The organisms that form the base of the food web are called autotrophic (auto = self, trophic refers to feeding). They use solar energy to transform carbon dioxide (CO2) and water (H2O) into glucose (C6H12O6) and oxygen (O2) in a process called photosynthesis.
Autotrophic organisms, also called producers, are consumed by heterotrophs (hetero = different or other). Marine autotrophs include phytoplankton (example: diatoms), macroalgae (example: kelp), seagrasses.
Primary (1º) consumers are heterotrophs that eat autotrophs.
Secondary (2º) consumers eat primary consumers.
Tertiary (3º) consumers eat secondary consumers.
And so on... Until you reach the final or apex consumer.
Each of these types of consumers represents a different trophic level or position. Displaying this in a triangular form, represents that the abundance of autotrophs must be great than the consumer level depending upon them for food.
Laying out the relationships of who eats whom within an environment is the process of diagramming a food web. During the consumption of other organisms, nutrients, as well as any contaminants in the organism are transferred to the consumer.
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