Plankton

Plankton are organisms that drift through water or air. They are defined by their ecological niche and their level of motility. This means they lack the ability to actively propel themselves against currents or wind.

While some, like jellyfish, can swim slowly, they cannot overcome the influence of major currents. This distinguishes them from nekton, which are animals like fish that can swim against the flow. They also differ from benthos, which live on the deep sea floor. Plankton inhabit many environments, including oceans, estuaries, rivers, lakes, and ponds.

Scientists categorize plankton into several distinct groups based on their life cycles. Holoplankton are organisms that spend their entire lives as drifters. This group includes most algae, copepods, salps, and certain jellyfish. In contrast, meroplankton are only planktic for part of their lives. They usually begin as larvae before graduating to a different existence. Some become nekton, while others become benthic organisms living on the floor. Examples of meroplankton include the larvae of starfish, sea urchins, crustaceans, and most fish.

Microplankton make up a massive portion of the ocean's total biomass. These organisms are typically smaller than one millimeter in size. They are often categorized into two main types: phytoplankton and zooplankton. Phytoplankton, such as diatoms and dinoflagellates, are photosynthetic organisms. Zooplankton include various creatures like radiolarians, foraminifera, and copepods.

These microscopic life forms are considered ecological linchpins. They drive primary production and cycle essential nutrients like carbon, nitrogen, and phosphorus. They also sequester large amounts of carbon and produce much of the world's oxygen.

Plankton distribution is heavily influenced by the availability of light and nutrients. Most plankton ecosystems are driven by solar energy. This confines much of the primary production to surface waters where light is abundant. Nutrient availability, such as nitrate, phosphate, and silicate, is also a critical factor. In some tropical regions, light is plentiful but nutrients are limited due to water column stratification. This results in lower primary production at the surface. In these areas, production may occur at greater depths where light is reduced.

In certain ocean regions known as HNLC regions, productivity is low despite high macronutrient levels. These areas lack the micronutrient iron. Iron reaches the ocean primarily through the deposition of dust on the sea surface. Interestingly, areas near arid land often have abundant phytoplankton because of this. For example, trade winds carry dust from the Sahara Desert to the eastern Atlantic. This dust provides the necessary iron to trigger phytoplankton blooms.

The term "plankton" was coined in 1887 by the German marine biologist Victor Hensen. He derived the name from the Greek words "halos," meaning sea, and "planaomai," meaning to drift or wander. The study of these organisms is called planktology. A single planktonic individual is referred to as a plankter. While most plankton drift horizontally with currents, some exhibit diel vertical migration. This allows them to swim hundreds of meters vertically in a single day.

Plankton are not limited to aquatic environments; they also exist as aeroplankton. These are organisms that drift in the atmosphere, carried by wind currents. Aeroplankton includes plant spores, pollen, and wind-scattered seeds. It also contains microorganisms swept up from land by dust storms. Marine microorganisms can be propelled into the air via sea spray.

These airborne microbes can travel long distances to coastal regions. Once they land, they can impact the health of humans, animals, and vegetation. This shows how deeply connected the air and ocean systems are through these tiny drifters.