Exploring Milford Sound's deepwater emergence phenomenon

Most deep water species found in Milford Sound can be seen by divers in recreational diving depths making this area an explorers dream.

The 14 fiords in the southwest of New Zealand’s South Island are totally unique and host habitat and biological communities that can’t be found anywhere else in the world.

Glacial origin

20,000 years ago glacial action carved out the deep fiords found in the southwest of New Zealand. Once retreated, these glaciers left behind rock debris at the entrance of each fiord. This shallow rock barrier restricts the seawater in and out of the fiords. The circulation of water is confined to the top 20-40 metres, leaving the deeper waters of the Fiord undisturbed for centuries.
For example Milford Sound greatest depth is close to 400m, where the shallow fiord entrance reaches up to about 80 metres.

Freshwater top layer

Fiordland is one of the wettest places in New Zealand as it receives over 7.5 metres of annual rainfall. All this water plummets into the fiord from the surrounding mountains in endlessly cascading waterfalls. The colour of the freshwater carries with it tannin from the rotting leaf litter in the forest and essentially stains the top few metres brown-green. This top layer averages a couple of metres thickness and reduces light levels enough for deepwater species to emerge from the depths to live and grow in the shallows.

Deepwater Emergence explained

It is this reduced level of light that makes the fiord waters so special and there is only limited seaweed growth on the walls of the fiords. With less competition for space from seaweeds, encrusting and sessile animals like colourful sponges and corals flourish on the underwater walls.

Just off the coast of Fiordland, the continental shelf falls away sharply to 6300m in the Puysegur Trench, and a number of deep-water animals are carried from the Tasman Sea into the dark sheltered waters of the fiords. Here, conditions are similar to those of the deep ocean, and species such as black corals, sea pens and lampshells thrive on underwater cliff walls. Scientists label the phenomenon of animals surviving at depths much shallower than their normal range ‘deep water emergence’.

Deep Water species in shallow depths

Most deep water species found in Milford Sound can be seen by divers in recreational diving depths making this area an explorers dream.

Black coral, usually live between 200 and 1000m depth. Fiordland is home to the largest black coral colonies in the world and some trees reach up to 5 metres tall. This coral grow as shallow as nine metres in the fiords. Underwater, the colonies appear brilliant white – it is the skeleton that is black and gives the coral its name.

Red corals are members of the order Anthoathecatae (hydrocorals or stylasterid corals). The most familiar species is the native red coral Errina novaezelandiae (family Stylasteridae), which may be encountered in relatively shallow water (24-50m depth).

Fiordlands conditions are also host to the rare temperate water fish, the bright orange Spiny Sea Dragon (Solegnathus spinossissimus) that can be found in diveable depths in the inner part of the Fiords.

Lampshells or Brachiopods are common throughout most of Fiordland. In some areas it is estimated that they reach densities of 1,000 per square metre. Although they resemble seashells, brachiopods are not molluscs but a distinct group of very ancient lineage. The name lampshell refers to the fact that, in typical forms, it resembles an ancient Roman lamp. These ancient shells are said to have remained unchanged in over 300 million years.

The glass sponge Symplectella rowei is found at depths of 30–50 metres, attached to submarine cliff walls. They are the largest sponges in Fiordland, and an example of deepwater emergence; glass sponges are usually found only in deep oceans or the polar regions.

Final Frontier

Besides all the known deepwater species resident in Fiordland, new species are still being discovered every time a research team enters the area. It is said from what so far has been discovered, it had some of the most unique and diverse marine ecology in the world.

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