Phytoplankton are microscopic single-celled algae that drift at the surface of the ocean. They contain chlorophyll which allows them to use the energy of sunlight to transform CO2 to sugars and oxygen by means of photosynthesis. As they need sunlight, they are found in the upper part of the ocean.  Phytoplankton productivity varies according to seasons and regions.  Spring is the peak season for the development of phytoplankton as there is plenty of light, and nutrients have been transported to the surface of the ocean through the deep water mixing during the winter.  Because of the distinct green colour of the chlorophyll pigment, optical satellite sensors can be used to visualise the distribution of chlorophyll and thus the phytoplankton in our oceans.
Phytoplankton are essential for many reasons:
- They generate around half the atmosphere's oxygen ;
- They form the bases of ; and
- They absorb and consume CO2 in photosynthesis. While much of that CO2 is released again through respiration of zooplankton eating the algae and bacteria consuming the remains, some of this carbon is released from the surface to the deep ocean. This is referred to as the and it constitutes an important driver in our climate system.
However, when too many nutrients are available, eutrophication takes place and phytoplankton blooms are observed which can negatively affect entire ecosystems.  Nitrogen and phosphorous are the primary inorganic nutrients responsible for the eutrophication of marine waters. Human sources of nitrogen include, for example, agriculture, shipping, aquaculture, waste water treatment plants and industrial water while phosphate comes from domestic and industrial sewage and waste water as well as run-off from land where fertilizers have been used. 
Eutrophication can have multiple impacts: reduction in the depth distribution of macroalgae and sea grasses due to the reduced amount of sunlight received by bottom waters, oxygen deficiency in bottom waters due to increased decomposition of organic matter (dead algae) and a shift in the biodiversity and ecosystem balance.  In addition, some phytoplankton produce toxic compounds that have harmful effects on fish, shellfish, mammals, birds and people. Toxins released from potential can have socio-economic impacts causing shellfish poisoning in humans  resulting in fisheries or tourism closures and related loss of income. of harmful algal blooms is therefore important to manage related risks. The European Union has been taking steps towards the reduction of nitrogen and phosphorous loads in the environment, notably through the adoption of several crucial pieces of legislation. 
Research and monitoring activities have a key role to play in our understanding of phytoplankton and their essential role for marine ecosystems, for the climate system and for us, as well as risks they can generate. A healthy ocean is key to our health and to the sustainable development of the blue economy. Dive into the Map of the Week to learn more about global ocean chlorophyll!