Freshwater ecosystems

While Borneo’s freshwater ecosystems remain in relatively good condition, the impacts of mining and logging are putting several places at risk – with serious consequences.

The freshwater giants

Some of the major waterways that stand out in the Borneo landscape include the Kapuas (in Kalimantan, Indonesian Borneo), which at 1,143 km is slightly shorter than the Rhine.¹ The Kapuas flows to the west coast, draining some 2/3 of West Kalimantan province - a watershed of 100,000 km2 (an area slightly larger than Hungary).

Other major rivers in Kalimantan include the Barito (900 km), which flows south, and the Mahakam (775 km), which empties into the Makassar Strait to the east of Borneo.²

In Borneo’s inland basins and in the lowlands, several major rivers form extensive lake systems. The Mahakam, Barito, Kapuas and Baram rivers form oxbow and seasonal lakes in their lowland reaches, some of which are of economic importance for fisheries.³

The journeys of Borneo’s rivers

Like sponges, Borneo’s tropical rainforests regulate the quantity of water that drains into streams in the uplands. These merge with others to form larger waterways, which swell and subside in water volume depending on the seasons. During the rainy season, water levels can change very fast. For example, much of the Kapuas can rise 10-12 m overnight during heavy rainfall.⁴

From trickles to roaring torrents

From the fast and clear headwaters to the wide-bodied and slow-flowing rivers of the lowlands, the ecology of Borneo’s rivers varies enormously during their course.

• Montane streams
  High up in the Heart of Borneo’s montane forests, waterways mostly consist of cold-water torrents. Few plants populate the river banks and wildlife is dominated by aquatic insects.
  
  In these streams, evolution has endowed invertebrates with useful adaptations such as flattened bodies and the presence of hookers, to avoid being swept away by the strong current. One fish genus in particular has evolved a handy ‘tool’ - the sucker fish (Gastromyzon species) has suckers to attach itself to the bottom of fast-flowing waters.
  
  
• Upland streams
  Between 100 m and 1,000 m, waterways become cool-water torrents. Some plants have evolved to be streamside specialists. Aroids (family Araceae) such as Piptospatha species, for example, appear on the sides of smaller tributaries while Saraca species, small trees of the legume family, grow along larger streams.
  
  Compared to montane streams, upland streams show more activity. Fish found here are often strong swimmers, with well-adapted streamlined bodies.  Dragonflies, mayflies and stoneflies may be each represented by more than 20 species.
  
  
• Lowland rivers
  Follow the course of a river below 100 m and a noticeable ‘wall’ of tall trees begins to form on either side. Underwater, sedimentation often causes the water to become turbid. This favours fish that have adapted barbells - such as catfish - to identify prey in low visibility conditions.⁵

The quest for living space in Borneo’s freshwater ecosystems

For Borneo’s freshwater wildlife, life is shaped by many factors (e.g. temperature, water flow, chemicals and competition with other species) that restrict their distribution in a very precise way. As a result, wildlife of river and lake ecosystems is heavily compartmentalized, with each species occupying a well-defined niche – a species place in an ecosystem.

Seemingly insignificant factors can affect species distribution. For example, the location of freshwater prawns and crayfish can be determined by a small detail: how large sediment particles are.

More obvious factors also define the ecological make-up of a particular freshwater habitat. Plant and invertebrate food supply, shade provided by overhanging vegetation and the presence or absence of riffles (shallower, faster moving sections of a stream) and pools are factors that are all considered when a species seeks a home.

In particular, top predators such as the false gharial (Tomistoma schlegelii) and the estuarine crocodile (Crocodylus porosus), river turtles, monitor lizards and otters shape the ecology of other species lower in the food web.⁶

Rivers: generally healthy, but not immune to risk

Unlike Borneo’s tropical rainforest ecosystems, most of the island’s rivers remain in relatively good condition. However, localized impacts from goldmine pollution are affecting the quality of water bodies in some places. 

Soil erosion and subsequent heavy sedimentation of rivers is very obvious now in many parts of Borneo. This can be a noticeable problem where logging activities take place, and where forest is converted for alternative land use, such as industrial plantations.

Such incidents have come at a cost. There is evidence that some populations of freshwater fish have declined over the last few decades in many Borneo rivers,⁷ such as in the Barito.⁸ Lakes too are bearing the brunt of logging and conversion of forests and new settlements.⁹

However, new species of freshwater fish continue to be discovered. Seemingly, this is one of the few groups of vertebrate animals in Borneo which have yet to fully investigated.