Phytoplankton Study: Taking water samples to study tiny, precious particles
estimated reading time: 5 minutes
One of the scientific objectives of the SWINGS campaign was to understand the evolution and development of phytoplankton in the Southern Ocean.
Phytoplankton refers to all single-celled organisms found in the water. The majority of these organisms are plants that develop in a way similar to terrestrial vegetation. They produce their own organic matter through photosynthesis using light, CO2 and nutrients from the water.
But the nutrients in the ocean differ from region to region. The Southern Ocean, for example, is rich in nitrate, phosphate and silicate but lacks iron. These elements in the water have several sources: abyssal hydrothermal sources, the seabed or the sky through atmospheric dust.
Scientists are therefore still trying to analyse these flows of material, which is a relatively complicated process since material can disperse and dissolve at different rates in water. This is where tracers – also called ‘trace elements’ – come in to help with the analysis of nutrients in water. This is the case for Beryllium 7, which the SWINGS scientists measured on a massive scale in order to deduce the quantities of iron, zinc, nickel and copper that have ‘fallen from the sky’ into the ocean.
Bellyrium 7: a rare and valuable radioactive tracer to estimate the amount of iron in seawater
Beryllium 7 is a radioactive tracer that is produced in the atmosphere when cosmic ray particles interact with oxygen and nitrogen atoms. This tracer is deposited on the surface of the oceans, like atmospheric dust, and eventually enters the ocean.
Beryllium 7 is valuable to science because its short-lived radioactivity assures its origin. Observed on the surface of the water, we can be sure that Beryllium 7 cannot have been transported by sea currents and that it necessarily comes from the atmosphere.
In the atmosphere, scientists are well aware of the proportion of iron in atmospheric dust in relation to the quantity of Beryllium 7. By measuring the quantity of Beryllium 7 in seawater, scientists can deduce the quantities of iron, as well as zinc, nickel and copper that have ‘fallen from the sky’ into the ocean, using a simple rule of three.
But while Beryllium 7 is valuable, it is also rare and its collection requires the deployment of large-scale instruments. During the SWINGS campaign, two American researchers from Florida State University (FSU) and Florida International University (FIU) – William Landing and Christopher Lopes – were specifically responsible for collecting Beryllium 7 samples for analysis. On board the oceanographic vessel Marion Dufresne, a pump attached to a submersible hose allowed the sampling of 600 litres of water at various depths up to a maximum of 200 metres. Carried out 5 to 6 times, this manipulation made it possible to collect a maximum of water and Beryllium 7 which, as it moves away from the surface, can rapidly disappear by radioactive decay.
The study of particles: a complex and meticulous protocol
The water collected by the pump is then stored in large 700 litre tanks. It then flows gently from the base of the tanks through glass wool cartridges impregnated with iron oxide, which absorb the Beryllium 7. Then it is stored on board in refrigerators and freezers.
The analysis of the samples is only carried out once back on land since the instruments (such as the gamma spectrometer) needed for these analyses are both very fragile and too expensive to be taken on board during the campaign.
Studying the development of plankton therefore requires a whole upstream process that requires a great deal of technical skill and patience, in order to better understand the composition of the ocean and thus advance science.
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