An Introduction to Plankton
Planktos is the Greek word for drifting or wandering, which is exactly what plankton do throughout all of the world’s oceans. When we think of marine animals we think of fish, octopus, whales and sharks, but plankton, in fact, account for a much larger percentage of the ocean's biomass! These are tiny, mostly microscopic organisms, some of which will always be that way, and some that will grow and transform into larger animals. A few drops of sea water can hold many different types of plankton, most of which, are invisible to the naked eye1.
Like life on land, plankton can be divided into plants and animals. The plants, or phytoplankton, can photosynthesise just like terrestrial plants can. This means they use the sun’s energy to convert carbon dioxide, water and other minerals into organic matter and oxygen. So, while trees make 50% of the oxygen we breathe, phytoplankton are busy making the other 50%? Of course, plankton exist in freshwater lakes and rivers too2.
A diatom captured under a dark field microscope. Diatoms are microscopic algae.
A diatom (top-view) captured under a dark field microscope.
A decapod crustacean captured under a dark field microscope. Decapod crustaceans are the larvae or young form of animals such as lobsters, crabs and shrimp
Animal plankton are called zooplankton. Some zooplankton spend their entire life as plankton, while other zooplankton are just a stage in the life cycle of a bigger animal. For example, crustaceans such as lobsters, crabs, and shrimp all spend some time floating in the plankton as tiny larvae.
Lots of things eats plankton. Zooplankton eat phytoplankton and they also eat other zooplankton. Many animals including shrimp, mussels, small fish and sea anemones rely on plankton as their main food source. This sea soup is the base of the food chain, meaning that they are essential to our marine eco-system. The species which eat plankton are, in turn, important prey items for species of fish, turtle, shark and so on.
Plankton can vary greatly in size; the smallest organisms like bacteria and viruses are only microns in length (smaller that the width of a strand of hair!), while the largest, Siphonophores, can reach up to 50 metres in length. Even jellyfish, which are classified as zooplankton, range from the Barrel jellyfish (Rhizostoma pulmo), which can reach up to 90 cm in diameter and weigh 35km, to other species that never grow larger than 1cm in size and must be observed under a microscope. [3,4] Researchers use a specialised plankton net which is often towed from a boat or vessel so that they can collect and study marine plankton.
Plankton, as drifters, are influenced by currents and atmospheric processes. Plankton grow in abundance when there are changes in conditions, such as differing temperatures, changes to salinity and increases or decreases in food. They can also increase and decrease throughout seasons and can be influenced by climate and pollution pressures.
As they are the base of the food chain, the entire eco-system relies heavily on plankton, and they are essential to a healthy ocean. Therefore, monitoring how plankton changes in our environment is really important to understand how our eco-system works and to keep it safe5.
References:
1 Plankton - an overview | ScienceDirect Topics
3 https://oceanservice.noaa.gov/facts/plankton.html
4 Plankton: Wonders of the Drifting World by Christian Sardet
