Phumdis are a series of floating islands, exclusive to the Loktak Lake in Manipur state, in northeastern India. They cover a substantial part of the lake area and are heterogeneous masses of vegetation, soil and organic matter, in different stages of decay. The largest single mass of phumdi is in the southeastern part of the lake, covering an area of 40 sq km (15.4 sq mi). This mass constitutes the World’s largest floating park, named Keibul Lamjao National Park. The park was formed to preserve the endangered Eld's Deer subspecies, called Sangai in the Manipuri language, indigenous to this area.
Phumdis are used by the local people for constructing their huts for fishing and other livelihood uses, and are inhabited by about 4000 people. Athapums are artificial circular phumdis, built by the villagers as enclosures for fish farming; aquaculture has caused proliferation of the phumdis in the lake.
Although phumdi vegetation has existed for centuries, it was not until 1886 that the Manipur Gazetteer recorded that wetlands with floating islands were used by inhabitants for fishing. Before the Itahi barrage was constructed in 1986, 207 khangpoks (huts or sheds) were reported on the phumdis, but after the dam was completed in 1999, the Loktak Development Authority (LDA) reported 800 such structures. Many of the huts are reported to have been converted into permanent dwellings and about 4,000 people live in these floating huts, earning their living as fishermen. The huts are constructed using plastic ropes, heavy rocks, wood, bamboo, zinc plates and iron rods. Athapums, artificial circular phumdis, which were built by the villagers as enclosures for fish farming, are present on the lake, and this aquaculture has caused further proliferation of the phumdis. A tourist lodge has been built on one of the phumdis in Sandra Island.
The floating mass of matted vegetation, organic debris, and soil that constitutes a phumdi has a thickness that varies from a few centimetres to two metres. Its humus is black in colour and porous, with a spongy texture. Only 20% of a phumdi's thickness floats above the water surface; the other 80% remains submerged. Before the construction of the Loktak Hydroelectric Project, the park area containing phumdis was merely marshy land, but since the commissioning of the project, two ecosystems have emerged. One, the body of open water, covers one-third of the area and the other, the phumdi, covers the remaining two-thirds.
The life-cycle of the phumdis has generally been subject to seasonal variation. During the monsoon season when the water level is high, the phumdis float, but during the dry season, as the water level falls, the phumdis touch the lake bed and absorb nutrients from it. When the wet season returns, they again float, and the biomass, which has enough nutrients stored in the plants' roots, survives. However, the contemporary situation, with high water levels in the lake throughout the year, has meant that the process of 'feeding' on lake–bottom nutrients has been seriously disturbed, resulting in a loss of biomass and a thinning of the islands each year. In January 1999, it was reported that a large section of phumdi in the north of the park had shattered into pieces and drifted away from the park area, theatening the habitat of the Sangai.
The largest of all the phumdis in the lake is situated in the southeastern region of the Loktak Lake, which forms the Keibul Lamjao National Park. This park is the last natural refuge of the endangered Manipur brow-antlered deer (Cervus eldi eldi), locally known as the Sangai, one of the three sub species of the eldi's deer listed as an endangered species by the International Union for Conservation of Nature. Apart from the brow-antlered deer, which is the flagship species of the park, other fauna found in the park are mammals, reptiles, and migratory and resident avifauna species.
Mammal species consist of Hog deer (C. porcinus), Wild boar (Sus scrota), Large Indian Civets (Viverra civetta, Viverricula indica), common otter (Lutra lutra), fox, jungle cat, Asian Golden Cat, Bay bamboo rat, musk shrew, common shrew, flying fox, and Sambar (Cervus unicolor). Reptile species found in the park are the Keel back Tortoise, Viper, Krait, Cobra, Water cobra, krait (banded) Bungarus fasciatus, Asian rat snake (Beauty rat snake), Russels’ viper (Daboia), checkered garter snake, Python and Common Lizard (Viviparous Lizard). Python molurus is an endangered species found in the park. Prominent bird species seen in the park are both migratory and resident avifauna species. Some of the avifauna are the East Himalayan pied Kingfisher, Black kite, lesser sky-lark, Northern Hill Myna, 5) Burmese Pied Myna, North Indian black Drongos, Lesser eastern jungle crow, Yellow headed wagtail, Spotbill duck, Blue-winged teal, Ruddy Shell duck, Threatened Hooded crane, Burmese sarus Sarus Crane, Indian White-breasted Waterhen and Crimson-breasted pied wood pecker.
The proliferation of phumdis, coupled with severe infestation of the lake by water hyacinth, has substantially impeded water circulation and caused an increase in siltation and deposit of pollutants in the lake ecosystem. The building materials used to build huts on the phumdi blocks sunlight from reaching the lower depths of the lake water, which has resulted in formation of vertical profiles of the lake water body and decomposition. Further, pesticides and insecticides are used for catching fish or as insect repellent. Degradation is in the form of benthal, which, as it decays, releases toxic gases such as methane, hydrogen sulfide, and reduces dissolved oxygen (DO). This causes the lake water to degenerate into a eutrophic condition, creating a dead water zone called the hypolimnion. Above the hypolimnion is a thin layer, known as epilimnion, where fish survive to some degree. The benthal is becoming increasingly thick, causing not only pollution of the lake water, but an increase in the shallow part of the lake.
A detailed study has been conducted by the Loktak Development Authority (LDA) in collaboration with Wetlands International-South Asia, supported by the India-Canada Environment Facility, implementing a project on Sustainable Development and Water Resources Management of the Loktak Lake. The project addresses the issues relating to water management, sustainable fisheries development, community participation and development, catchment area treatment and conservation of wildlife. The Planning Commission of the Government of India, decided in September 2008 that these policies would be implemented over a period of 5-6 years at an estimated cost of over Rs500 crores (US$100 million), and extended the area under management to also incorporate the water of Nambul and other rivers and their tributaries, which are primarily responsible for polluting the Loktak Lake.