Marine Atlas: Maximizing Benefits for Tonga

VOYAGE TO THE BOTTOM OF THE SEA: GEOMORPHOLOGY

Tonga’s sea floor is rich in physical features of different shapes and sizes that affect the distribution of biodiversity, fishing grounds and deep-sea minerals.

an area of raised sea floor—that perches several thousand metres above the surrounding ocean sea floor. Within this area, there are several small, spreading ridges, with small rift valleys forming in their centres. There are 43 seamounts in Tonga’s waters, with the majority to the west and north of the main islands. A distinct chain of seamounts lies to the immediate west of the main islands, running in a north–south direction. A single large guyot—a sea- mount with a flat top—lies to the east of the main islands. Seamounts are large (over 1,000 metres high), conical mountains of volcanic origin, while guyots are seamounts with flattened tops (see also chapter “Underwater mountains”). The steep sides of all these features interact with currents and cre- ate important habitats for many species. Immediately to the east of the main islands lies the Tonga Trench, a deep ocean trench reaching depths greater than 8,000 metres. It’s deepest point, known as Horizon Deep, measures 10,880 metres. These deep ocean trenches are likely to support a suite of unique species compared with other parts of the sea floor. The main islands are perched on the large Tonga Ridge, which marks the eastern boundary of a large plateau. Adjacent areas of slope and the margins of the plateau are incised with numerous large submarine canyons. These canyons are char- acterized as areas of high biodiversity due to their steep sides featuring rocky slopes, strong currents and enhanced access to food. They also act as a conduit between the deep-sea floor and the shallow shelf areas. On all these features, areas of steep sea floor (escarpments) are likely to contain hard substrate which, coupled with increased current flow, create ideal habitats for filter-feeding organisms such as sponges and cold-water corals.

The Samoa Tsunami One recent example of such effects was the 2009 Samoa Tsunami, which caused substan- tial damage and the loss of 189 lives in Sa- moa, American Samoa and Tonga (see graph-

ics). A 76 millimetre rise in sea levels near the earthquake’s epicentre transformed into a wave up to 14 metres high when it hit the shallow Samoan coast.

Emerging Giant – A Tsunami Races across the Ocean

The nation’s seascape is as diverse underwater as its landscape above, including towering underwa- ter mountains (seamounts) that attract migratory species from hundreds of kilometres away, and deep-sea canyons that carry nutrient-rich water from the deep ocean to the shallow areas. Geo- morphology (the study and classification of these physical features) reveals both the geological origin of the features as well their shape (morphol- ogy), size, location and slope. The geomorphology of the sea floor influences the way the ocean moves (see also chapter “Go with the flow”), the way the wind blows and the distri- bution of water temperature and salinity (see also chapter “Hotter and higher”). These factors affect the distribution of biological communities, resulting in different biological communities being associat- ed with different types of sea-floor geomorpholo- gy. For example, seamounts generally have higher biodiversity and a very different suite of species to the adjacent, deeper abyssal areas.

Similarly, different economic resources are often associated with different features. Many fisheries operate on certain features, such as the shelf, slope or over seamounts, based on where their target species occur. In Tonga, important deep- sea snapper is mostly found on outer reef slopes and around seamounts (mainly in depths from 100 to 400 metres; see chapter “Fishing in the dark”). Furthermore, different types of deep-sea mineral deposits are also associated with different features, such as the sea-floor massive sulfide de- posits found along mid-ocean ridges (see chapter “Underwater Wild West”). Tonga’s waters harbour 18 different geomorphic features, which are presented in this map and associated figures. The distribution of geomor- phology reflects many of the patterns observed in the bathymetry map, as geomorphology is primar- ily a classification of the shape of the sea-floor features. The Tongan islands and the western part of Tonga’s waters sit on top of a large plateau—

0 km

200

400

Ridge

2

Pinnacles

Guyot

Seamounts

Abyssal Hills

Trough

4

km depth

Seamount

Continental Crust - Granite

6

Sediment

Ocean Crust - basalt

Subduction Zone

Sediment Drifts

Upwelling lava

MAXIMIZING BENEFITS FOR TONGA

SUPPORTING VALUES

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