Danau Sentarum, Kapuas River Basin, West Kalimantan, Indonesia

Environment

Environment

Affected by human activity

Affected by human activity

Deforestation

Deforestation

Initial

Surrounding area

The southern part of Danau Sentarum is a unique type of floodplain forest, mainly made up of peat swamps that stay underwater for most of the year. Beyond the Amazon, it is one of the largest inland peat ecosystems in the world. Between November and April, the whole forest floor is flooded. During the dry season, it changes into scattered wetlands and damp forest areas. This long flood period is known as the hydroperiod and plays a key role in shaping the forest’s yearly rhythm.

The land environment is shaped by tall tropical trees from families including Dipterocarpaceae, Euphorbiaceae, and Pandanaceae. These trees form thick canopies that create a dark, humid microclimate below, providing ideal conditions for a wide variety of species. In the branches, flying squirrels (Petaurista spp.) and tarsiers move around, while bearded pigs (Sus barbatus) wander through the undergrowth. The forest also hosts many bird species, such as kingfishers, drongos, woodpeckers, and serpent eagles.

The moist conditions support a wide variety of insects and arthropods, such as leafcutter ants, swarming termites, aquatic grasshoppers, and giant moth larvae. Reptilian life includes freshwater turtles and small forest snakes, which thrive in the warm, damp climate. Many of these insects are part of the decomposer group, breaking down leaf litter and supporting detritivore diversity within the food web. Isolated from human development, this region remains one of the least disturbed and most biodiverse areas in Southeast Asia.

Underwater landscape

In the southern peat swamp forests of Danau Sentarum, seasonal floods extend lake and swamp waters deep into the forest, transforming the landscape into a shallow but vast blackwater ecosystem. During these high-water periods, most of the forest floor becomes submerged beneath a layer of acidic, mineral-poor water.

The flooded ground is covered with decaying leaves from trees like Shorea, Macaranga, and Terminalia, along with twigs, roots, and organic silt. This mix creates a soft, sponge-like layer rich in tannins and humic acids. These compounds stain the water a dark brown and increase turbidity while reducing how much light can pass through. Illumination rarely exceeds a few dozen centimeters, and the deeper layers remain dim and shadowy.

The water is highly acidic (with a pH ranging from 4.0 to 5.5), extremely soft (GH 0-2°dH), and has virtually no buffering capacity (KH 0-1°dH). Water temperatures fluctuate between 24°C and 34°C throughout the year, with an average of approximately 29.5°C during the flooded season. Submerged plants are absent, but sunken wood surfaces are covered with thin biofilms and microalgae that have adapted to low-light conditions. The layer of leaf litter on the bottom often reaches several centimeters in depth, contributing to high levels of dissolved organic carbon in the water.

These conditions create a low-energy, low-visibility habitat, ideal for stealth-adapted species like Sphaerichthys vaillanti, also known as the Samurai Gourami. Its laterally compressed, leaf-shaped body and shades of brown, copper, and black allow it to blend almost perfectly into the forest floor. It stays near leaf debris and twigs, holding its fins close to the body and drifting gently in the current.

This behavior is a form of crypsis, a camouflage strategy that reduces movement and makes the fish nearly invisible to predators. It helps the fish conserve energy and survive in an environment where sight is limited. This cryptic behavior may also serve as a defense mechanism against visually hunting predators such as Belontia hasselti and Nandus nebulosus, which share the same flooded habitat.

Fish list:

• Sphaerichthys vaillanti (Osphronemidae)
• Desmopuntius rhomboocellatus (Cyprinidae)
• Betta dimidiata (Osphronemidae)
• Betta enisae (Osphronemidae)
• Belontia hasselti (Osphronemidae)
• Nandus nebulosus (Nandidae)
• Pectenocypris balaena (Cyprinidae)
• Desmopuntius endecanalis (Cyprinidae)
• Kottelatlimia pristes (Cobitidae)
• Luciocephalus pulcher (Osphronemidae)

Terrestrial vegetation list:

• Cyperus brevifolius (Cyperaceae)
• Cyperus imbricatus (Cyperaceae)
• Lindernia hyssopifolia (Linderniaceae)
• Shorea balangeran (Dipterocarpaceae)
• Pandanus amaryllifolius (Pandanaceae)
• Macaranga gigantea (Euphorbiaceae)
• Mangifera griffithii (Anacardiaceae)

Threats to ecology

The peat swamp forests of southern Danau Sentarum face increasing ecological threats due to both local and global pressures. One of the most significant dangers is deforestation caused by illegal logging and land conversion for agriculture and oil palm plantations. These activities not only damage forest vegetation but also alter the flood dynamics that support the blackwater ecosystem.

Another major concern is peatland degradation. Draining peat soils to make the land suitable for farming releases large amounts of carbon and increases the risk of peat fires, which are very hard to extinguish. These fires produce a thick haze that spreads across West Kalimantan and beyond, reducing air quality and affecting human health. Healthy peatlands usually store carbon through a process called carbon sequestration, but when they are disturbed, they release it back into the atmosphere, contributing to climate change.

In addition, changing climate patterns are disrupting rainfall, which is critical for the annual flood cycles. Shorter or weaker flood seasons reduce the lifespan of aquatic habitats and limit the breeding periods of blackwater-dependent species.

Without effective protection and cooperation from local communities, the rich biodiversity of Danau Sentarum’s blackwater forests, from ancient trees to rare species like Sphaerichthys vaillanti, may quietly fade away.

Riparian zone

Trees near the aquatic habitat

Many -

Bibliography

• → The Raffles Bulletin of Zoology, Supplement No. 13, 2005 - The Fishes of Danau Sentarum National Park and the Kapuas Lakes Area, Kalimantan Barat, Indonesia by Maurice Kottelat, Enis Widjanarti
  ISBN Not applicable. journal supplement, not a standalone book.
  
• → Borneo Research Bulletin, Volume 31, 2000 - FLORA AND VEGETATION OF DANAU SENTARUM: UNIQUE LAKE & SWAMP FOREST ECOSYSTEM OF WEST KALIMANTAN by Wim Giesen
  ISBN Not applicable. peer reviewed journal article
  
• → Borneo Research Bulletin, Volume 31, 2000 - Introduction to Danau Sentarum National Park by W. Giesen, J. Aglionby
  ISBN No ISBN available, as this is a journal article published in the Borneo Research Bulletin.
  

Sitography

Comment by the expert