Marine Atlas: Maximizing Benefits for Fiji

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 coast.

The Samoa tsunami

One recent example was the 2009 Samoa Tsu- nami, which caused substantial damage and the loss of 189 lives in Samoa, American Sa- moa and Tonga (see graphics). A 76 millimetre rise in sea level near the earthquake’s epicentre

turned into a wave up to 14 metres high when it hit the shallow Samoan coast. Owing to its submarine ridges to the east, Fiji merely experienced large waves with no major damage caused, highlighting the influence of bathymetry (ISC, 2015).

Emerging Giant – A Tsunami Races across the Ocean

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 deepest parts of the ocean. Bathymetry influences the en- ergy, direction and timing of a tsunami. As a ridge or seamount may redirect the path of a tsunami to-

wards coastal areas, the position of such features must be taken into account by tsunami simulation and warning systems to assess the risk of disaster. As the bathymetry map shows, Fiji’s main islands are located on a raised plateau less than 2,000 metres deep, which extends to the south as the Lau Ridge. There are several other extensive ridg- es that run south-west from the main Fijian islands, including the Denham, Moore, Colwyn and Herald Ridges. Just like mountain ridges, these subma- rine ridges rise several thousand metres above the deep ocean floor. To the north-west of Viti Levu is a series of small ridges and troughs, and to the west of Rotuma is a series of shallow banks that rise from the deep. These include Charlotte, Alexa, Louisa, Morton and Hazel Holme Banks. Aside from these shallower areas, the majority of the Fijian national waters are deeper than 2,000 me- tres, with a mean depth of around 2,700 metres, extending down to the deep ocean floor to exceed 6,000 metres. 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 sublit- toral zone encompasses the sea floor from the coast to the shelf break—the point at which the sea floor rapidly drops away. The bathyal zone extends from the shelf break to around 2,000 metres depth. 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 photosynthesis. 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.

Standing on Fiji’s shore and gazing into an alluring turquoise lagoon, it is hard to imagine how deep the ocean truly is. Less than 3 per cent of Fiji’s na- tional waters are shallower than 200 metres, while the other 97 per cent are up to 6,000 metres deep. Changes in ocean depth, also known as bathym- etry, affect many other dimensions 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).

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 FIJI

SUPPORTING VALUES

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