Phytoplankton vs Zooplankton

Plankton makes up some of the smallest organisms in the ocean. In the world of the microscopic, two microorganisms form the foundations of the entire marine food web – Phytoplankton and Zooplankton.

Plankton derives their name from the Greek word “planktos,” meaning “drifter.” This means these microorganisms can’t swim or propel themselves forward.

Apart from this and their microscopic size, there are vast differences between Phytoplankton and Zooplankton.

Credit: Department of Marine Resources (Left) & NOAA (Right)

Main Differences Between Phytoplankton Vs Zooplankton

• Kingdom: Phytoplankton are mostly found in the Protista and Plantae kingdoms, while zooplankton are commonly found under the Animalia kingdom
• Appearance: Phytoplankton varies in shape but represents simple single or multi-celled organisms. Zooplankton look like small animals, such as worms or shrimp
• Diet: Phytoplankton are autotrophic (self-feeding) and energize by means of photosynthesis or chemosynthesis 
• Predators: Phytoplankton is at the bottom of the food chain and is fed on by zooplankton, as well as coral and larger fish or mammals. Zooplankton is fed on by many of the same aquatic species barring phytoplankton.

Phytoplankton

Phytoplankton is the autotrophic division of the plankton community. Although they are often referred to as microalgae, it is both microalgae, as well as cyanobacteria, that are part of the phytoplankton family.

These two groups are further broken into five main phytoplankton types – cyanobacteria, diatoms, dinoflagellates, green algae, and coccolithophores.

Appearance

As there are different types of phytoplankton, it can be expected that they will represent different appearances. 

The table below compares the physical appearances of the four major divisions of phytoplankton.

	Size	Shape	Color	Other
Cyanobacteria	0.5 – 60 µm	Spherical, rod, and spiral	Green, blue, pink, orange, and various shades in between
Diatoms	20 – 200 µm, although occasionally up to 2mm long	Circular, triangular, square, elliptical	Brown to yellow-brown
Dinoflagellates	5 µm to 2mm	Various shapes with two dissimilar flagella. Transverse flagella look like a wavy ribbon	Red, blue, green, yellow, and brown	Most commonly related to red tides and bioluminescence
Green algae	Averagely 2 – 7 µm, but can be as small as 1 µm or as large as 3m  for multi-celled seaweeds	Cylindrical, oblong cocoids, complex segmented cells. Cells form in long, branched, or unbranched filaments	Most commonly bright green, but can be found in red-orange, and yellow
Coccolithophores	2 – 75 μm	Circular shape covered in hubcap-like plates	White due to their calcium carbonate (chalk) plating	Plankton blooms cause milky blue/ white or opaque turquoise coloring in the ocean

Habitat

Phytoplankton can be found in both freshwater and seawater environments. 

Freshwater phytoplankton is often found near lakes, streams, swamps, and various other wetland areas. Phytoplankton, for the most part, cannot survive in rivers and often streams, as the current carries them down to lower water bodies.

Saltwater phytoplankton are, as expected, inhabitants of the ocean. As phytoplankton can’t propel themselves in any direction, they are subject to the movements of the ocean and are therefore found in areas where currents end or in drifting tides.

Ocean phytoplankton thrives along coastal regions, along continental shelves, and along the equator in the Pacific and Atlantic oceans.

Phytoplankton tends to be more abundant in areas north of the equator.

Diet

Phytoplankton are plants, and as it is with other land or aquatic plants, they create energy through the process of photosynthesis. 

This means that the plankton needs a good light source to convert energy.

Apart from sunlight, phytoplankton needs essential nutrients such as phosphate, calcium, iron, silicon, and nitrate, with nitrogen and phosphorus (like many plants) being two of the most critical for survival.

These nutrients are drawn in from the surrounding waters. If they are not present, the plankton will struggle to grow.

Reproduction

Phytoplankton primarily reproduces through asexual reproduction; however, some species can reproduce sexually.

Depending on the species, this asexual reproduction will occur differently but is completed through one of the following processes:

• Binary fission: A single parent cell divides into two identical cells, which continue to divide
• Multiple fission: A single nucleus divides into multiple identical organisms
• Fragmentation: The separation of filaments that then begin to grow separately
• Budding: Buds grow that mature and then break off, similar to making a clone of a plant

Some phytoplankton, such as some diatoms and green algae, can reproduce sexually when conditions support it.

Predators

Phytoplankton is at the base of the ocean’s food web, and as a result, they are preyed upon by primary feeders.

These most commonly include zooplankton as well as soft and hard corals; however, larger fish such as basking sharks, ocean mammals such as blue whales, crustaceans, and sea slugs all feast on these microorganisms. 

Zooplankton

Zooplankton makes up the microscopic animal division of the plankton world. These organisms are found in a large size range with either single or multi-celled construction.

Zooplankton can be both holoplanktonic, meaning they spend their entire life cycle in the plankton form, or meroplanktonic, where they grow and spend part of their life in the benthic zone.

Appearance

There are estimated to be over 30,000 species of zooplankton, which makes describing the appearance of each an impossible task.

That said, the table below gives a brief breakdown of the physical appearances of some of a few common zooplankton.

	Size	Shape	Color	Examples
Radiolarians	<100 μm for single-celled and up to 1-2 mm for colonial species	Spherical or cone-shaped with multiple needle-like protrusions known as pseudopods	Vary greatly in color and can be found in any color between white and black; however, they are most commonly found in gray, brown, gray-brown, green-rusty brown	Acrosphaera australis Acrobotrys cribosa Acanthosphaera actinota
Foraminiferans	Average around 100 μm; however, they can be found in sizes ≤32 μm and as large as 20cm	A flexible and non-solid body that is able to change shape	White, black, gray, brown, red-brown, red-yellow	Globigerina bulloides Globigerinoides sacculifer Globigerinoides ruber
Cnidarians	<1 mm to larger than 8 feet for larger jellyfish	Vary greatly in shape, but most commonly found in a bell shape	Vary in color, but mostly semi-transparent	Jellyfish Anemones Corals Hydras
Crustaceans	<1 mm to over 14 ft (for some crabs)	Represent small shrimp-like creatures, worms, or flat round objects	Semi-transparent, yellow-brown, red-brown	Mysids Copepods Decapods

Habitat

Zooplankton is found in both fresh and seawater, inhabiting areas such as lakes, ponds, swamps, or drifting in an ocean current.

Zooplankton can be found near the water’s surface or in deep ocean areas. Most zooplankton have a preferred depth; however, some species move in a vertical column.

You will be able to find zooplankton in most of the areas where phytoplankton is present.

Diet

Most zooplankton feed on phytoplankton, various species of algae, bacteria, and in some cases, other zooplankton.

Some zooplankton species can be parasitic as they attach and feed on a host.

Reproduction

Depending on the species, zooplankton can produce both sexually and asexually. 

Asexual reproduction usually occurs through fission; however, some zooplankton species can produce parthenogenetically (when an egg can form without the need for external fertilization by sperm).

Predators

Zooplankton is preyed upon by a huge variety of sea life, including bacteria, small fish, sea slugs and snails, aquatic worms, corals, other zooplankton and their adult counterparts, as well as larger fish and mammals such as the blue whale and whale sharks.

Ecological Importance Of Plankton

As plankton form the base of the aquatic food chain, it could be said that it is through these tiny micro-organisms that the rest of the ocean can thrive.

Without a healthy population of phytoplankton, zooplankton would not be able to survive.

With no zooplankton, the ocean’s ecosystem would fall to pieces, leaving smaller fish and corals short on food, as well as killing off larger animals such as krill feeding whales, which in turn would leave sharks and killer whales with no food source.

As a building would collapse if you removed its foundation, so would the ocean if you removed plankton.

Furthermore, as phytoplankton feeds through photosynthesis, they constantly take carbon dioxide and release oxygen, just as plants on land do.

Plankton is estimated to be responsible for 70% of the earth’s oxygen supply.

Plankton numbers have significantly decreased over the past 70 years, which, to some scientists, is the result of changing ocean temperatures and acidity levels.

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