Marine Atlas: Maximizing Benefits for Tonga

STILL WATERS RUN DEEP: OCEAN DEPTH SUPPORTING VALUES

It is important to understand how ocean depth influences both the distribution of life below the surface and the man- agement of human activities along the coasts of Tonga.

Standing on Tonga’s shore and gazing into an alluring turquoise lagoon, it is hard to imagine how deep the ocean truly is. Only 1 per cent of Tonga’s national waters are shallower than 200 metres, while the other 99 per cent dive to 10,800 metres deep, and further. Changes in ocean depth, also known as bathymetry, affect many other dimen- sions of human life and natural phenomena. Bathymetric maps were originally produced to guide ships safely through reefs and shallow pas- sages (see chapters “Full speed ahead” and “One world, one ocean”). Since ocean depth is corre- lated with other physical variables such as light availability and pressure, it is also a determining factor in the distribution of biological communities, either those living on the bottom of the sea (ben- thic), close to the bottom (demersal) or in the water column (pelagic). In addition, bathymetry significantly affects the path of tsunamis, which travel as shallow-water waves across the ocean. As a tsunami moves, it is influenced by the sea floor, even in the deep- est parts of the ocean. Bathymetry influences the energy, direction and timing of a tsunami. As a ridge or seamount may redirect the path of a tsu- nami towards coastal areas, the position of such features must be taken into account by tsunami simulation and warning systems to minimize the risk of disaster. As the bathymetry map shows, Tonga’s main islands are located along a large ridge less than 2,000 metres deep, which extends to the south as the Tonga Ridge. To the west lies an area of shallow abyssal sea floor between 2,000 and 3,000 metres

deep. To the north and west of the main islands, there are numerous seamounts. Several if these, such as the Zephir Shoal (see also chapter “Un- derwater mountains”), have peaks approaching the sea surface. To the east of the main Tongan islands lies the Tonga Trench, the second deepest trench in the world. This trench goes to depths in excess of 8,000 metres along much of its length. Horizon Deep, in the southern part of Tonga’s waters, is the deepest part, with a maximum depth of 10,880 metres. The abyssal sea floor is much deeper to the east of the Tonga Trench, with depths between 5,000 and 6,000 metres. The Capricorn Guyot, a seamount with a flat top, rises up from this deep- sea floor to within 500 metres of the sea surface. The sea floor can be divided into several differ- ent zones based on depth and temperature: the sublittoral (or shelf) zone, the bathyal zone, the abyssal zone and the hadal zone. The sublittoral zone encompasses the sea floor from the coast to the shelf break—the point at which the sea floor Horizon Deep Tonga’s ocean is deep. Very deep. It boasts the deepest point in the southern hemisphere and the second deepest on Earth. With a maximum depth of 10,880 metres, Hori- zon Deep is the deepest point in the Tonga Trench. The submarine trench in the floor of the South Pacific Ocean is about 1,375 kilometres long and 80 kilometres wide and forms the eastern boundary of the Tonga Ridge. Together, the Tonga Ridge and the

rapidly drops away. The bathyal zone extends from the shelf break to around 2,000 metres. The lower limit of the bathyal zone is defined as the depth at which the temperature reaches 4°C. This zone is typically dark and thus not conducive to photosyn- thesis. The abyssal zone extends from the bathyal zone to around 6,000 metres. The hadal zone, the deepest zone, encompasses the deep-sea floor, typically only found in ocean trenches. Tonga Trench constitute the northern half of the Tonga-Kermadec Arc, a structural feature of the Pacific floor, completed to the south by the Kermadec Trench and Ridge. The trench exists where the Pacific plate slips under the Indo-Australian plate. And this is another re- cord: it is the fastest plate tectonic velocity on Earth. While 24 centimetres per year may not seem particularly speedy, for geologists, this is breakneck speed!

2 0 0 m

S h e l f

B a t h y a l

4 ° C

A b y s s a l

6 0 0 0 m

H a d a l

MAXIMIZING BENEFITS FOR TONGA

SUPPORTING VALUES

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