New research investigates global carbon cycle from phytoplankton satellite imagery

A recent study has produced a 20-year time series of primary production by marine phytoplankton, one of the largest fluxes of carbon on our planet. Studying phytoplankton primary production is important because it provides useful information about ocean biology, climate, and global carbon cycle. Observing primary production over long-time scales or quantifying its small variations can help the scientific community to determine carbon dioxide concentrations, as well as the effect of climate variability on these processes. The study uses in -situ measurements and satellite data from the European Space Agency’s (ESA) Climate Change Initiative,

The study assesses global annual phytoplankton primary productivity between 1998 and 2018 and aims at identifying its long-term patterns and its variability with seasons and locations. It uses a global database of in situ measurements of photosynthesis versus irradiance (P-I) parameters and a 20-year record of climate quality satellite observations. Primary production was measured using models based on ocean-colour remote-sensing observations that display the relationship between phytoplankton biomass and Photosynthetic Active Radiation (PAR, 400–700 nm). The data was obtained from the European Space Agency (ESA) Ocean Colour Climate Change Initiative project and contained merged products of observations from the Sea-viewing Wide Field-of-View Sensor (SeaWiFS, 1997–2010), the Medium Resolution Imaging Spectrometer (MERIS, 2002–2012), the Moderate Resolution Imaging Spectroradiometer (MODIS, 2002–present) and the Visible Infrared Imaging Radiometer Suite (VIIRS, 2012-present).

Although researchers suggest to wait at least 30 years to be able to successfully identify any clear climate trends, the research has already revealed some important findings. First, it shows that global annual primary production varied around 38 to 42 gigatonnes of carbon per year. Second, it highlights that inter-annual changes in global primary production does not follow a linear trend, rather it varies location to location, season to season, and year after year. In particular, it was found that there is high production in coastal areas and low production in the open oceans. The paper also shows that phytoplankton productivity levels increase and decrease with major Earth system processes – such as El Niño, the Indian Ocean Dipole, and the North Atlantic Oscillation. Moreover, the study reveals that trends in primary production follow directly from changes in chlorophyll-a and are related to changes in the water's physicochemical conditions (such as temperature) due to inter-annual and multidecadal climate oscillations. 

The study was published in Remote Sensing and contributes to ESA’s BICEP (Biological Pump and Carbon Exchange Processes) Project. BICEP is a platform that aims at bringing together research that use space data, in situ measurements, and model outputs to better understand components of the ocean biological carbon pump, its pools and fluxes, its variability in space and time, and its processes and interactions with the Earth system.

Marine phytoplankton are microscopic, free-floating plants in aquatic systems. They are one of the largest fluxes of carbon on our planet. Studying their natural processes is important to determine the global carbon dioxide concentrations and any related variations. Even a small variation in their primary productivity can influence biodiversity and fisheries. Along with marine phytoplankton, other marine plants can have harmful impacts on coastal ecosystems, public health, fisheries, and coastal economies. Harmful algal blooms (HABS), for instance, can produce toxins that can result in animal poisoning, respiratory irritation in humans, shellfish contamination, and changes in aquatic vegetation. HABS occur when algae — simple photosynthetic organisms that live in the sea and freshwater — grow out of control. 

Spaceborne technology can help monitor marine plants, identify any changes in their concentrations and behavior, determine any factor that might affect them, and prevent any risks associated with them. By measuring the reflectance of the ocean and by providing information on ocean color (based on light intensities), satellite imagery can track the frequency and severity of marine plants, provide information on the plants’ location and, therefore, help scientists and the public health community to target specific areas and to make informed decisions about bloom prevention and minimization of exposure risk. For more information on how satellite technologies can be used to monitor marine plants, click here.