Zooplankton Vs. Phytoplankton

Phytoplankton are a valuable part of our ecosystem - they account for over 50% of the oxygen produced by plant life, and are a primary source of nutrition for many marine species.

Plankton is a term used for life forms that drift on water currents. Although a few of them can swim a bit, they cannot counteract the currents. The term plankton itself is taken from the Greek word planktos, meaning wanderer or drifter. They occur in fresh as well as saline water, and are found in ponds, lakes, and oceans.

The demarcation between the different types of plankton is not very clear, as they are very diverse, with many similarities and variations. But generally, they are divided into three types - phytoplankton: consisting of plants, zooplankton: animals, and bacterioplankton: bacteria.

This classification is based on their trophic levels, i.e. depending upon whether they produce or obtain their energy, and their energy sources. For example, zooplankton can be said to be the carnivores, since they subsist on other plankton, whereas many bacterioplankton are like dung beetles, in that, they breakdown the waste and decaying matter.

Phytoplankton are primarily producers, and obtain their energy by photosynthesis. Like all plants, they absorb sunlight, carbon dioxide, and nutrients, and convert them into oxygen and carbohydrates. Although, some like the dinoflagellate also ingest microorganisms to obtain energy. Zooplankton, on the other hand, obtain their energy by eating phytoplankton, zooplankton, bacterioplankton, and even some nektonic organisms.

Phytoplankton are mostly plants like diatoms, dinoflagellates, and various algae like the blue-green algae. Zooplankton are rather diverse, and consist of krill, jellyfish, arrow worms, and even the larvae or eggs of larger organisms.

Most phytoplankton, being autotrophs, need sunlight to live, and hence are found near the water surface. On the other hand, zooplankton prefer cold, shaded regions, but might drift towards the surface during the day.

All plankton cannot swim against ocean currents. But, a few members of zooplankton, such as the jellyfish and comb jelly, do have the ability to swim feebly in order to avoid predators or to grab their catch. This ability is absent altogether in phytoplankton.

Phytoplankton stay phytoplankton all their lives, but some zooplankton are the larval stage of fish, crustaceans, or sea worms. While some of them metamorphosize into free-swimming creatures like sea stars, some settle down to a sessile lifestyle, such as the tunicates (sea squirts), and to some extent, the sea urchins. These transforming organisms are termed meroplankton.

As mentioned earlier, phytoplankton contribute to more than 50% of the oxygen produce by plants on the Earth. Serving as the primary food source of many marine animals, plankton play a major role in the ecology of oceans. 

Fish larvae are especially dependent on plankton for food, and an entire breeding season can be affected by the presence or absence of these microscopic organisms.

They are also used by scientists as indicators of pollution - they bloom or deplete fast in response to various chemicals and nutrient levels, and serve as the first signs of any change, as they are often brightly colored or bio-luminescent and easy to detect when found in large numbers.

Zooplankton, on the other hand, are usually transparent, to avoid being detected by predators, do not produce oxygen, but are subsisted on by many species, including the baleen whale.

These are the differences between phytoplankton and zooplankton. Some organisms exhibit a few characteristics of each other, which has the scientist community divided over their classification. The immense size of the oceans, diversity of these organisms, and microscopic size, do not help the cause.

To complicate the matter further, researchers are constantly finding new species, challenging the definition of what constitutes a phytoplankton, zooplankton, or bacterioplankton.