Seasonal variations in the water quality of a tropical wetland dominated by floating meadows and its implication for conservation of Ramsar wetlands
Introduction
An array of ecosystem services provided by wetlands such as ground water recharge, maintenance of water quality, sinks for nutrient and carbon sequestration is well established (Verhoeven et al., 1996, Wetzel, 2001, Bassi et al., 2014). These wetlands are increasingly under threat due to various human induced factors such as agricultural encroachments and drainage, overexploitation of resources, poaching, pollution and linear developments. The discharge of untreated waste into wetlands and agricultural runoffs adversely affect the plant and animal communities in it by enriching the organic content, leading to eutrophication (Beeby, 1995, Marsden et al., 1997, Sukumaran, 2002). In recent years, wetlands across the globe have received scientific as well as political attention (Kimmel et al., 2010) as most of them are under threat due to anthropogenic pressure (Turner and Rabalais, 1991, Marsden et al., 1997, Carpenter et al., 1998). A better understanding of the physical, chemical and biological characteristics of wetlands and their impacts is necessary for the successful conservation of these important aquatic systems (Carpenter et al., 1998).
The Loktak Lake is located in the Barak - Chindwin River basin in Manipur, India. The Lake is characterized by floating meadows of various thickness that form the last remaining habitat of the endangered Eld's deer (Rucervus eldii) locally known as Sangai (Hussain et al., 2006). The floating meadows are formed due to the accumulation of biomass and organic debris at various stages of decomposition, which are colonized by primary colonizers like Salvinia cucullata, Salvinia molesta, Azolla pinnata, Cyperus spp. The formation of meadows is enhanced due to the extent of nutrient (such as nitrogen and phosphorous) available in the lake. Approximately 15% increase in meadows area was observed between 1989 and 2002 (Singh and Khundrakpam, 2009) which affected the lake surface area reducing the ecosystem services value of the Lake. The southern part of the Lake is occupied to a large extent by thick floating meadows, which is protected as Keibul Lamjao National Park (KLNP) for the conservation of this small and isolated population of Sangai (Tuboi and Hussain, 2016).
The Loktak received national and international attention when it was declared as a Wetland of International Importance (Hussain et al., 2006). At present, Loktak is experiencing severe degradation due to anthropogenic pressures. In 1983, a barrage constructed in the downstream of Manipur River, the Ithai barrage, altered its hydrologic regime. The Lake was a seasonally flooded wetland with several small wetlands comprising the whole Lake which used to be separated during low water levels and merge into one during monsoon. However, after the construction of the barrage, the water level in the Lake is maintained at 769.12 m msl so as to support the hydro-power project. Therefore, the natural regime of the Lake is altered putting the Lake under stress due to permanent flooding. This permanent increase in water level inhibits the floating meadows from settling down on the ground to get replenished with nutrients and soil essential to maintain its thickness. This has led to the thinning of the meadows (Tuboi and Hussain, 2016). The thin meadows are not able to support the weight of the Sangai threatening its survival.
The increased inflow of nutrients and change in the hydrology of the lake due to the construction of Ithai barrage is a matter of concern as it affects the survival of endangered Eld's deer (Rucervus eldii) and hog deer (Axis porcinus) occurring in the meadows. To maintain the thickness of the floating meadows, it is important to allow major portion of the meadows to settle during lean seasons by reducing water level of the Lake and minimize burning, walking, trampling, resource extraction and grazing on the meadows. Fire as a management practice leads to loss of base material for the formation of meadows. Hence, cutting of the vegetation and leaving it as such is recommended so as to maintain the thickness of the meadows (Tuboi and Hussain, 2016). The population of the two cervids viz the Eld's deer and the hog deer is less than 100 breeding individuals, hence they exert negligible grazing pressure on the meadows. Although more than 80% overlap was observed between the diets of Eld's deer and hog deer, there appears to be little competition among them for forage due to an abundant availability of high quality forage across all seasons (Tuboi and Hussain, 2016).
Moreover, due to deforestation, nutrients from its catchment, pesticides from agricultural fields and domestic sewage are also being discharged into the Lake. Encroachments through the construction of fishponds, roads and settlements have gradually degraded the Lake ecosystem (Trisal and Manihar, 2004). Due to these changes, the Lake was included in the Montreux Record in 1993. The Montreux Record is a register of wetland sites on the List of Wetlands of International Importance where changes in ecological character have occurred, are occurring, or are likely to occur as a result of technological developments, pollution or other human interference. It is maintained as part of the Ramsar List. It is with this background that the present study was undertaken to assess the water quality of the Loktak Lake and KLNP and its implication for the long term conservation of the single isolated population of Eld's deer. The primary objective of this study was to examine the seasonal variation in physicochemical characteristics of the Lake water from 11 sampling sites and to investigate the suitable monitoring strategy to contribute to current and future management of the Lake for restoration.
Section snippets
Study area
The Loktak is the largest natural freshwater lake in northeastern India located in the State of Manipur. Apart from being a Ramsar Site, the Lake is one of the 115 wetlands identified under the National Wetland Conservation Programme in India. National Wetland Conservation Programme in India was initiated in 1985 for the conservation and wise use of wetlands so as to prevent further degradation of wetlands. It lies between 93°46′E and 93°55′E longitude and 24°25′N and 24°42′N latitude (Fig. 1).
Physicochemical features of the lake water
The ranges of the physico-chemical parameters varied within and among the seasons (Table 2, Table 3). Among the water quality parameters examined, ten parameters varied significantly between the Lake and the Park (Table 2). Temporal CA rendered a dendrogram, where the twelve sampling months were grouped into three statistically significant clusters (Fig. 2). The first group constitute the summer months (February–April), second the monsoon months (May–September) and third the winter months
Discussion
The Loktak Lake lies in the Indo-Burma biodiversity hotspot. Due to its importance in the socio-economic and cultural life of the local people, it is considered as “the lifeline” for the lakeshore communities. In recent years, the Loktak Lake is facing both natural and cultural eutrophication due to anthropogenic pressure and land use change. As the local communities are highly dependent on the Lake and its resources for their livelihood and other domestic uses, the degeneration of the water
Conclusions
The problems of the Loktak ecosystem can be traced to the loss of vegetation cover in the catchment and construction of the Ithai Barrage in the southern part of the Lake. The data available from earlier studies suggest that the vegetation composition of the floating meadows is changing (Tuboi et al., 2015). Only 185 plant species were recorded by Tuboi et al. (2015) as compared to 223 reported by Singh (1991). The natural properties of the Loktak Lake are changing due to accumulation of
Acknowledgements
This study was conducted under the project “Conservation Ecology of Eld's deer and its wetland habitat” sponsored by the Wildlife Institute of India. We are grateful to the Department of Forests, Government of Manipur for granting us logistic support. We thank the Director and the Dean at the Wildlife Institute of India for their support. We thank Dr(s). Ruchi Badola, Sangeeta Angom, Niladri Dasgupta and Shivani Barthwal for all the discussions and critical inputs at different stages of the
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