THE UNDERWATER WORLD OF FIORDLAND

Forest and Bird, Volume 17, Issue 3, 1 August 1986, Page 10

THE UNDERWATER WORLD OF FIORDLAND

by

Oceanographic

Institute

Ken Grange,

~iordland, stretching almost 200 km +. from Milford Sound south to Preservation Inlet, is internationally acclaimed for its spectacular scenery, isolation, unmodified environment and as home for a large number of rare plants and animals. However, few people probably stop to think that the name ‘Fiordland’ is composed of two words, ‘‘fiord’’ and ‘‘land’’. The fiords

are often thought of merely as the long ribbons of water which separate the more familiar mountains. Many visitors to the area return with memories of constant heavy rain, waterfalls, swarms of sandflies and dense forest from the snowline to the water’s edge. A few may wonder at the incredible forces generated by the glaciers as they cut through the mountains on their way to the sea; but until recently very few thought about the marine life in the fiords themselves, apart from when they may have been seals, dolphins and penguins during a launch trip. The land and climate are important in understanding the marine life beneath the dark waters of the fiords; in few other places do these mesh together so intricately to produce such unique environmental conditions that here support one of the richest and most unusual marine ecosystems in the world.

Towering mountains

The New Zealand fiords are the drowned

lower reaches of valleys which were last occupied by glaciers approximately 20,000 years ago. Between the sheer sides of towering mountains rising to heights of 2,000 metres or more, the sea now penetrates on average 16 km inland. Beneath the water these steep mountains continue down as rock walls until they reach the mud-covered flat floors of the fiords. The water depths range from around 100 m to more than 450 m in the deeper basins. A shallow ridge near the fiord entrance marks the seaward extend of the former glacier; this is called a sill, and it partially cuts off the water in the fiord from the sea. Freshwater flows into the fiords continuously in prodigious amounts as a result of rainfall (almost 7,000 mm a year) and the spring-summer melt, but as it rushes down through the surrounding dense bush it rarely washes away the soil since the bush has a thick carpet of mosses and ferns. With little erosion, sediment is not carried into the fiords to smother the plants and animals there. Rather, the

freshwater picks up large quantities of leaves and other plant detritus as well as dissolved organic material from the undergrowth, carrying this down into the fiords to provide nutrients to plants and animals. The dissolved organic material stains the freshwater and by the time it enters the fiord, it is the colour of weak tea, or pale ale, coloured but not cloudy. Because this freshwater is less dense than the seawater in the fiords, it sits as a less salty surface layer which is normally around 3 m thick. Even during periods of strong winds this layer does not fully mix with the higher-salinity seawater below since large waves or swells cannot form in the enclased fiords. The low salinity layer continually flows seaward over the top of the seawater, but as it moves it entraps and carries some higher salinity water from below, developing a two-layered circulation with high-salinity seawater flowing into the fiord below an outflowing low-salinity layer. This circulation is, however, confined to the upper 20-40 m.

Below this, the water in the deep basins may remain undisturbed for years, particularly in fiords with very shallow sills. Water in these basins can be renewed only when nearby coastal water at the sill depth is denser than the basin water so it can spill over the sill. If the basin water is not renewed, it becomes isolated. Eventually dissolved oxygen is consumed through animal respiration and oxidation of organic material, sulphur is reduced, and hydrogen sulphide is liberated. This has profound effects on the sediment and organisms living at that depth resulting in generally low species diversity. The dominant animals are heart urchins, tube worms, bivalves, and tusk shells, but the actual species are the same as those found in bathyal depths (around 1,000 m) in the open ocean outside. Different coast A further effect of the freshwater layer is to lower salinities (salt levels) to near zero in the intertidal region, so that at low tide one does not see the usual abundant masses of mussels, barnacles and seaweed characteristic of open coasts, although some brackish-water species can tolerate the conditions and colonise the fiord walls between tide levels. These include small snails, barnacles and blue mussels. On most rocky shores throughout New Zealand large seaweeds are found at low tide and for some distance sub-tidally, depending on light penetration and water density, but in the fiords the low salinity of the surface layer prevents most seaweeds growing at low tide level, and its dark brown colour does not allow sufficient light to penetrate down to the seawater for luxuriant seaweed growth to occur. Fur-

thermore, , the steep walls of the fiords cast long shadows, again reducing light levels. The low salinities in the top 5 metres reStrict this zone to organisms which can live in brackish water for long periods, such as green sea lettuce, blue mussels and barnacles. In contrast to the sparse life of the deep muddy and inter-tidal habitats, the steep rock walls between 5 and 40 metres support extremely diverse communities, and there is a marked change in species composition with depth, as shown in the accompanying diagram. Below the light-absorbing low salinity layer, the seawater is still, very clear and relatively warm with annual temperatures of 11-15 °C, the smallest range anywhere on the New Zealand coast. Between 5 and 15 metres deep the rock walls are completely encrusted with tubeworms, sponges, soft corals, colonial sea squirts and file shells, so abundant are the nutrients here. A variety of starfish, urchins, sea snails and colourful sea slugs prey upon these immobile, attached species. Below 15 m there is perpetual gloom and here the tubeworms are replaced by large sponges, sea squirts, red and pink hydrocorals, soft corals, black coral, horny coral, solitary corals and brachiopods. Even the few patches that appear superficially as bare rock have a thin covering of encrusting pink algal ‘‘paint’’. Fish are abundant; over 50 species have so far been recorded in the top 40 m. The communities deeper than 40 m on these rock walls have not been examined intensively because they are beyond the range of scuba. However, remote television cameras lowered to around 100 m show colonies of black coral, sponges, and brachiopods, although there appears to be less diversity at these depths. Unique marine life The marine communities in the fiords are unique. This is mainly due to a large number of species that are very common there, but generally considered as rare elsewhere in New Zealand, or which live in much deeper water elsewhere. The perpetual gloom beneath the low-salinity layer and the shading caused by the surrounding mountains simulates the conditions usually found at far greater depths in the open ocean and allows species that normally live in deep water to colonise these shallow areas without competition from the faster growing seaweeds, while the lack of wave action permits delicate, branching species to become established without being damaged. Several animals that had been collected previously only from the continental shelf in depths between 100 and 200 m have been found quite commonly in water less than 30 m deep throughout the fiords. These include the saucer sponge, shrimps, red and pink hydrocorals, starfish, feather stars, sea pens, brachiopods and fish such as the orange-lined perch and spiny sea dragon. Other species, such as the large tube anemone, black coral, and horny coral which have been reported within diving depths only occasionally elsewhere (usually deeper than 30 m) are abundant

from 6 m in the fiords. Since the fiord waters below the low salinity layer have narrow seasonal fluctuations, species which generally occur in either warm or cool water have become established. Warm water species, those considered more at home in water around the northern half of the North Island, include molluscs (for example the large trumpet shell and several sea slugs), and fish (for example the blue-dot triplefin). Cold water species include copper moki, trumpeter, banded wrasse and pigfish. There are also species of fish, sea slugs, snails, anemones, and sponges which have been collected in the fiords for the first time and are new to science. Living fossils . The southern fiords are now recognised as supporting the largest populations in the world of several brachiopod species. These animals, commonly called lamp shells, superficially resemble bivalved molluscs since they have two shells hinged together. The soft parts of the animal are, however, entirely different. They attach to hard substrates by a pedicle, a horny, stalked structure that protrudes through a hole in the top shell, and they feed with a spiral, ciliated structure called a lophophore. Brachiopods are known as fossils from rocks dated around 600 million years old and were once the dominant animals in the world’s seas, but they are not common today. The presence of large populations of at least five species in shallow water throughout the fiords provides a unique opportunity for scientific study of these living fossils. Brachiopods are the underwater equivalent of Fiordland’s best known land-dwelling representative of a former age, the takahe, and they deserve the same study. Black coral There are several animals with economic potential living in the fiords, including rock lobsters, scallops, blue cod, and paua. The potential economic rewards from harvesting, polishing and fashioning the skeleton of black coral into jewellery however probably exceed all others. The living population of black coral in water less than 35 m deep in the fiords has been estimated at over 7.5 million colonies, although most are too small to harvest — the largest resource of black coral known in the world. Money can be made from this without harvesting, however. Research is showing that the colonies are extremely slow growing so rather than risk over-fish-ing, the living colonies may be worth even more as a potential diving/recreational/ tourist resource. Protection needed At present there is no legislation preserving the underwater habitat of the southern fiords; Fiordland National Park stops at high-water mark. Only the black coral is protected by Fisheries legislation. The underwater ecology of the fiords is finely balanced by the unique set of environmental conditions above water in the surrounding mountains. Interference to this balance could potentially have disas-

trous effects; for example, should freshwater be prevented from running through dense forest on its way to the fiords, the major source of nutrient input would be lost. Similarly, if the bush and undergrowth is removed, sediment would be brought into the fiords by the freshwater runoff and smother the slow-growing, immobile marine organisms attached to the rock walls. If the dark-stained low-salinity layer with its important light-absorbing properties is interfered with, then light may reach the marine communities encouraging seaweed growth, eventually displacing the deep-water animals. The fiords may seem isolated and under little threat but recently there have been proposals to export freshwater, using super-tankers which possibly could mix the low and high salinity layers, and there have been requests to harvest black coral colonies. Even too many deer in the surrounding mountains could increase erosion and have some effect. Fortuitously, the establishment and management of the Fiordland National Park above water has probably also been beneficial to the survival of the marine communities by ensuring that modifications to the vegetation are minimal. Fiord inclusion essential In a recent issue of Forest and Bird (Vol. 16, No. 4, Nov. 1985) an excellent article on the case for south-west New Zealand to be included as a World Heritage area mentions many of the geological and biological wonders of the area, but nothing is said about the fiords themselves and their unique attributes. It is essential that the fiords are included in any submission, but we must also have due regard for the rock lobster fishermen who rely on the area for their living without significantly modifying its existing natural beauty. As the marine wonders of the southern fiords become better known, more people will join in the quest to protect the huge 300-year-old black coral colonies, the tube-anemones, sea pens, brachiopods and unusual fishes. Should it proceed, a recent proposal to build an underwater viewing platform in Milford Sound will enable non-divers to gain an impression of the underwater world of the fiords. The marine communities with their living fossils and deep-water representatives are just as unique in the world as the land communities of Fiordland which harbour species such as the takahe and kakapo. Surely the marine environment is worthy of the same status and protection. Ken Grange is a biologist with the DSIR’s Division of Marine and Freshwater Science, based at the N.Z. Oceanographic Institute in Wellington, and specialising in shallowwater marine ecology. For the past five years he has been examining the unique underwater environment of the southern fiords and, more recently, has directed his research specifically towards the abundance, distribution, growth rates, and reproduction of black coral. He is also a scientific adviser to the Fiordland National Park, overseeing marine research in the area, and is a keen diver and underwater photographer.