTD bottles. The CTD is only at each depth for a few seconds as it closes each bottle, so to catch doliolids in the bottles would mean the density is very thick.

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Preserving the bottles.

The CTD water, the MVP LOPC, and the UVP all saw the same thing – doliolids at the same concentration at the same depth. But we can’t truly compare the results of how they recorded the bloom until Tristan manually checks all the images. And that will take many days. Like so many things we do at sea, there is a lot of analysis on the backend after the instrument comes on board. So until then, we wait and wonder how all our instruments compare. On to Cycle 4!