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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?

Phytoplankton

'Plant-drifter'

Phytoplankton consist of microscopic marine algae, residing up the upper 200 metres of the ocean where sunlight is abundant enough that photosynthesis can occur. Through this process, the phytoplankton form the base of the entire marine food web, providing food for a diverse range of creatures. They are the primary producers of the Epipelagic Zone.

primary production in the depths

Though photosynthesis cannot occur in the deep, there is another form of nutrient production that takes its place at hydrothermal vents. Here, there are hot upwellings of dissolved chemicals and minerals that form towering chimney structures on the sea-floor, and release heated minerals from deep within Earth into the ocean. The chemicals expelled within the superheated vent fluid would be toxic to us humans. But to the life here, they are vital, containing nutrients that bacteria are able to convert to energy in a process that mirrors the conversion of sunlight during photosynthesis.

 

In place of Photoautotrophs, we find Chemoautotrophs. In place of plants and plankton, there is bacteria. In place of photosynthesis, chemosynthesis. The bacteria are the primary producers, a major source of nutrients for the deep sea ecosystem.

 

Like manatees grazing sea-grass in the shallows, there are grazers here too. Bottom-feeders, like limpets and shrimps, graze on the microbial mats. Yeti crabs farm colonies of the bacteria on their legs as a source of food. These creatures, that rely on the bacteria for food, are the deep ocean’s primary consumers.

 

Predators at the vents, such as crabs, eels and octopuses, are the secondary consumers. But here’s where things get interesting. Though organisms at these isolated vent ecosystems cannot stray from their respective habitats, nutrients are still cycled between communities. The creatures here are demersal, meaning they live on or near the sea floor. They may be confined here, relying on the bacteria and unable to survive away from the vent ecosystem, but a number of other processes contribute to the transfer of chemical energy from vents to the rest of the open ocean.

 

The first of these processes involves visitors to the vents. From hagfish to deep sea skates, these are creatures that can survive away from the vents, but must seek out these hotspots of life in order to feed, or lay their eggs.

Deep Sea Shrimp

Mirocaris fortunata

Chemosynthetic bacteria lives inside the mouth and specially evolved gills of these shrimp. The bacteria enables the shrimp to survive on the energy released by these bacteria in the absence of sunlight.

the nutrient supply chain

When visitors to the vents depart, they take any energy that they have assimilated with them, and form a kind of supply chain - moving energy from the source to the open ocean where there are no producers. In doing so, these creatures are supporting another form of life in the deep sea. The pelagic ocean wanderers, ever-moving and never settling as part of any one community.

 

They get their energy through predation, feeding on the hagfish, skates and other organisms. Energy is lost during its transfer from the vents to the open ocean; it is used up by the creatures as they move, grow and reproduce, so only a fraction of the nutrients they assimilated will make it to their predators. Thus, wanderers like the Humboldt squid tend to be larger, as they must be in order to become more efficient. This is because although nutrients from vents and seeps are brought to them, they are far less abundant so far from the producers at the source.

 

Pelagic wanderers can go for very long periods of time without feeding. Their slowed metabolisms are a useful adaptation down here where nutrients are scarce. Even so, wanderers cannot rely entirely on residual nutrients brought to them from vents and seeps. They must become opportunistic feeders, and be able to take advantage of any kind of food they come across.

Humboldt Squid

Dosidicus gigas

Like many other cephalopods, Humboldt squid help maintain the marine ecosystem by eating enormous quantities of food. They are formiddable predators, whose group hunting techniques often resemble a feeding frenzy. As they hunt, they communicate with each other using changing patterns of light and dark pigment.

whale-fall communities

Occasionally, amidst the constant trickle of marine snow from above, something larger will sink to the deep sea. A sunken whale carcass - known as a ‘whale-fall’ - that can support a complex biological community for up to 50 years.

 

Within minutes of it reaching the sea floor, pelagic wanderers gather out of the barren darkness to feed. The first to arrive are 'mobile scavengers' - these are the larger ocean wanderers, including hagfishes and gigantic sleeper sharks, who take on the role of detrivores, breaking down the dead matter.

 

Whale-falls attract such an abundance of life because their nutrient content is equivalent to 2,000 years worth of marine snow, providing a rich supply of nutrients concentrated in one small area. Even once the soft tissue is stripped, the remaining bones and scraps do not go to waste. During the sulfophilic stage of the whale-fall ecosystem, chemosynthesis occurs here too, during which specialised bacteria break down the lipids present in the bones to produce sulphides. The presence of sulphides allows other organisms to thrive, including dense mats of bacteria, mussels and tube worms.

 

But unlike hydrothermal vents, whale-falls are temporary. Life cannot specialise to rely on them for nutrients; there is simply not enough time to allow creatures to evolve here, so these sunken carrion remain to be transient feasts for the pelagic wanderers who, once the flesh has been stripped, dissipate once more into the barren expanse in search of prey.

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