Limnological Characteristics Of Liuxihe Reservoir

Liuxihe, located at the Tropic of Cancer, is a typical large valley-type reservoir. Here, we summarize limnological features of this water-body based on observations that started in 2000. Two streams feed the reservoir. Mean water residence time is about 170 days but residence time in the wet season is only one-quarter of that in the dry season. Thermal stratification is monomictic; a short and full mixing occurs in winter in January, promoting nutrient regeneration from bottom to surface.The mean concentrations of total nitrogen (TN) and total phosphorus (TP) are low, about 0.6 and 0.02 mg/L, respectively. Average chlorophyll a concentration is 1.93 mg/m(3), and transparency (SD) is 2.9 m. These four variables indicate that the reservoir is oligo-mesotrophic. However, a tendency toward eutrophication occurred across the 10 years of observation. Mass ratio of TN/TP and dissolved inorganic nitrogen (DIN)/dissolved inorganic phosphorus (DIP) are 30:1 and 78:1, respectively; such high ratios mean that growth of phytoplankton is surely limited by phosphorus. The high N/P ratio is attributed to tropical red soil in which a high iron content tightly binds phosphorus, the well-vegetated watershed, and low human population activity. Nutrients as well as other water quality variables show considerable temporal and spatial variation. In the early phase of the flooding season (April to May), nutrients and chlorophyll a concentrations are markedly higher than in the other periods and decrease from the riverine to the lacustrine zone. Spatial variability of nutrients shows a clear longitudinal gradient.