A basic understanding of the Indonesian Throughflow and “Sverdrups” is essential if you want to know why the scuba diving can be so be good in certain parts of the vast Indonesian Archipelago and less-so in other areas…

Excellent descriptions of both Sverdrups and the Indonesian Throughflow can be found in David Pickell & Wally Siagian’s book Diving Bali and on Wikipedia, but the Readers Digest version is that to the northwest of the Indonesian archipelago lies the Pacific Ocean where the sea level is 200 mm (8 ins) above average, whilst to the south lies the Indian Ocean where the sea level is 100mm (4 ins) below average.

This massive disparity is caused by the trade winds and associated oceanic currents that act in opposite directions in the northern & southern hemispheres, but the overall result is the largest movement of water on the planet which flows through the Indonesian archipelago from the Pacific Ocean to the north-east to the Indian Ocean in the south-west.

Map of the Indonesian Throughflow

Map of the Indonesian Throughflow

So huge is the volume of water associated with the Throughflow that traditional measurements such as cubic meters & gallons are inadequate to describe it in an easily understandable way, so the Norwegian scientist Harald Sverdrup invented the Sverdrup – one million cubic meters of water per second.

David Pickell visualizes it best in Diving Bali – think of a river 100m wide, 10m deep and flowing at 2 knots, then imagine 1,000 of those rivers all combined together and that is one Sverdrup!

It is estimated that the total amount of seawater that passes through the Indonesian Throughflow is 15 Sverdrups, or 15,000 of those rivers. A massive volume of water which has to make it’s way around the chain of islands that runs along the bottom part of the Indonesian archipelago called the Lesser Sundas which stretch from Bali in the west to Timor in the east.

There are a limited number of channels between the islands of the Lesser Sundas and the main ones that carry the majority of the water flow are the Lombok Strait between Bali & Lombok, the Sape Strait between Sumbawa & Komodo and the Ombai Strait between Alor & Timor.

Of these the 35km wide Lombok Strait offers the most direct path to the Indian Ocean and it is estimated that about 20% of the shallow water flow of the Indonesian Throughflow passes through that conduit, which in terms of rivers means 3,000 of them.

Map of the Lesser Sunda Islands showing main Indonesian Throughflow exit points

Map of the Lesser Sunda Islands showing main Indonesian Throughflow exit points

Diving Indonesia: Biodiversity and the Throughflow

Indonesia’s amazing underwater biodiversity can start to be understood when the contents of the Throughflow are considered, because carried along in the huge volume of water are  the eggs & larvae of the marine life of the Indo-Pacific region – an incredibly diverse area with over 4000 identified species (compared to around 1000 in the Red Sea & 400 in the Caribbean).

As the Througflow touches land in the north-east of the archipelago around Raja Ampat in Irian Jaya, Halmahera and North Sulawesi it brings with it the eggs & larvae, along with the rich detritus of the sea that is swept up from the offshore deep basins by upwellings created by the huge volume of water flow.

Virtually everything that dies is the sea descends to the bottom where is decomposes and creates a rich layer of phosphorous & nitrogen based nutrients which are sucked upwards by the upwellings.

When this mechanism is considered it starts to become clearer why the areas around Raja Ampat in Irian Jaya, Halmahera and North Sulawesi are so rich and biodiverse.

A similar mechanism occurs to the south, with the deep basins of the Banda Sea and  the Lesser Sundar Islands, also explaining why that area is so rich and biodiverse.

Map of the Indonesian Archipelago and Indonesian Throughflow

 Map of the Indonesian Archipelago and Indonesian Throughflow

Diving Indonesia: The Wallace Line and the Throughflow

Alfred Russell Wallace was the 19th century British naturalist who spent who spent the 8 years from 1854 to 1862 exploring and classifying both the flora & fauna and the people of what he called the Malay Archipelago – the results of which he subsequently documented in a book of the same title that was first published in 1869.

The Malay Archipelago is still in print to this day and provides a remarkable insight into the man himself and the areas he explored in more than 60 separate journeys, covering over 14,000 miles and collecting a total of 125,660 specimens of natural history – much of which he bequeathed to British Museums.

The time spent and the distance traveled, combined with the vast specimen collection, lead Wallace to the very clear conclusion that just as the island of Madagascar had “distinct & peculiar” species because of the 300 mile deep water channel that separates it from Africa, and Corsica & Sardinia had different species to Italy for similar reasons, there were striking differences between the flora & fauna in the eastern & western parts of the archipelago.

Wallace postulated that an imaginary line could be drawn roughly north to south through the Malay Archipelago and to the west of that line the flora & fauna clearly belonged to what he called Indo-Malayan division, while everything to the east belonged to the Austro-Malayan division. This also “lead irrestively to the conclusion” that the land masses to the west had at one point been part of the continent of Asia, while those to the east were once part of a much larger Australian continent.

That imaginary line became known as the Wallace Line and is often referred to as the most profound zoological barrier on the plant, but the basic mechanism that creates the incredible barrier is the Indonesian Throughflow – something that Wallace would have had no idea of at the time…

Map of the “Malay Archipelago” & the Wallace Line

Map of the “Malay Archipelago” & the Wallace Line

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