Phytoplankton: Small in size, large in importance.

Phytoplankton are simple-celled plants, often transparent, that have little resistance to the ocean current. There are two classifications: Nannoplankton which are too small to be caught in a net, and Net Plankton which are large enough to be caught in a net. They vary in shape; some make chains, some grow wings and some grow spines. There are 5,000 known species including diatoms (algae), dinoflagellates (marine plankton) and cyanophyta (bacteria).

Phytoplankton can reproduce both sexually and asexually. The process they use is called simple-celled division when the "parent" cell splits itself into two "daughter" cells, dividing everything inside. When it splits, the parent cell ceases to exist. Phytoplankton have rapid reproduction dividing once a day, producing a billion copies of itself in a single month.

Phytoplankton are essential in the food web. They produce 95% of the sea's oxygen therefore sustaining nearly every living creature in the ocean and making them primary producers. They create energy through photosynthesis, using sunlight to create organic materials such as sugar, starch, fat and protein. For this reason they are required to stay near the surface of the water in the "euphotic" zone, which is where photosynthesis can occur. Zooplankton, also at the base of the food web, are organisms that convert plant material into animal tissue providing a vital link between phytoplankton and the rest of the food web. Phytoplankton is a staple food for numerous creatures ranging from sea stars to shrimp to snails and whales.

Since they are slightly denser than water and need to stay near the surface despite their tiny size, phytoplankton have adapted several mechanisms. They will sink lower to obtain nutrients in deeper water but they need to control their fall from and return to the surface. The rate of the fall is proportionate to the amount of resistance their body has to the gas or liquid in which it moves. The temperature and the salinity of the water affect the water density and are factors in their buoyancy. Keeping their body size small is important in their resistance to sinking, which is especially important for tropical plankton because the water is warm and therefore less dense. Their buoyancy is controlled by altering their body fluids to be less than the density of water or equal to the volume. To achieve this, some phytoplankton will develop a gas-filled float. Others will use excess oils and fats to form oil droplets under their armor to aid buoyancy. Some may even change their body shape to become more buoyant by evolving a flattened body or growing appendages, projections or a spine.