Sailing the Southern Ocean – nPOP Research Cruise

Sarah Le Besque

University of Oxford

 

Caption: Sarah Le Besque fighting the wind of the Southern Ocean aboard the RSS Discovery. They are wrapped up warm in a red coat, wearing their yellow hardhat and smiling at the camera. 

In December 2023 I departed the grey clouds of Oxford and arrived in sunny Cape Town to mobilise the RRS Discovery ahead of the nPOP research cruise (new Perspectives on Photosynthesis). nPOP is funded by NERC (project number: NE/T00746X/1) and the main aim of the project is to understand how light harvested by photosynthetic organisms in the surface ocean is used to provide metabolic energy and to fix organic carbon. The cruise travelled south to 59°S (the HNLC region of the Southern Ocean), then northwards to 30°S (the South Atlantic Subtropical Gyre). 

My specific role on the ship was to observe how the phytoplankton community structure in this under-sampled region of the global ocean is related to changes in environmental forcing (physical and nutrients) as we traversed across a range of oceanographic conditions over a latitudinal gradient. As we sailed along our transect, we aimed to profile the water column every day using two CTDs over the month-long cruise - a stainless steel cast in the euphotic zone to collect samples to observe the biology, and a deeper titanium cast for the trace-metal-clean samples. We also sampled from the non-toxic underway and from the trace-metal-clean tow-fish, every 4 hours, collecting a suite of data on the biochemistry of this region.

For my work, a variety of samples were collected to determine the community structure, pigment composition, optical properties, and abundance of marine phytoplankton.  I also used a specialised flow cytometer, the CytoSense instrument, to provide detailed imagery of the cells, and their fluorescence and scattering characteristics. The device, generously provided by the University of Southampton, was plumbed into the RRS Discovery’s underway system and was set to analyse a seawater sample every 4 hours.  The Cytosense auto-sampling capabilities allowed for real-time observations of the taxonomic and size structure of the phytoplankton cells to be collected as we sailed from subpolar to subtropical waters.  Seawater was filtered to harvest cells for both High Performance Liquid Chromatography (HPLC) analysis to detect various algal pigments, and spectrophotometric analysis (PABS) to determine how these pigments absorb light energy, both of which will be used to ground truth satellite observations.  The data collected on this cruise will form the basis of a large portion of my PhD thesis and will allow for multidisciplinary collaborations with international scientists from the US, Germany, and South Africa.
Caption: Collecting water for “bucket science” – Sarah is squatting down next to the CTD rig, with a plastic tube attached to the CTD tap. The tube leads to the plastic jerry can they fill with seawater to sub-sample from. 

Throughout the cruise, opportunities to conduct unplanned science were presented to us – fondly called “Random Acts of Science”. For example, whilst we waited for storms to pass, we conducted diel sampling to observe how the proteins, DNA, and fluorescence of phytoplankton varied as the light intensity changed over the course of a day. We also sailed around an iceberg, sampling the surrounding waters, and carrying out 7 CTD casts in 36-hours while we traversed an Agulhas ring. While not part of the original proposal, they were opportunities too good to miss.
Caption: Christmas Dinner aboard the RSS Discovery – 6 people are seen sitting around a table, decorated for Christmas. They wear paper hats, and some are letting go of balloons. 

The data collected on this cruise will form the basis of a large portion of my PhD thesis and will allow for multidisciplinary collaborations with international scientists from the US, Germany, and South Africa.
Caption: Iceberg hunting – Standing outside the bridge of the RSS Discovery, Sarah is looking away from the camera through binoculars towards a large iceberg on the horizon.

The science was what inspired me every day, but the kindness of the crew and fellow scientists was what made each day a joy. Throughout the cruise, I was reminded how incredibly lucky I am to be at the start of my career in oceanography, and that this could be my office for several weeks of the year. I am eternally grateful to the organisers of nPOP for finding space for me on the ship, and to my fellow PhD students who I look forward to leading our own cruises together with in the future.
Caption: Sunrise CTD deployment – The CTD rig is hanging from the winch as it is lowered off of the side of the boat into the water as the sun rises behind it. 

Finally, I would like to acknowledge the Challenger Society for Marine Science, your generosity made my involvement with this cruise possible, an experience that will shape my career as an oceanographer.

Profile
I am a second year PhD student at the University of Oxford, UK, working with Prof Heather Bouman and collaborators at the University of Southampton, UK. I am funded through the NERC Environmental Research DTP, as well as grants awarded from the Burdett-Coutts fund, the Challenger Society, and St John’s College, Oxford.
Using a combination of in-situ methods, I am researching how phytoplankton community structure changes seasonally and temporally in the Southern Ocean. I hope to use remote sensing to observe the community structure across all regions of the Southern Ocean.