Simply stated, the Indonesian Throughflow is the largest movement of water on the planet. It is also the key to understanding the amazing biodiversity of the huge Indonesian archipelago.
The Throughflow is an incredibly powerful force of nature. With so much water flowing that a special measurement had to be developed to quantify it – the “Sverdrup”!
Excellent descriptions of both Sverdrups and the Indonesian Throughflow can be found in David Pickell’s book Diving Bali and also on Wikipedia, But the basic mechanism is the difference in sea-level between the Pacific Ocean to the north-east of the archipelago and the Indian Ocean to the south-west.
This massive disparity is caused by the trade winds and associated oceanic currents. These act in opposite directions in the northern and southern hemispheres. But the overall result is the largest movement of water on the planet which flows through the Indonesian archipelago.
The 17,000+ islands of the archipelago act as a hydraulic brake on the Indonesian Throughflow. And it is said that if Indonesia did not exist, there would only be 23 hours in each day. Because that hydraulic brake effectively slows down the rotation of the Earth!
The Indonesian Throughflow and “Sverdrups”…
So huge is the volume of water associated with the Indonesian Throughflow that traditional measurements are inadequate. Cubic meters and gallons are simply too small 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 – 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 its way through the the Lesser Sundas. The chain of islands that runs along the bottom part of the Indonesian archipelago.
There are a limited number of channels between the islands of the Lesser Sundas. The main ones that carry the majority of the water flow are the Lombok Strait between Bali and Lombok. The Sape Strait between Sumbawa and Komodo and the Ombai Strait between Alor and 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.
Biodiversity and the Indonesian 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 and larvae of the Indo-Pacific region marine life. An incredibly diverse area with over 4000 identified species (compared to around 1000 in the Red Sea and 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 those eggs and larvae. Along with the rich detritus of the sea, swept up from the deep offshore basins by upwellings created by the huge volume of water flow.
Virtually everything that dies is the sea descends to the bottom, where it decomposes. Creating a rich layer of phosphorous and 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.
The Wallace Line and the Indonesian Throughflow
Alfred Russell Wallace was a British naturalist who explored the “Malay Archipelago” (Indonesia as we now know it…) in the 1850’s. Wallace spent 8 years classifying both the flora and fauna and the people of the 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 it provides a remarkable insight into the man himself and the areas he explored in more than 60 separate journeys. Wallace traveled over 14,000 miles and collected 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, led Wallace to a very clear conclusion. Just as the island of Madagascar had distinct species because of the deep water channel that separates it from Africa. There were striking differences between the flora and fauna in the eastern and western parts of the archipelago.
Wallace postulated that an imaginary line could be drawn roughly north to south through the Malay Archipelago. To the west of that line the flora and fauna clearly belonged to what he called Indo-Malayan division. While everything to the east belonged to the Austro-Malayan division. This led to the conclusion that the land masses to the west had once been part of the Asian continent. While those to the east were once part of a much larger Australian continent.
That imaginary line became known as the Wallace Line. Which 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!