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Phytoplankton: Small cells with a big impact

Citation: Kulk, G., et. al., 2020. Primary Production, an Index of Climate Change in the Ocean: Satellite-Based Estimates over Two Decades. Remote Sensing 12, 826. https://doi.org/10.3390/rs12050826

The power of phytoplankton

Phytoplankton make up less than 1% of the photosynthetically active biomass on Earth, yet they are responsible for half of the total global net primary production. Primary production is the rate at which phytoplankton remove carbon dioxide from the atmosphere and convert this carbon dioxide into sugar. This process is called photosynthesis. Lucky for us humans, oxygen is produced from photosynthesis so we get our oxygen from the ocean! Pretty neat.

An example of a phytoplankton community in the ocean. Cells come in many different shapes and sizes! Photo credit: Richard Kirby, Smithsonian

Primary production in the ocean is one of the largest fluxes of carbon on the planet. Therefore, measuring primary production (PP) is of great interest to the scientific community. Production rates are affected by physical parameters in the water such as temperature, light and nutrients. Phytoplankton need light for photosynthesis and nutrients for various cellular processes. The ocean has already experienced environmental change, such as increases in sea surface temperature, sea ice melt and enhanced precipitation. These physical changes will affect nutrient concentrations in the sun-lit layer of the ocean, where phytoplankton thrive. Therefore, it is important to monitor how physical changes affect primary production rates.

Global net primary productivity across the ocean basins. Image credit: earthobservatory.nasa.gov

Primary production is measured in different ways. Scientists can either collect direct measurements from water sampling or they can use satellite-based methods. Wonder how we can use satellites to look ay microscopic organisms from space? Well, all phytoplankton use a pigment known as chlorophyll-a to help them harvest energy from the sun during photosynthesis. Wavelengths of chlorophyll-a can be picked up by satellites in space and are used to get an estimate of global phytoplankton biomass. In this study scientists used a database of chlorophyll-a concentrations and surface light measurements in the ocean to establish global primary production and its changes over the last two decades.

How did they do it?

The scientists obtained monthly surface chlorophyll a concentrations from 1998 to 2018 from the European Space Agency Ocean Color Climate Change Initiative project and separated into different oceanic regions. For the primary production model, the authors used a model that can separate chlorophyll-a wavelengths from other light that is reflected off the ocean surface and they were able to look at how chlorophyll-a changed with depth. This model incorporates changes in photosynthesis as a function of light. The scientists retrieved photosynthesis-irradiance curve parameters from a global database. A photosynthesis-irradiance curve is a graphical representation of the relationship between photosynthesis and available sunlight. Since photosynthesis requires light, there is a positive relationship between these two variables; however, the exact relationship and thus parameters, depends on physical properties such as temperature and nutrients. Parameters for the photosynthetic-irradiance curve change based on location in the ocean so the scientists turned to a database to incorporate different parameters in their model. The database covers 53 provinces and represents about 97% of the world’s oceans. So by combining measurements and parameters from the database, the scientists were able to understand how various photosynthetic parameters affected the sensitivity of primary production over two decades from 1998 to 2018.

What did they find?

Using the models they created with photosynthesis versus light parameters, the scientists found that summer was the most productive (11.6-12.9 gigatons of carbon over three months). For comparison purposes, the Amazon rainforest fixes 86 gigatons of carbon per year. On more regional scales, the Pacific Ocean had the highest production and general trends in production rates across all regions varied considerably between 1998 and 2018. There was a large variability in global production on an annual scale where production increased during some years (1998-2003), was stable during others (2003-2011), and decreased at other times (2011-2015). The variations in global primary production were associated with trends in the El Niño-Southern Oscillation and Atlantic Multidecadal Oscillation climatic events. These climatic events work on larger scales and affect wind patterns and currents. Changes in wind and currents can affect nutrient conditions and thereby affect phytoplankton growth and production rates.

Satellite image of a phytoplankton bloom in the Southern Ocean. Image credit: NASA

What is the point of this?

Phytoplankton production rates are difficult to measure directly and can vary significantly between methods. Since phytoplankton production is a key process in global carbon cycling, it is important to understand the environmental parameters that affect the ability of phytoplankton to take in carbon dioxide from the atmosphere. This study is the first to combine high-quality, multi-sensor ocean color observations over two decades to examine the magnitude and variability in marine primary production on a global scale. The model developed in this study led to a more accurate assessment of global annual primary production and its trends over the past 20 years. Modeling these production rates using high resolution models is a good approach to estimate how rates will change under continued global environmental change.


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