Image of small floating objects underwater

Twenty years of data contribute to global phytoplankton study


For the first time, research led by PML brings together satellite ocean colour observations from across two decades with high-coverage in situ observations to compute primary production on a global scale.

Marine phytoplankton are a hugely important component of the ocean, particularly so in the vital role they play in transferring carbon from the atmosphere to the sea, as they fix carbon in the process of photosynthesis, producing energy from sunlight, which is passed up food chains as one organism consumes another.

Estimating this crucial energy production (or primary production) is, therefore, necessary in understanding how oceans and climate change are linked.

Visualisation showing global annual primary productivity from 1998-2018
Credits: Ocean Colour CCI, Plymouth Marine Laboratory/ESA

The recent study, published in Remote Sensing, involved researchers from across the world and used a global database of in situ records, measuring photosynthesis against light levels, as well as 20 years of satellite observations, to assess global primary production and how it varied across years, seasons and locations. It also investigated how sensitive the primary production calculations were to potential changes to phytoplankton photosynthesis under changing environmental conditions.

Using, and enhancing, this global in situ measurement database meant that photosynthesis calculations could be assigned to specific areas all across the globe, improving the confidence with which primary production could be estimated in those regions.

Looking at global primary productivity from 1998-2018, researchers found a wide range of variation, including in different areas, with high production near the coast and lower production in the open oceans, as well as changes relating to major Earth system processes such as El Niño events. Globally, annual primary production ranged between 38 and 42 gigatonnes of carbon per year.

PML scientist and lead author, Dr Gemma Kulk, said: “Everyone understands why the rainforests and trees are important – they are often called the lungs of the Earth, taking up carbon dioxide from the atmosphere. What is overlooked is that the oceans are of equal importance – every second breath you take comes from the oceans, and research like this helps us to better understand the changes.”

Co-author Dr Shubha Sathyendranath, also from PML, said: “Although the data records span 20 years, it is important to wait at least 30 years to be able to identify any clear climate trend with sufficient confidence.

“It is critical that the ocean colour dataset as part of the Climate Change Initiative be extended and maintained on a regular basis, so that we have an empirical record of the response of ocean biota to changes in climate. From this, we can develop reliable models, so we can accurately predict change in order to adapt to the impacts of a changing world.”

The study involved researchers from PML, University of Oxford, Laboratoire d'Océanographie de Villifranche, University of Exeter, University of Technology Sydney, Institut de Ciències der Mar, Instituto de Investigaciones Marinas, University of Tokyo, Universidad Rey Juan Carlos, Marine and Freshwater Research Institute, Xiamen University, Hokkaido University, University of Split, Instituto Nacional de Investigacion y Desarrollo Pesquero, Universidade de Vigo, Space Applications Center (ISRO), University of Copenhagen, University of Groningen, Australian Institute of Marine Science and the National Centre for Earth Observation.

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