The Amazon rainforest is often called the “lungs of the planet” as it produces up to 20% of the oxygen we breathe. However, a much more important source of oxygen, the phytoplankton in our oceans, often goes overlooked. Phytoplankton are miniscule single-celled algae that drift at the surface of the ocean and account for up to 50% of the oxygen in our atmosphere and form the basis for all marine life1. Just like plants, they contain the green pigment chlorophyll, which allows them to use the energy of sunlight to transform CO2 to sugars and oxygen. As such, they are the primary producers at the base of the marine food chain.
As they require both nutrients and sunlight to grow, phytoplankton are not uniformly distributed across the ocean. Because of the distinct green colour of the chlorophyll pigment, we can use optical satellite sensors to visualise the distribution of chlorophyll and thus the phytoplankton in our oceans. These satellite observations are then calibrated with in-situ chlorophyll concentration measurements like the ones offered by EMODnet Chemistry. The map of the week features the long-term average concentration of chlorophyll at the ocean’s surface in milligrams per cubic meter of water.
The average distribution of the phytoplankton mainly reflects the presence of nutrients like iron and the ocean currents that carry them around. You can see large pools of low chlorophyll concentration in the subtropics. These result from the presence of the subtropical gyres, wind-driven planet scale whirlpools that push the low-nutrient surface water to their centres. As they pull the surface water away from the equator, deeper nutrient-rich water is allowed to rise, resulting in mass phytoplankton blooms and higher-chlorophyll bands around the equator. In the mid-latitude and polar regions, strong winter cooling causes the surface waters to sink – a phenomenon known as thermal winter convection – that seasonally replenishes the surface water and nutrients, allowing more phytoplankton to bloom. The highest concentrations of chlorophyll occur at the edge of the continents like the west-coasts of Africa, North and South Amerika. Here, alongshore winds push away the surface water and allow deep water, rich in nutrients, to rise along the steep continental margins in a process called coastal upwelling.
As phytoplankton require sunlight in addition to nutrients, there are also significant daily and seasonal changes which are not apparent from the long term average concentration map. However, if you are interested, you can check out the near real-time daily and monthly averages of chlorophyll concentration from Marine Copernicus which are also available in the European Atlas of the Seas. Note that the absence of data in the daily map is caused by cloud coverage.
The data in this map were provided by the Joint Research Centre (JRC).