2. Trench (hadal) environments
Map ID Number: 2
Location: Two trenches are located in the New Zealand region: the Kermadec and Puysegur, which lie to the northeast and southwest of the New Zealand landmass respectively
Approximate area: 381,424km2
Description of area:
Trenches are massive linear depressions found throughout the world’s oceans. They are formed at subduction zones, regions where one tectonic plate slides beneath another and is recycled into the earth’s interior. Bottom sediments in trenches are dominated by muds and fine sands of mixed pelagic/terrestrial origin. The “hadal” zone is defined operationally by biologists as depths exceeding 6,000m; only trenches contain ocean this deep. The Kermadec Trench trends northeast from East Cape. It is approximately 2,000km long, with a maximum depth exceeding 10km in places. The Kermadec Trench is narrow and steep in comparison with other trenches, being about 40km wide on average, with a wall gradient of about 1:6. The Puysegur Trench originates off southern Fiordland and trends southwest. The hadal portion of this feature is not as deep or as extensive as that of the Kermadec Trench.
Trench faunas, in general, exhibit a high degree of regional endemism, possess specialised ecological and physiological adaptations, and exist within a community structure that differs from adjacent areas of abyssal sea floor.
Trenches are widely separated from each other, and in geological terms are long-lived features. These characteristics have significant evolutionary and biogeographical implications for trench faunas, including the generation and maintenance of species endemic to a single trench. It is reasonable to expect trench endemism to be high among hadal benthic taxa. On this basis, New Zealand’s trench habitats probably contain many species found nowhere else in the region.
Levenstein (1991) reviewed the world’s known hadal polychaete fauna (120 species) and found that about 30% of species recorded from trenches were restricted to hadal environments (the rest co-occur in the abyssal zone). Of this fraction, about half were endemic to a single trench. For hadal invertebrates found in multiple trenches, intra-specific differences among populations have been demonstrated (France 1993).
Trench organisms rely on particulate fluxes from the upper ocean as well as large food falls, including terrestrially derived plant matter. Deep camera and trap deployments indicate that highly mobile scavenging invertebrates adapted to exploit large food falls are an important component of the trench epifauna. Chemosynthetic communities are likely to be present in New Zealand trenches.
Trenches contain the ocean’s deepest habitats. By definition they represent the lowest possible limits of depth distribution for aquatic life. Special conditions in trenches include extreme pressure and cold, and a highly variable food supply, which are conducive to chemoautotrophic organisms. Most higher metazoan taxa cannot survive deeper than mid-hadal depths, so deeper trench environments do exceed genuine biological endurance limits for many groups. Fishes, for example, have never been recorded deeper than 8.2km.
Physiological adaptations to extreme hydrostatic pressures have been documented for a number of trench bacteria, a number of which only survive under high environmental pressures (e.g. Yayanos and Dietz 1983). Other hadal microbes are reported to have extreme thermophilic or cold-optimised adaptations. Similar adaptations are hypothesised to exist for hadal invertebrate taxa.
No data exists for the New Zealand region about the proportion of biomass contained in trenches. Comparative evidence, however, indicates that trench communities generally differ considerably from surrounding deepsea environments in this regard. For example, anomalously high concentrations of organic matter in the Atacama Trench contributed to meiofaunal densities an order of magnitude higher than those of the surrounding abyss (Danovaro et al. 2002).
Endemism; trophic/functional diversity; extremities of range and adaptation to environment; degree of disturbance; special conditions and specialised organisms; unusual degree/proportion of biomass.
Status and management:
Trenches are perhaps the most pristine of New Zealand’s marine environments in terms of direct anthropogenic disturbance. Indirect impacts are harder to gauge, but potentially could be significant. For example, the removal of large epipelagic fish biomass from the world’s oceans by fishing (cf. Christensen et al. 2003) could reduce the food supply to hadal scavengers, many of which appear adapted to exploit large natural food falls. Hadal environments in the New Zealand region lie partly within, and partly outside the Exclusive Economic Zone. They are not spatially managed or protected by focused management plans, and have no commercial fishing potential. Presently there are no direct activities that pose a special risk to ocean trenches. In addition, benthic fisheries are severely constrained in some parts of this area by nominal catch limits and special permit access provisions.
State of information:
Poor. Hadal zone research in the New Zealand region has focused on the Kermadec Trench. The Danish Galathea and Soviet Vityaz Expeditions sampled fauna from this trench in 1952 and 1958 respectively, trawling to a depth of 10,000m. Hadal collections included many hundreds of specimens, among them polychaete worms, hydroids, gastropod and bivalve molluscs, echinoderms, crustaceans and fishes (e.g., Belyaev, 1966). An unpublished Australian Navy collection of bottom photographs of the Kermadec Trench to a depth of 10km may contain further ecological data.
References and further reading:
Belyaev (1966), Bourne and Heezen (1965), Christensen et al. (2003), Danovaro et al. (2002), France (1993), Galathea committee (1957), Levenstein (1991), Lewis and Marshall (1996), Nielsen (1964), Yayanos and Dietz (1983).