The deep ocean

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.

A 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 kg².

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 dark².

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)³, to deep-sea species of worms, squat lobsters and sea urchins (eg Uroptychus tracey, Caenopedina porphyrogigas)⁴.

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 tubeworms⁵ to survive.

• Protected coral species
• Marine species, habitats and protected areas

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. 

*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.

References

¹ Baseline seabed habitat and biotope mapping for a proposed marine reserve

² Giant squid and glow-in-the-dark sharks surprise scientists | NIWA

³ Chronicles of the deep | NIWA

⁴ Uncovering secrets of our seamounts | NIWA

⁵ Voyage reveals extraordinary life around deep-sea gas seeps | NIWA

More information

Biodiversity of the Kermadec Islands and offshore waters of the Kermadec Ridge: report of a coastal, marine mammal and deep-sea survey (TAN1612) (PDF, 7,862K)