Spatiotemporal shifts in macrophyte diversity and water quality of Nilnag Lake, Kashmir: impacts of dredging and anthropogenic pressures over four decades

Abstract

High-altitude forest lakes are crucial ecological and economic resources. This study examined the physicochemical characteristics of water and changes in aquatic macrophytes in Lake Nilnag, Kashmir, in response to dredging by the Department of Irrigation, Jammu and Kashmir, and anthropogenic pressures. Five sites were studied during 2019–2020 across three seasons (summer, autumn, and winter) in both the littoral and limnetic zones. Eleven macrophyte species were identified, comprising emergent (36%), rooted floating-leaved (36%), and submerged (27%) forms, with their distribution restricted to the littoral zones. Historical comparisons revealed the loss of five species and the gain of three emergents, indicating a shift toward emergent-dominated communities. Phytosociological analyses showed peak metrics in summer for rooted floating-leaved species (e.g. Potamogeton natans: IVI 47.66, density 3.55 plants/m²), with moderate diversity (Shannon H' overall 2.15; highest in autumn, at 2.18). Statistical analyses, including Pearson's correlations and PCA (explaining ~ 99% variance), revealed strong positive links between temperature and rooted floating-leaved species (r > 0.8, p < 0.01) and nutrients (especially phosphorus) with emergents (r up to 0.883, p < 0.01), driving eutrophication and accelerated succession. Nutrient concentrations averaged 201.5 ± 1.55 μg/L nitrate-N, 18.56 ± 0.76 μg/L ammoniacal-N, and 22.53 ± 3.24 μg/L. Dredging, agricultural runoff, and tourism-related eutrophication have significantly altered macrophyte composition and lake dynamics. Collaborative efforts among researchers, the Jammu and Kashmir Department of Irrigation, and non-governmental organizations are essential for the long-term conservation of this forest lake.