Indonesia: Tackling impact of invasive water weeds on a volcanic lake

Breathing New Life Into Dying Lake
Dwi Lusiana Jakarta Globe 18 Dec 11;

Lake Ranu Pane, East Java. With its mineral-rich, emerald green waters and tree-lined shores, the crater lake of Ranu Pane on the slope of Mount Semeru has always been an attraction for visitors and an important source of water for locals.

But now this geological marvel inside the Bromo Tengger Semeru National Park in Lumajang, East Java, is under grave threat.

In the past year, the water level in the lake has dropped drastically, reducing its surface area by more than 30 percent.

The water itself has become more turbid and no longer fit for human consumption, locals say.

“Before, the water here used to be clean, as clean as at Ranu Regulo,” said Bunoyo, a resident of Ranu Pane village, referring to another lake inside the national park.

“But in the past year it’s become dirty, all because of the weeds.”

Weeded out

The sinister weed in question is the Salvinia molesta aquatic fern, also known as kariba weed, an invasive species from South America whose explosive growth over the past year has seen it carpet much of the lake’s surface.

Because of its thick cover, in some places 20 centimeters deep, it starves the algae in the water of sunlight, which in turn prevents them from photosynthesizing and producing oxygen.

As a result, the water turns brackish and the lake’s native plant and animal life die out. The weed itself thrives on the rich mineral content of the volcanic water and on pollutants, and continues to spread and absorb more water.

Sukodono, a Ranu Pane villager, said that the quality of the lake water has deteriorated so much, that it is no longer fit for bathing or washing.

“When the lake’s all covered in weeds like this, what use is it to us,” he said, adding that villagers now have to pay to have clean water brought to their village by tankers.

The villagers are not the only ones suffering, though. Scientists say that whistling ducks, native to the volcanic lakes in the area, are no longer seen at Ranu Pane, whereas large populations of the birds still thrive at Ranu Regulo and Ranu Kumbolo.

Real culprit

Lukman Hakim, a biologist at Brawijaya University in Malang, East Java, says the culprit in all this is not the kariba weed itself, but the factor that allowed it to bloom in the first place: pollution of the lake by humans.

“The falling water level and the appearance of the weeds are signs that the lake is polluted,” he said.

“Most of it comes from the leaching of pesticides and fertilizers from local farming activities, but there’s also the problem of people dumping trash in the water and bathing and washing there.”

Lukman said his own research has highlighted high nitrate levels in water samples from Ranu Pane, pointing to human pollutants. What aggravates the problem, he said, is the farming system used there.

“They don’t use terracing, despite having to farm on an incline, so what happens is that during heavy rains, all the chemicals get flushed directly into the lake downhill, along with large amounts of soil,” he said.

“If this keeps up and the lake is not restored immediately, our fear is that Ranu Pane will dry out within 10 years.”

Five-year plan

Officials in charge of the national park have taken key steps in response to the threat.

Emi Endah, head of the park’s conservation and restoration unit, said officials are clearing away the weeds and mulching it for use as organic fertilizer, thereby addressing both the leaching of chemical pollutants and the growing weed cover.

They are also planting more trees by the shoreline and setting up mud traps to prevent loose soil from washing into the water.

Emi said the restoration process, which also involves the villagers, is expected to take five years to complete.

Sri Utama is one of the Ranu Pane villagers getting in on the act. She and her neighbor are growing 400 tree seedlings that they plan to later plant by the lake. A total of 10,000 trees are expected to be planted during the restoration process.

With help from the park officials and Brawijaya scientists, as well as increased awareness of how their own actions have threatened the very lake they depend on, the villagers are making sure the lake sticks around for several generations yet.