Ninety four percent of Earth’s life forms inhabit the aquatic environment and the sea is so vast that a million or more as yet unknown species may live in its waters. The quantity of marine species is enormous and they all require energy in order to survive and carry out their life processes, such as growth, reproduction and for their metabolism. This energy comes from the organism’s ecosystem and in many cases from the food that organism eats. The set of linkages that show who eats who in an ecosystem and the transfer of energy that takes place is commonly called food chain. In the following paragraphs we will described the characteristics of this concept.

Basis of the food chain

Since primary producer organisms are able to capture solar energy and convert it into a form that other organisms can use (see the primary producer section), they represent the basis of food chain. While seagrasses and seaweeds are oceanic primary producers, 95% of the production derived from microscopic phytoplankton such as the diatoms or the dinoflagellates. Thus, a typical food chain starts with a phytoplankton basis.

Upper levels of the food chain

For organisms that cannot make their own food, they must ingest other organisms to fulfill their energy requirements. These organisms are then called consumers. Consumers are subdivided into different categories according to their diet: there are either herbivores that feed on plant material (e.g. zooplankton), carnivores that feed on other animals (e.g. tuna, shark), scavengers and detritivores that feed on dead plants and animals (e.g. crabs), Omnivores that feed on both plants and animals (e.g. sea turtle) or decomposers that chemically break down organic matter such as bacteria.

A food chain and its inclusion in a food web

A typical food chain could be described as following: phytoplankton (primary producer) is consumed by an herbivore primary consumer (such as zooplankton). The herbivores are then consumed by the secondary consumer represented by small carnivores such as little fishes. The tertiary consumers could be carnivores such as mackerel eating the secondary consumer while the quaternary consumer eats mackerels such as Tunas. Since the quaternary consumer is not the prey of another predator, it is called super-predator. In reality, most ecosystems are more complicated than a simple chain of feeding interactions. Many species consume more than one type of species, creating a complex web of interactions known as a food web. Besides, along its life, a species could change its position in the food web from secondary to tertiary predator for example (a Tuna is the super-predator but its juvenile or eggs are food for mackerel).

Each step of the food web or chain is called a trophic level. Primary producers are then always the first trophic level. The average amount of energy transferred from one trophic level to the next is 10%. For example, 10% of the solar energy that is captured by phytoplankton gets passed on to zooplankton (primary consumers). Ten percent of that energy (10% of 10%, which is 1%) gets passed onto small fishes (secondary consumers) that eat the zooplankton. Thus, the more trophic levels exist between the primary producer and a consumer, the more the latter will require a large primary production to support it. Sustaining a super predator requires luxuriant primary productivity. Since consuming 1 kg of tuna corresponds to approximately 10 kg of mackerel (because of their trophic level difference) we we should question the effectiveness of our fishing system in feeding a growing population.

Hydrotherma vents ecosystem. NOAA.

Alternative food chains

The primary marine food web, which is based on plant productivity, includes many of the sea’s species— but not all of them. There are other deep-ocean ecosystems that are entirely independent of the sunlight energy that kick-starts the main marine ecosystem. At their roots, these unique ecosystems are fuelled by chemical energy, which enters the ocean from sources like seafloor hydrothermal vents. These primary producers do not produce sugar through photosynthesis but chemosynthesis.

Moreover, the “primary consumer, herbivore and a carnivore sequel” presented previously is quite simplistic. Indeed primary consumer could be either herbivore or omnivore (feeding at least in part on primary producers) as well as the upper level could be either omnivores, scavengers or carnivores.