In an ecosystem, phytoplankton are _____.


B.Tertiary consumers.


D.Secondary consumers.

E.Primary consumers.

Diagram of two food chains showing producers: plants and phytoplankton at the base
Diagram of two food chains showing producers: plants and phytoplankton

The correct answer is C. Producers.

An ecosystem consists of a group of organisms that all interact with one another in some way and interact with the physical environment. A hallmark of ecosystems is the presence of food chains.

Food chains are groups of individual organisms that interact by feeding, that is, by trophic interactions. Generally, these interactions take the form of trophic pyramids in which the most energy available is at the base of the chain with less energy available as you move up to higher trophic levels.

The producers are the organisms that can use chemicals or light energy to form sugars. These individuals include such life forms as bacteria, plants, and phytoplankton.

These organisms are then fed on by other organisms such as zooplankton in the aquatic environment, or animals in the terrestrial ecosystem.

These primary consumers consume and take in the energy from the producers. Additional consumers, known as secondary consumers then prey on these creatures at this next level. These are carnivorous protists and animals that actively capture the primary consumers.

Tertiary consumers then prey on the secondary consumers and may be at the top of the food chain, or there may be higher levels. Many food chains can link together in complex ways to form large food webs with many individuals interacting with each other.

Food chain

A food chain is a group of organisms that interact through feeding activity. The base of the chain includes organisms that are producers that can use chemicals or sunlight to make sugar.

Phytoplankton use sunlight to make sugars by the process of photosynthesis and are thus known as photoautotrophic producers. Some bacteria are able to make sugar by simply using chemicals in the environment and thus they do not need sunlight. These bacteria are producers that are known as chemoautotrophic organisms.

The next level of the food chain is the organisms that feed on the producers. These are consumers which are also known as heterotrophs.

These include the animals which are unable to make their own food using chemicals or sunlight. They, therefore, have to feed on some other organism in order to take in the nutrients and energy that they need to survive.


The animals that feed on the producers are known as primary consumers. In aquatic ecosystems, the primary consumers are usually the zooplankton that feeds on the phytoplankton.

Primary consumers are then in turn eaten by secondary consumers which are often carnivorous. In a terrestrial food chain a producer may be grass and then the primary consumer could be a grasshopper which is preyed on by a bird such as a sparrow.

The sparrow takes in the nutrients and energy from the grasshopper when it feeds on it. At the same time, it then becomes a source of food for an animal that is higher up the food chain.

A tertiary consumer feeds on a secondary consumer, so for example, a red-tailed hawk may feed on the sparrow.


These are organisms that help to recycle waste material and dead plants and animals by breaking down the material. Fungi are one such life form that is very important, and along with bacteria, helps to recycle nutrients. Water molds along with bacterial species act in aquatic environments to break down detritus.

Detritivores are also commonly referred to as saprotrophs which take in the material after secreting enzymes to digest the substrate they are growing on. Minerals and nutrients released by saprotrophic activity are then returned to the water or soil.

These same chemicals can then be taken up by plant roots and used for growth. In an aquatic environment, the minerals can help promote the growth of other primary producers that rely on minerals for growth.

Trophic pyramids

Food chains are often also called trophic pyramids because they consist of feeding or trophic levels. The pyramid shape comes from the fact that the numbers of individuals and amount of energy available at each level decrease through the food chain.

In other words, there will be large numbers of producers to supply food for primary consumers. There will be less primary consumers than producers or else the animals would run out of food and starve to death. At each level, the amount of available energy also decreases.

Types of producers

Autotrophic producers include bacteria and various plants and Protista. The phytoplankton are the Protista that occurs in water and photosynthesizes to produce food.

Phytoplankton generally includes the green algae, dinoflagellates, and diatoms, and they are organisms that contain various chlorophyll and accessory pigments in order to photosynthesize.

Diatoms are particularly abundant in marine ecosystems where they are believed to be major contributors to the primary productivity in ocean environments. They are important as a food source for many creatures in the ocean, including various small zooplankton species.

Not all producers in the ocean are microscopic in size in the way that diatoms and dinoflagellates are. In fact, seaweeds are also primary producers that use sunlight to make sugar.

These species can even be very large and can grow fast, for instance, kelp, which can grow up to 5 inches a day.

In terrestrial ecosystems, plants are the dominant producers. They contain primarily chlorophyll a and b which captures radiant energy from the sun for use in photosynthesis.

Primitive plants such as moss and ferns along with giant trees all play a role in providing food and shelter for many other organisms.

Types of consumers

Consumers include all species of animal and some protists. Protists are single-celled organisms which can sometimes switch between being a producer and a consumer.

Euglena is such an example in which the organism is able to photosynthesize when the light is available, but can also take in food particles if need be when no light is present.

Animals do not have this ability to switch between feeding modes, and all are heterotrophs that have to take in solid particles of food.

The animal species that feed only on vegetation are known as herbivores and these often include grazing type animals such as deer or insects like caterpillars and crickets.

The animals that feed on other animals are at the secondary consumer level and may be known as carnivores. These organisms actively hunt and prey on other animals in order to take in food. This predatory behavior even occurs in unicellular protists such as Didinium which actively preys on Paramecium.

However, there are types of consumers that eat both plant and animal material, these are known as omnivores. Omnivory is quite common and is seen in many species of birds and also in primates.


  1. AW Wilson (2018). Saprotroph. Retrieved from Encyclopedia Britannica.
  2. S Carpenter (2018). Trophic cascade. Retrieved from Encyclopedia Britannica.
  3. Editors for Encyclopedia Britannica (2018). Trophic pyramid. Retrieved from Encyclopedia Britannica.
  4. JN Thompson, TF Flannery (2018). Community ecology. Retrieved from Encyclopedia Britannica.
  5.  RL Dorit, WF Walker, RD Barnes (1991).  Philadelphia: USA, Saunders College Publishing.


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