By Laetitia Drago, PhD student at Sorbonne Université / NORTHERN ATLANTIC OCEAN /
As a child, I used to spend my summers on the rocks near the water in Villefranche-sur-mer, France, my hands busy with a bucket and a small net. I was fascinated by the organisms surrounding me both on the rocks and in the water. Little did I know that I would have the chance to explore the open ocean with a bigger hand net, and multiple imaging instruments on a 231 foot (70.5 meter) long vessel.
I started my PhD in October at IMEV in Villefranche-sur-mer, France, on the impact of zooplankton on the biological carbon pump through an in-situ imaging approach. It’s in this context that I had the privilege to join this impressive EXPORTS campaign onboard the Sarmiento de Gamboa research vessel. This vessel’s scientific team consisted mostly of people coming from the Woods Hole Oceanographic Institute, researching the ocean twilight zone, the layer of water between 656 and 3,280 feet (200 and 1,000 meters) below the surface of the global ocean. It is a very important layer of the ocean for the biological carbon pump, the process which is at the core of my PhD.
The biological carbon pump moves carbon from the surface to the intermediate and deep oceans. This process starts at the surface of the water where small plantlike organisms called phytoplankton do photosynthesis, the process of using light to transform carbon dioxide into organic matter. This phytoplankton is then eaten by zooplankton, which transfer the carbon from the surface to the intermediate and deep oceans through multiple processes such as producing fecal pellets and daily migration up and down throughout the ocean. These organisms constitute an important source of food for fish, making them an important link in the food webs supporting fisheries all around the world.
To look more closely at the ocean twilight zone, I brought imaging instruments to observe which organisms live in this layer. These included Underwater Vision Profilers (UVP). These instruments were developed in my lab in order to study large particles and zooplankton up to nearly 20,000 feet (6000 meters) in depth! The instrument counts and measures particles greater than 0.1 millimeters and saves images of the ones greater than 0.6 millimeters because those are the ones with a clear enough resolution to determine which taxonomic group we’re looking at. To do that, it uses a camera and a dedicated red light flashing system. On the image of the UVP6 you can see that there is a light. It can flash every few seconds depending on how you program the instrument. For the UVP6 for example, it was programmed to flash once every two seconds. This way, it illuminated a volume of water every two seconds below the camera, which can then take a picture of the illuminated field of view.
The UVP5 has already performed more than 10,000 profiles in the ocean throughout the 10 years since its creation. It has been used in all the oceans fixed on CTD rosettes like the one used during this cruise. CTD rosettes are submerged in the ocean to measure temperature, depth and salinity in the ocean.
I also used two UVP6s, a more versatile, small and powerful version of the instrument. Each one was in a cage, fixed to a drifting line which was deployed at sea. We hope that the images taken by these two instruments will help improve our knowledge of the biological carbon pump.
I also brought with me a Planktoscope. This microscope platform was designed at Stanford University by Plankton Planet and the Prakash Lab in the context of frugal science, which aims to bring science to the maximum number of people. It can be customized, redesigned and mounted aboard a ship by anyone in the world at a very affordable price!
Using a net or the water from the Niskin bottles (as seen in the second picture), I imaged the organisms living in the water and watched as the composition of organisms changed between the different parts of the ocean that we sampled.
Here a few images acquired by these instruments:
As you might know, this journey was not an easy one. Three storms came our way during our mission at the PAP site. Nevertheless, we managed to do 11 profiles with the UVP5 and get six and a half days of images from each UVP6 with one image every two seconds. This amounts to around 148,500 vignettes for the UVP5, 323,000 vignettes for the UVP6 and 79,200 vignettes for the Planktoscope.
The storms were unfortunate for our life on board and the conditions which stopped us from sampling during half of our presence at the PAP site. However, it was fortunate in the sense that we have a unique dataset containing data before the first storm as well as data between the three storms. This will hopefully give us an idea on the potential impacts that one or multiple storms can have on zooplankton and particle flux.
Our hard work was of course rewarded by the data acquired but also by a wonderful sunrise at the end of a very long last night of sampling followed by a 15 minute visit from a group of common dolphins on our way back to Vigo.
Finally, I want to deeply thank the team in Villefranche-sur-mer, France, who trusted me with handling the instruments and supported me from afar as well as the very motivated team of scientists and the ship’s crew support who helped us acquire very important data which will hopefully help us to understand a little bit more the carbon processes at hand in the ocean twilight zone.