An overlooked carbon source to an important freshwater fishery may be under threat

By Ben Miller, SAFS student

Floating village in Tonlé Sap Lake

When you first arrive at the community of Kampong Phluk, your neck cranes up bamboo stilts to meet the chatter of families in houses high above. From the top of what guidebooks call “bamboo skyscrapers,” locals gaze over the tops of submerged trees, a glittering, island Buddhist temple, and clusters of floating fishing villages in the distance. This is the shoreline of Tonlé Sap Lake, or “The Great Lake,” a landscape in which the boundaries between land and water are constantly shifting.

Located in the center of Cambodia, Tonlé Sap Lake is part of a much larger lower Mekong watershed. Each year, the monsoon increases water levels in the Mekong by up to 15 meters. This increases discharge from six Olympic-sized swimming pools to 16. Per second. As all of that water flows downriver, past Laos, Thailand, Cambodia, and the delta in Vietnam, it backs up at the edge of the South China Sea. This causes flows in one tributary, Tonlé Sap River, to change course. Instead of flowing south, from Tonlé Sap Lake into the Mekong, the Tonlé Sap River begins to flow north, from the Mekong into Tonlé Sap Lake. Over the next several months, the lake expands to four times its normal surface area over the surrounding floodplain and under the bamboo stilts of Kampong Phhluk, carrying with it fish, people, and floating villages. Where children walked to school yesterday, they paddle in dugout canoes and even giant rice bowls today.

Fish catches skyrocket during this annual flood pulse. Markets at the water’s edge receive a steady stream of boat traffic starting at 5:00 a.m. as fishermen unload catches from a variety of ingenious traps and mazes deployed overnight and designed to funnel fish on the floodplain into nets. It is estimated that 60 million people in Cambodia and the surrounding countries rely on protein from the Tonlé Sap Lake fish catch each day. These markets provide a unique opportunity for scientists in SAFS’s Holtgrieve Ecosystem Ecology Lab to sample a variety of species and size classes of fish without ever leaving the shore.

The Holtgrieve lab looks at chemical signatures within fish muscle and bone to determine their role in local food webs, how they migrate, and where their carbon—which is the building block of all life—ultimately originates. Conventional wisdom says that most carbon in lake food webs originates from plants. However, scientists analyzing samples from Tonlé Sap Lake noticed that carbon in several fish species is low in one of two stable isotopes of carbon. This is an unequivocal sign that some of the carbon within fish comes from methane.

Fish trap in Tonlé Sap Lake

Methane is produced by bacteria in water-logged soils. As bacteria that thrive on oxygen metabolize carbon in soils, oxygen begins to run out. Eventually, a group of bacteria known as methanogens take over and slowly but steadily metabolize carbon into methane gas. As methane gas rises out of soils and into floodplain waters, it encounters yet another group of bacteria that metabolize methane, known as methanotrophs. Methanotrophs are consumed by protozoans, rotifers, and zooplankton in the water column, which are in turn consumed by fish. This makes methane a potentially important carbon source to the Tonlé Sap food web. And for a fishery that happens to expand with monsoonal flooding each year, when 9,000 square kilometers of floodplain soils become water-logged.

To determine how much carbon from methane becomes available to the fishery when waters rise in Tonlé Sap Lake, scientists in the Holtgrieve Lab are leaving the shore on long, wooden fishing boats with fellow researchers from the Cambodian Ministry of Agriculture’s Inland Fisheries Research and Development Institute in order to sample water and mud on the floodplain. Production of methane by methanogens and consumption by methanotrophs is being measured at different flood stages in hopes of associating these rates with the lake’s flood pulse. Alteration of this flood pulse and the amount of carbon from methane available to the fishery is imminent as the lower Mekong is dammed.

Until recently, the Mekong remained one of the world’s wildest rivers. But three dams have been built on the upper Mekong in China, with a fourth—Xayaburi—under construction along the Laos-Cambodia border. These hydropower development projects are undertaken with Chinese funding, and will primarily benefit power consumers in that country rather than the riverside nations of Laos and Cambodia. Such hydropower is projected to tightly control future flooding downstream. Modeling suggests that, under current scenarios of hydropower development, Tonlé Sap Lake will experience shorter periods of maximal flooding. If predictions by scientists in the Holtgrieve Lab are correct, this means that the amount of carbon from methane available to the Tonlé Sap fishery, and perhaps the region’s food security, will be threatened by the dams.

Hydropower is set to increase globally by 56% over the next 20 years, with the majority of new dams built in the tropical, developing countries of Southeast Asia, South America, and Africa. What happens in the lower Mekong, Tonlé Sap Lake, and communities like Kampong Phhluk therefore has far reaching implications. Many large, tropical rivers such as the Irrawaddy, Amazon, and Congo follow the same pattern of predictable, annual flooding long known to the scientific literature. In countries along these rivers, freshwater fish is a crucial, easily available protein source for the poor. If the flood pulse is an important ecosystem service that maintains fisheries, communities, and traditional cultures, this service should be evaluated against the hydropower boom occurring throughout the developing world today.

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