On the 10th of January 1882, Charles Darwin wrote in his diary about building a contraption that he towed behind the HMS Beagle and which caught a large mass of small animals. These animals are what we now call plankton, which are fundamental to life in the ocean.
Planktos is the Greek word for drifting or wandering. Plankton refers to the multitudes of living creatures which drift and float throughout the world’s oceans. When we think of marine animals we think of fish, octopus, whales and sharks, but these large animals only make up about 2% of the ocean’s species. The remaining 98% are planktonic. These are tiny, mostly microscopic organisms, such as bacteria, crustaceans, molluscs and jellyfish. A few drops of sea water can hold many different types of plankton, all which are mostly invisible to the naked eye. 
Like life on land, plankton can be divided into plants and animals. The plants, known as 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.
Did you know that trees make 50% of the oxygen we breathe, but phytoplankton are responsible for the other 50%? Meaning half of the oxygen you breathe is produced by phytoplankton. Phytoplankton rely on nutrients found in the ocean to survive, such as phosphates and nitrates. Of course, plankton exist in freshwater eco-systems such as lakes and ponds too. 
Figure 1: A diatom captured under a dark field microscope. Diatoms are microscopic algae.
Figure 2: A diatom (top-view) captured under a dark field microscope.
Figure 3: 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 an animal. An example of this are decapod zooplankton, which are the larval or young life stage of crustaceans such as lobsters, crabs, shrimp.
Lots of things eat plankton. Zooplankton eat phytoplankton and they also eat other zooplankton. Many animals including shrimp, clams, crustaceans, 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 within the jellyfish, which are classified as zooplankton, we can see large size ranges. The Barrel jellyfish (Rhizostoma pulmo) can reach up to 90 cm in diameter and weigh 35km, whereas other species of jellyfish can be < 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.
A multitude of plankton: Microscope footage of a few millilitres of seawater viewed under a zeiss dark-field stereomicroscope without being dissected apart. When studying plankton, scientists have to work with smaller drops of water and separate the plankton apart so that they can view them more clearly.
Watch this short video of plankton under a microscope
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 safe.
Obelia feeding: Microscope footage of a species of gelatinous zooplankton known as Obelia sp. Obelia is the most common gelatinous species in Ireland and all over the world. Here you can view its mouth and oral arms, searching for prey within the water.
Watch this short video of obelia under a microscope
 https://oceanservice.noaa.gov/facts/plankton.html  Plankton: Wonders of the Drifting World by Christian Sardet