Today’s image is the answer to Earth Observatory’s September Puzzler.
Popular among water gardeners for its showy flowers and glossy leaves, water hyacinth (Eichhornia crassipes) is one of the fastest-spreading plants in the world. As a result, the floating flower—which is native to the Amazon but now thrives on every continent except Antarctica and Europe—has become one of the most widely reviled, especially in Africa.
People living along Lake Victoria, the world’s second largest lake, have a particular loathing for water hyacinth. Since the plant became established in the 1990s, occasional outbreaks have caused serious problems for communities bordering the lake, particularly those along Winam Gulf, a shallow inlet in Kenya.
“This plant has at various times covered so much of the lake, especially in Winam Gulf, that it completely blocked out local fishing, clogged water supplies, and harbored pathogens harmful to local people and animals,” explained University of Nevada–Reno conservation biologist Thomas Albright. “At times, it has been an economic calamity at local and even regional levels.”
In 1997, an outbreak along the eastern and southern shores of Winam Gulf, carpeted 172 square kilometers (66 square miles) of water with hyacinth. In 2006-2007, heavy rains and nutrient-rich runoff fueled an even more extreme outbreak. The plant-covered area increased from about 40 square kilometers in March 2007 to more than 400 square kilometers just a month later—about one-third of Winam Gulf.
Aggressive control efforts—including removal by hand or with harvesters on boats, as well as the release of weevils that eat the plant—followed both outbreaks. Ecologists think the weevils can help keep the plant in check, but several other factors affect water hyacinth’s abundance as well. “In the 2000s, we saw big reductions in water hyacinth coverage that we attributed in large part to the effects of an El Niño year with winds, water levels, and wave action possibly uprooting the plants,” Albright said. “The declines we saw in some parts of the lake pre-dated the release of weevils.”
Though its numbers are down, water hyacinth has hardly been eradicated in Winam Gulf. The Advanced Land Imager (ALI) on the Earth Observing 1 satellite captured this image showing mats of water hyacinths on May 17, 2014, in Osodo Bay, off the Sondu-Miriu Delta. An analysis of nearly a decade of satellite imagery found that this area was one of the most common places for the plant to grow.
In addition to water hyacinth, blue-green algae and other opportunistic aquatic vegetation—water lettuce, papyrus, and tussock grasses—likely contribute to the green in the image. The water is likely brown with sediment and runoff from recent rains.
NASA Earth Observatory image by Jesse Allen, using EO-1 ALI data provided courtesy of the NASA EO-1 team. Caption by Adam Voiland, with information from Thomas Albright (University of Nevada, Reno) and Thomas Moorhouse (Egerton Unviversity).