The deep ocean

Exploring below our shallow coastal waters uncovers the secrets of the deep ocean and its unusual biodiversity.

The deep parts of our ocean stretch from 30 m below the surface, to the bottom of the deepest trenches which lie between 6-11 km below sea level.

The deep is difficult to explore due to the tremendous pressure that comes with increasing ocean depth – caused by the weight of water above.

As you go deeper, the characteristics of the ocean change. As well as the pressure increasing, light and temperature decrease, and food becomes scarce. However, life in the cold, dark, deep ocean is incredibly varied.

What’s down there

The creatures of deeper ocean zones are highly adjusted to this less-forgiving environment. In the mesophotic zone we have recently discovered incredible sponges and coral reef systems¹, and the twilight zone contains the greatest number of fish, providing food for seabirds, seals, dolphins, whales, and leatherback turtles. The Kaikōura Canyon is renowned internationally as a biodiversity hotspot with sponge gardens, abundant fish, and very dense populations of seabed invertebrates – worms, sea cucumbers and urchins – in their thousands per square metre.^{2 & 3} Biomass in the canyon between 900 and 1100 m has been estimated to be 100 times higher than similar deep-sea canyon habitats around the world.

At greater depths, deep-sea gigantism has been observed, allowing animals to reach enormous sizes, eg spider crabs (Jacquinotia edwardsii) that have a leg span of up to a metre, and colossal squid (Mesonychoteuthis hamiltoni) as heavy as 500 kg⁴.

There are also large deep-sea sharks including sleeper sharks (eg Somniosus antarcticus), sixgill sharks (eg, Hexanchus griseus), and smaller sharks that even glow in the dark⁴.

There are some familiar species that undertake vertical migrations into the depths. Ihupuku (southern elephant seals) and parāoa (sperm whales) can hold their breath for 2 hours and dive to 1500 and 2000 meters when they venture deep in search of prey.

In Aotearoa, we have more than 800 seamounts, supporting a variety of marine life. From deep-sea corals that can live for hundreds to thousands of years (eg bubblegum coral, Paragorgia arborea)⁵, to deep-sea species of worms, squat lobsters and sea urchins (eg Uroptychus tracey, Caenopedina porphyrogigas)⁶.

As well as seamounts, Aotearoa’s deep ocean has hydrothermal vents and methane seeps, supporting unique communities of bacteria that enable larger animals like clams, mussels and bizarre tubeworms⁷ to survive.

Physical zones of the deep ocean

The mesophotic zone - from 30 to approximately 200 m, is the ‘middle light’ zone between shallow and deep waters.

The twilight zone - the layer of the ocean between 200 and 1000 m below the surface. This zone still receives sunlight, but so little that it would appear pitch black to your eyes.

The midnight zone - the largest part of our ocean, stretches from 1000 – 4000 m and is so deep that no sunlight can reach its depths. The temperature throughout the midnight zone is 4°C.

The abyss - from below the midnight zone to around 6000 m lies the abyss. We have been exploring the abyss for over 60 years but, due to our difficulty getting to such depths it remains largely a mystery.

The trenches - deeper than the abyss are the trenches, narrow steep sided depressions in the sea floor. The deepest trench in Aotearoa New Zealand is the Kermadec Trench (10,047 m deep). If you placed Mount Everest at the bottom of the Kermadec Trench, the summit would still be over 1000 m below the ocean surface.

Why it's important

The deep ocean supports the health of our blue planet. It stores carbon in deep-sea sediments and recycles and pumps nutrients from deeper, colder parts of the ocean to surface waters, where they are consumed by plankton and small fish, driving marine food webs.

Every night, the earth's largest migration occurs from the twilight zone, as creatures small and large swim to the ocean surface to feed, before descending again to deeper waters. This draws down carbon from the surface to the depths, where it can be stored for millennia.

Cumulative threats

The biodiversity of our deep ocean is more vulnerable to threats than shallow water ecosystems. This is because species are typically slower growing and often delicate, eg, habitat-forming deep-sea corals and sponges. Once impacted, these habitats could take hundreds of years to return.

Bottom impacts, climate change and plastic pollution are some of the biggest threats to our deep ocean biodiversity. The cumulative effects of these threats can destabilise entire ecosystems.

Our work

We collaborate with other organisations to explore the deeper regions of Aotearoa’s ocean.

The focus of our work is to:

identify, map and develop seafloor classifications of deep ocean habitats for future protection planning
identify the distribution of protected species and the threats they face

We also monitor our marine reserves, many of which include deepwater habitats and species.

Deep water habitats and species monitored and managed by DOC
Reserve	Maximum depth (m)	Examples of deeper dwelling species
Poor Knights Islands, Northland	110	Hāpuku (Polyprion oxygeneios) Frostfish (Lepidopus caudatus) Eyebrow perch (Hypoplectrodes coronatus)
Tūhua (Mayor Island), Bay of Plenty	160	Pink maomao (Caprodon longimanus) Giant boarfish (Paristiopterus labiosus) Bigeye thresher shark (Alopias superciliosus)
Te Paepae o Aotea (Volkner Rocks), Bay of Plenty	379	Smalltooth sand tiger (Odontaspis ferox) Black coral (Antipatharia) Longfin boarfish (Zanclistius elevatus)
Fiordland marine reserves	365	Red coral (Errina novaezelandiae) Sea pens (eg, Sarcoptilus bollonsi, Virgulariidae) Bluntnose sixgill shark (Hexanchus griseus)
Hikurangi, Kaikōura	1193	Parāoa (Physeter macrocephalus) Barnard’s lantern fish (Symbolophorus barnardi) Frilled crab (Trichopeltarion fantasticum)
Subantarctic marine reserves*	3043	Southern elephant seal (Mirounga leonina) Arrow squid (Nototodarus sloani) Dark ghost shark (Hydrolagus novaezealandiae)

*Antipodes Islands, Auckland Island, Bounty Islands, Campbell Island.

Through exploring and monitoring our deep ocean, we can better understand the intricate links between different ocean zones. We can also uncover more about how the creatures of the deep support life in our shallow, sunlit ecosystems.

This increased understanding will also enable us to prioritise conservation efforts to best protect the deep ocean’s incredible biodiversity and maintain the essential ecosystem services it provides.