Study of primary production magnitude under the thermocline (20-25 m) and the continuous supporting mechanism of production in Lake Pumayum Co, Tibetan Plateau

To study the primary production magnitude under the thermocline (20-25 m) and the continuous supporting mechanism of the production in Lake Pumayum Co (a high alpine lake) located on the southern Tibetan Plateau (altitude 5,030 m), sediment traps were deployed at 50.5 m water depth in the eastern part of the lake. Based on source analysis of sinking particles trapped at depths 27 and 45 m, the following results are suggested: 1) The average production rate (0.63 mg-C m^-3 d^-1) of sinking phytoplankton-derived organic carbon in the 27-45 m -3 -1 water layer was at least four times higher than that (0.15 mg-C m^-3 d^-1) in the upper water layer (0-27 m). The ratio of increase (4.2) was equivalent to that (4.0) of the theoretical primary production between 13.5 and 36 m (midpoints of the above layers) (13.5 m: 0.042 mg-O₂ l^- h, 36 m: 0.17 mg-O₂ l^- h; Murakami et al., 2007). Thus, the subsurface primary production maximum appears to have formed in the 27-45 m water layer of the lake. Moreover, although this high alpine lake is generally classified as oligotrophic and is located well above the forest line, primary production in the subsurface layer probably continued to exceed the mesotrophic magnitude. 2) Because of the transport and supply mechanisms of nutrients supporting such high production under the thermocline, there can be deep intrusion of river water (5°C) colder than the lake epilimnion (10-13°C). This possibility is supported by the high average abundances of silt and clay, sinking terrestrial plant-derived organic carbon, and sinking soil-derived organic carbon in the 27-45 m water layer, as compared with the 0-27 m layer. The supply of nutrients in the 27-45 m water layer is believed to be several times higher than that of the 0-27 m layer. The supply of nitrogen nutrients by diffusion from pore water in bottom sediments is also considered one of the mechanisms.