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Open ocean

by  Janet Grieve

Life in the open ocean seems very remote to us because it is largely unseen. This mysterious world is home to a host of highly specialised creatures. The richness of ocean life is closely connected to the supply of nutrients. The weather brings nutrients to sunlit surface waters, where they fuel the growth of microscopic plants.


Plankton: the key to ocean life

Floating pastures

The open oceans around New Zealand are home to a great variety of plants and animals, from plankton to whales and sharks. Floating pastures of tiny plants with chlorophyll called phytoplankton sustain this ocean life. Where phytoplankton flourish, bigger creatures come to feed. When the sea is green and visibility is low, usually there are large quantities of phytoplankton. When the sea is deep blue and we can see a long way under water, phytoplankton is sparse. In recent years marine scientists have begun to understand some of the factors controlling the distribution and abundance of plant and animal plankton around New Zealand.

Seasons in the sea

The growth of plankton is governed by the seasons. Phytoplankton convert energy from sunlight into food for tiny animals called zooplankton. These in turn become food for larger animals. Phytoplankton need sunlight and nutrients, but for much of the year one or the other is in short supply.

In winter, nutrients are abundant, but light is limited. Around parts of New Zealand there is a small increase in phytoplankton growth at this time. In spring, more light penetrates the surface waters and phytoplankton grow rapidly, reaching a peak in New Zealand around September and October. However, as spring turns to summer and the sun warms surface waters, the cooler, denser depths are not stirred up so there is no mixing of the layers that could replenish nutrients. The bloom of phytoplankton dies away. In autumn and winter, surface waters begin to cool and, assisted by strong winds, mix with the nutrient-rich layers below. In this way surface waters are recharged with nutrients.

The vertical traveller

Neocalanus tonsus, a flea-sized zooplankton, is particularly plentiful in New Zealand waters in spring and early summer. At this time it is an important food for filter-feeding seabirds such as the broad-billed prion, and large animals such as basking sharks and sei whales. From late January until October Neocalanus tonsus completely disappears from waters down to 500 metres, moving to depths between 500 and over 1,300 metres.

In summer, Neocalanus tonsus feed and grow on smaller plankton. They put on weight and store extra food as a globule of red lipid (fat), which gives them energy to reproduce. In a diapause (resting stage) until September, they moult and become adult males or females, then mate and lay eggs. These hatch and the young probably migrate towards the surface without feeding. They arrive in time to eat from the spring phytoplankton bloom.

This remarkable life history is a strategy for getting by without food and escaping predation in winter.


Larger plankton in the food chain

All life in the open ocean ultimately depends on the growth of microscopic phytoplankton. But these tiny plants are too small to be eaten by larger marine animals. While small creatures eat the phytoplankton, larger ones are mixed feeders on plant and small animal plankton, or they are carnivores that prey on animal plankton. Moon jellies and salps are examples of mixed feeders, and arrow worms are carnivores.

Moon jellyfish

The moon jellyfish Aurelia is a common summer plankton. They are easily identified by four purplish oval rings (testes or ovaries) on transparent, bell-shaped bodies, which can be up to 40 centimetres in diameter. The creatures propel themselves through the water by rhythmic contractions of the bell. They are also carried along by ocean currents in the same way that parachutes are carried by the wind.

They feed primarily on small floating plankton which are caught in mucus on the umbrella’s surface and propelled to the rim by tiny beating hairs. Four long oral arms or lips then lick the food particles into the mouth.

The jellyfish has two stages. The first is a floating stage that reproduces sexually. Floating males and females produce tiny larvae that settle on the sea floor. Here, they begin an asexual stage, reproducing by budding off juveniles which grow into floating jellies. This capacity to produce numerous floating forms can result in a dense aggregation of jellyfish close to shore. Luckily these jellyfish do not sting.

A flash in the dark

Like many creatures in the ocean, moon jellyfish are bioluminescent. They produce blue light from a pigment called luciferin. When luciferin combines with oxygen it produces an unstable chemical that emits a flash of light. Most bioluminescent creatures are found in the upper 1,000 metres of the open ocean. Other common producers of bioluminescence in New Zealand waters are krill and dinoflagellates.

Salps

Salps are semi-transparent, hollow, barrel-shaped animals that move through the water by contracting bands of muscles around their bodies. The salp Thalia democratica forms long chains (up to 10 metres), and has been recorded in very large numbers in New Zealand waters during summer. Salps have a reputation for growing to swarm proportions when they encounter plankton-rich waters.

When salps propel themselves along, they feed from the stream of water they create. They appear to vacuum up small organisms which include phytoplankton, bacteria and ciliates. One individual is capable of clearing food from up to 440 millilitres of water each day. One of the fastest-growing plankton, Thalia democratica can grow 10−20% of its body length in an hour, or almost double its weight in a day. Salps are eaten by fish, marine mammals and seabirds.

Arrow worms

Arrow worms are found only in the ocean, and are carnivores. Sagitta tasmanica and Sagitta minima are two species of arrow worms most commonly found in New Zealand waters. Straight and thin, they range in length from 0.3 to 15 centimetres. They are perfectly transparent, except for two black eyes on the tops of their heads.

Arrow worms are important predators. They are capable of eating animals as large as themselves, and may consume up to one-third of their body weight in a day. They detect vibrations made by their prey and dart towards them. Arrow worms belong to a large group of invertebrates called Chaetognatha – the name means ‘hairy jaw’, and refers to the large moveable hooks around their mouths, used to catch food.


Fish larvae and the ocean environment

Hoki: a case study

Hoki (Macruronus novaezelandiae) is the most abundant commercial fish species in New Zealand waters. Since 1995, however, there has been a below-average survival of young fish that contribute to the hoki fishery. This low recruitment along with high levels of fishing has reduced the western stock of hoki to about one-fifth of its original level. A major study between 1979 and 1989 helped scientists determine the conditions that affect the growth and survival of hoki larvae.

Birth and growth

Mature hoki live in deep water and migrate to special spawning grounds around New Zealand. In winter one of the places they head to is the waters west of the South Island, where they release their eggs above the continental slope at a depth of about 200 metres. The fertilised eggs float to the surface and become part of the plankton. The eggs take three days to hatch, and it is another five days before tiny larvae develop mouths and begin to feed.

To escape predation hoki larvae gorge themselves and grow quickly: big mouths allow them to eat relatively large food items straightaway. They favour the tiny (1 millimetre) crayfish-like zooplankton called copepods, especially Calocalanus. Oceanic, warm-water forms of Calocalanus thrive just off the West Coast in some conditions.

Environmental conditions

In the Tasman Sea in autumn, surface waters mix with deeper waters, and the amount of phytoplankton increases rapidly. Copepods graze on this new organic matter and are in turn eaten by hoki larvae.

However, there are subtle, year-to-year differences – driven by the weather – in the timing and pattern of water mixing, which means that luck plays a large part in whether larvae get enough food. For example, in 1987, when mixing started early and progressed slowly to deeper water, phytoplankton and copepods were abundant. Hoki born in 1987 were strongly represented in the adult population some years later. In contrast, in 1990, when mixing started late and progressed rapidly to deeper water, it was a poor year for hoki larval survival. There was not enough phytoplankton to support copepod growth to the concentrations required by hoki larvae.

Warm seas

But this is not the whole story. From 1996 to 2002 the Tasman Sea warmed up to at least 800 metres deep. This coincided with a low recruitment of juvenile hoki into the adult stock. It is not known why numbers declined, but it may be connected to reduced nutrient concentrations, changes in the depth of mixing waters, or a combination of these factors.


Hononga, rauemi nō waho

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How to cite this page: Janet Grieve, 'Open ocean', Te Ara - the Encyclopedia of New Zealand, http://www.TeAra.govt.nz/mi/open-ocean/print (accessed 15 November 2019)

Story by Janet Grieve, published 12 Jun 2006