For millennia the deep ocean, beyond human reach, was imagined to be populated with fantastic beasts. The ancient Greek word abyss (meaning bottomless) is used to describe the ocean’s apparently infinite depths. In 1521 the Portuguese explorer Fernão de Magalhães (Ferdinand Magellan) reputedly lowered a cannonball to around 750 metres in the Pacific Ocean and concluded that it was immeasurably deep. This finding led to the theory that life in the depths was not possible because there were no currents or temperature shifts, and hence no oxygen exchange or nutrient supply.
Samuel Morse’s invention of a telegraph system led to the laying of transatlantic and other undersea cables from 1857. When broken cables were raised for repair, they were found to be festooned with life. The discovery of sea lilies in 1864, until then known only from 120 million-year-old fossils, raised the fascinating possibility that other supposedly extinct or unknown species were living in the deep.
Voyages of discovery
In 1872 the sailing ship Challenger undertook one of the greatest voyages of biological discovery. For three and a half years it circumnavigated the globe, taking soundings and putting down dredges and trawls. New Zealand waters were included in this amazing voyage. The later named Challenger Plateau, west of New Zealand in the Tasman Sea, was located and studied, and New Zealand marine biologists James Hector and Frederick Hutton described some of the fish collected there. The result of the project was the discovery of 4,717 new species globally, and a greater understanding of the depths.
No further study of the deep sea around New Zealand was undertaken until the 1950s when a retired fisherman, Richard Baxter, managed to catch the lantern dogfish (subsequently named Etmopterus baxteri), white rat-tails and basketwork eels. He used a hand line down to 1,000 metres from an open dinghy off Kaikōura.
It was not until the 1970s that exploratory trawling began in depths of 800–1,000 metres. This was quickly followed by commercial trawling of orange roughy, a deep-sea fish. Today, although we have some understanding of the creatures that live in the depths, there is evidence that a vast number of organisms remain unknown. The first intact specimen of the colossal squid (Mesonychoteuthis hamiltoni) was discovered as recently as 2003, south of New Zealand in the Ross Sea. This 6-metre specimen was a juvenile; scientists speculate that adult forms could be two or three times larger.
Zones of the ocean
Depending on how deep the sea is, there can be up to five vertical zones in the ocean. From the top down, they are:
- The epipelagic zone. Also called the euphotic zone, this is the top layer, where photosynthesis can still take place. Typically the epipelagic zone extends to 200 metres, but this partly depends on the presence of suspended and dissolved material of various kinds. Virtually all primary food production in the ocean occurs here.
- The mesopelagic zone. This encompasses any last vestige of light in its 200–1,000-metre depth range; there is not enough energy for photosynthesis. The main thermocline (a rapid change in temperature over a small depth range) is here. Below the thermocline the temperature is a relatively constant 2–5°C. At about 500 metres the water becomes depleted in oxygen (known as the oxygen minimum layer). Despite this, there is an abundance of life that copes with the shortage through more efficient gills, minimal movement, or both.
- Bathypelagic zone. This is the region from 1,000 metres to 4,000 metres deep.
- The abyssopelagic zone – from 4,000 metres down to the ocean floor.
- The hadopelagic zone. This lies at the bottom of ocean trenches, the deepest being 11,000 metres. Around New Zealand they can be 6,000–9,000 metres deep.