It is often the infinite little that affects the infinitely large, and so is the role of plankton in ocean life. Born nearly 1.5 billion years ago, plankton is the base of all life on earth. It plays a crucial role but it remains unknown to the general public.
Plankton comes from the ancient Greek "planktós" which means "wandering". It refers to all living things, animals and plants, evolving in the water and can not fight against the currents.
Plankton is divided into two main monarchies: the plant plankton, also called phytoplankton, consisting of almost all unicellular algae, called micro-algae or micropyla and the animal plankton called zooplankton. Phytoplankton is easily differentiated from zooplankton by its very simple forms (no legs, no antennae), often geometric (perfect square, rectangle, round, oval).
Zooplankton is also composed of two groups:
- permanent zooplankton: unicellular or multicellular organisms that are born, reproduce and die as zooplankton.
- temporary zooplankton: eggs and larvae of very varied animals (crustaceans, shells, fish ...) leaving the world of plankton by metamorphosing: the larva (plankton) turns into a juvenile by acquiring all the attributes of the future adult. It is similar to the transformation of the butterfly.
The vast majority is invisible to the eye. Indeed, its size varies from 0.2 micrometre, for some viruses and bacteria, to 200 micrometers (1 micrometer = 1 micrometer = 0.001 millimeter, it is 10 times smaller than a pencil lead). To fish for it, it is necessary to have a special net with very fine mesh called "plankton line". Sometimes the animal plankton can reach several meters, with, for example, some jellyfish.
Phytoplankton is called "primary producer" because it has the capacity to transform the inorganic matter (CO2, mineral salts, water, light ...) into organic matter. Zooplankton feed on phytoplankton. Like plants on Earth, phytoplankton are at the base of the marine food chain.
Plankton reproduction patterns are complex because of its great diversity and ability to change sexuality during its lifetime. The main mode is sexless mode:
- the mother cell buds
- either the mother cell divides into two or more daughter cells, as in diatom microalgae.
The other mode used is the classical sexual reproduction with fusion of male and female gametes in external environment. Plankton is renewed very quickly since its average life is only a few days.
Phytoplankton is the basis of feeding in aquatic herbivores, so the biodiversity of phytoplankton populations is an important factor. It is also a food source for many marine and freshwater animal species, forming the beginning of the food chain. Phytoplankton are eaten by herbivores and zooplankton which are then eaten by secondary consumers such as small fish. The latter will then make the happiness of larger fish that will themselves be the menu of birds and mammals that sometimes end up on our plates. Contrary to what we think, forests are not the lungs of the globe. Indeed, plankton produce almost 70% of the oxygen we breathe, so it's much more than the performance of our dear plants.
Even after its death, the plankton emits sulfur molecules that contribute to the nucleation of water drops, that is to say to the formation of clouds and rain, by moving this carbon in the column of water. Plankton has also been an excellent provider of fossil energy. When planktonic organisms die, they sink to the bottom of the sea creating a layer of sediment. For millions of years, these sediments have fossilized, producing our precious oil.
Since 1990, global plankton activity has been declining steadily. The causes are numerous. First, under the effect of global warming, many planktonic species adapted to temperate waters migrate to the increasingly cold waters of the north, bringing with them their predators, which modifies the balance of the system. There is also the ocean acidification, that is, the gradual decline in the pH of the oceans, induced by the increase in anthropogenic carbon dioxide (CO2) emissions into the atmosphere. This phenomenon likewise greatly affects plankton. Surface water stifled by agricultural effluents, drifting plastic microphones, prevent it from carrying out its mission. Overfishing, at the top of the pyramid, leads to the development of herbivorous species and therefore increased consumption of plankton.
Phytoplankton are mainly present in the euphotic layer, that is to say the zone to which light penetrates. It develops best in rather calm waters rich in nutrients: rivers, estuaries, marshes. It can even be found in fountains or seeps of rocks. At the scale of one year, it therefore presents a great seasonal variability. It develops preferentially from spring to autumn, when the conditions are optimal. This is called algal blooms or phytoplankton bloom that can sometimes color the waters to green or blue.
Zooplankton move to the surface at night to feed on phytoplankton and return to the deeper waters during the day. It escapes predators and saves energy because the temperature is then lower.
In conclusion, plankton is the lung of the oceans and it is time for the human species to begin to realize it. There are only a few years left to rectify the situation.