Measuring surfactants in the ocean

by Penny Holliday and Bita Sabbaghzadeh

If you want to know about how the ocean moves carbon and other gases around the world, you need to learn about how the gases move across the boundary between the air and the sea. The rate at which gases enter the ocean, or leave the ocean and enter the atmosphere, is called the “gas transfer velocity” and this is a hot topic in ocean science at the moment.

Bita is a PhD student from Newcastle University and is here on cruise JR302 to address the question of how natural surfactants in sea water affect the rate at which carbon dioxide crosses the boundary between air and the ocean. Surfactants are special molecules that can bind together molecules from substances that don’t naturally mix. We use them all the time – you will find surfactants in soap where their purpose is to bind oils to water so you can wash the oils off your skin. Surfactants in the ocean can bind air and water molecules together, reduce surface tension and change the interaction between air and water. They can reduce the mixing in the very top layer of the ocean by flattening the tiny waves caused by wind blowing across the surface. With less mixing, the very surface water becomes rich in carbon dioxide and the rate at which the gas is absorbed by the water slows right down.

The surfactants that Bita is interested in come mainly from phytoplankton (tiny algae) that grow in the sunlit surface layer of the ocean. When phytoplankton grow in large numbers they are called “blooms” and these blooms can create so much surfactant that the wind can whip the surface of the sea into a frothy layer.

Bita’s job here on the cruise is to find out how the concentration of natural surfactants changes across the huge region of the open ocean between Canada and the UK. Once a day she takes samples of the very thin sea surface layer, and from bottle samples of water collected on the CTD package from the depths of the ocean. She is expecting the amount of surfactant to change every time she collects samples, and is looking for the connections between the level of surfactant and a range of things like how windy it is, how warm the water is, whether there are a lot of phytoplankton on the water, whether there is a strong current. No-one else has attempted to make these connections over such a wide area of ocean before so the results should be very interesting.

Her task is not a simple one. It is very hard to capture water from the very surface layer of the ocean when the surface is moving and you are standing on a ship which is also moving. The ship itself is not free of surfactants and there is a risk of contaminating samples, so great care needs to be taken with the way water samples are captured and stored. On previous cruises, water samples have been collected, frozen, then analysed in a laboratory back on land. But it is not clear that the surfactants survive this process intact, so Bita has taken the innovative approach of bringing an analyser (a machine called a polarograph) to sea with her. Getting the analyser to work on the ship is a challenge in itself; it has been designed to operate in the controlled conditions of a laboratory not on a moving ship. We all love a challenge though, so between Bita, Brian and Simon the Deck Engineer the race is on to get the analyser working before the end of the cruise. Bita will be back here on the ship in September, and it will be a huge advance for her studies if a way is found to make it work by then. It seems that the machine is very sensitive to the movement of the ship and to the vibrations that are ever present onboard. Simon has built a special gimbal table for the analyser – this ingeniously allows the machine to stay perfectly horizontal while the ship moves around it. This has improved things greatly, but they are still working on ways to counteract the vibrations from the ships engines, thrusters and other equipment onboard.

By the end of the cruise Bita will have collected a large number of samples and data, and gained a huge amount of experience of making difficult measurements at sea. Along with her new-found sea legs these will stand her in good stead when she rejoins the ship for another long cruise in September.

Images: Bita collecting samples of the surface layer with the help of Ken, Bita collecting samples from 20m below the surface to measure how much phytoplankton chlorophyll is in the water, and the analyser on its new gimbal table (all by Penny Holliday)

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