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The Food Web of the Deep Sea

At first look, the depths of the ocean may appear barren, lifeless, and silent. The lonely expanse rolls out into a long horizon, while the great abyss plummets ever-downwards into darkness. Communities of life are solitary. They seem isolated from other habitats in the deep. But there is more going on here. 


primary production in the epipelagic

At the bottom of every food web, whether it is terrestrial or marine, there must be a source of nutrients. There must be organisms capable of synthesising organic molecules from energy. On land and in the shallows, this is the job the plants, and single-celled plankton. The Photoautotrophs. Photosynthesising organisms that use energy from sunlight to convert carbon dioxide into oxygen and chemical energy stored as glucose. Though they are tiny, these plankton play a large role, producing the carbon that supports nearly all marine life, as well as producing 50% of Earth’s oxygen.


Herbivores, or primary consumers, such as zooplankton or turtles and manatees, feed primarily on the plants and phytoplankton. They assimilate what’s left of the chemical energy, using it themselves to grow and move. The herbivores become food themselves for secondary consumers, which make up the top two levels of the food web. Carnivores, and top predators.


But amidst the productive frenzy of the shallow open ocean, animals excrete, and die, and moult, creating a trickle of organic material that begins to sink downwards. A nutrient-rich supply of food, known as marine snow. And so, we enter the isolated deep.


With no sunlight, photosynthesis cannot occur below 200 metres in the Twilight Zone. There are no plants here to fulfil the role of nutrient producers. There is only marine snow, bringing down the leftovers from above. It might be enough to sustain an abundance of detrivores, or small filter feeders like sponges, but this alone is not enough to support a complex biological community.

And yet, the deep sea is complex in its biodiversity. So if the energy is not coming from the sun, where is it coming from?