The plankton consists largely of small organisms such as bacteria, plants and animals that drift at the mercy of ocean currents. The phytoplankton is the component of the plankton that is able to photosynthesize and it therefore very important in marine food-webs as primary producers. Indeed, phytoplankton are important for almost all life on Earth as they carry out about half of all photosynthetic activity, and therefore produce much of the oxygen in the atmosphere.
|Diatoms, one of the most numerically dominant types of phytoplankton. Other important groups include dinoflagellates, cyanobacteria and algae.|
Phytoplankton are restricted to the surface water where sunlight can reach them. Their numbers there are limited by the availability of certain nutrients. When these nutrients become abundant, such as during the upwelling of water from the deep ocean, the phytoplankton numbers increase rapidly. Such events are called 'blooms' and can be large enough to be detected from space.
|A false-colour image of a phytoplankton bloom in the South Atlantic Ocean taken by the Earth-observing satellite Envisat. The colours represent the density (shade) and types (colour) of phytoplankton present.|
Different types of phytoplankton use different combinations of pigment for photosynthesis. Different pigments absorb different wavelengths of light and this allows satellites to make a coarse identification of the species of phytoplankton present in a bloom by detecting the reflected light. Using this information false-colour pictures of a bloom, such as the one above, can be constructed to identify when, where and what types of phytoplankton are blooming. This is important information that assists with our understanding of the effects of human impacts on the marine environment, such as pollution and climate change.
Like life on the deep seafloor, the diversity of species in the plankton presents a paradox. Phytoplankton exist in a seemingly uncomplicated environment and compete for a small number of limiting resources, a situation that should favour a limited set of species, yet there is a huge diversity. Perhaps small variations in the spatial and temporal availability of the resources and variations in temperature create the complex set of niches required to support high diversity, as they do in the deep.