In aquatic biology, the paradox of the plankton refers to a situation where limited resources support a large number of plankton species. This seems to go against the competitive exclusion principle, which states that if two species compete for the same resource, one will be forced to disappear.

Ecological paradox

The plankton paradox occurs because many types of plankton live together even though the competitive exclusion principle, also called Gause's law, suggests that only one species should survive when two species share the same limited resources. This principle states that if two species compete for the same resource, the stronger one will eventually use up all the resources, causing the weaker species to disappear. Despite this, phytoplankton exist in many different forms even though they compete for a small number of resources like light, nitrate, phosphate, silicic acid, and iron. The plankton paradox was first described in 1961 by G. Evelyn Hutchinson, who suggested that factors such as changes in light levels from the surface to deeper water, the movement of water, relationships between species, differences in how predators eat, or changes in the environment could explain how so many plankton species coexist.

Later research showed that other factors, such as the pressure from zooplankton eating plankton, random water movement, differences in how size affects grazing, changes in the environment over time and space, bacteria helping or harming plankton, or changes in environmental conditions, also help explain the paradox. Scientists generally agree that the interactions between ecological and environmental factors prevent the plankton habitat from reaching a stable state where one species would dominate.

It was once believed that water turbulence mixed plankton so thoroughly that they could not form groups smaller than a few meters. However, studies looking closely at plankton distribution found that plankton can form small groups (about 10 centimeters in size) that stay together for more than 10 minutes. These groups are large enough to allow plankton to eat, compete with each other, and spread diseases.

Resolution by viral lysis

One possible solution to the paradox is the control of plankton populations by marine lytic viruses. Marine viruses are important for the ecology of bacteria and plankton. They play a major role in biogeochemical cycles and the transfer of genes between bacteria and plankton. Viruses are the most common living things in the ocean and can quickly reduce the numbers of their host organisms. These viruses infect specific host species, so when a virus is abundant, it can rapidly change the structure of phytoplankton and bacterial communities. During the lytic cycle, a virus enters a host cell, reproduces until the cell bursts, and releases new viruses. Viruses can also enter a lysogenic cycle, where they insert their DNA into the host’s genetic material. When a phytoplankton species experiences a bloom, its cell numbers increase rapidly, making many viral targets available.

One explanation for the plankton paradox is the "Boom-and-bust dynamic" hypothesis, also called "Kill the winner." During a phytoplankton bloom, one species grows quickly under favorable conditions, increasing its population and outcompeting other phytoplankton. This rapid growth creates an opportunity for viruses to infect the host cells, leading to a sharp decline in the phytoplankton population. This decline creates space in the local phytoplankton community, allowing other species to grow. Viruses help control these populations, increasing diversity over time and space. Over the long term, viruses prevent the previously dominant species from growing too large during future bloom events.