A. Lami & G. Giussani (Guest Editors) Limnology of high altitude lakes in the Mt Everest Region (Nepal) Mem. Ist. ital. Idrobiol.: 57: 99-106 Organic carbon and microorganisms in two Nepalese lakes Roberto BERTONI, Cristiana CALLIERI and Mario CONTESINI CNR Istituto Italiano di Idrobiologia, L.go V. Tonolli, 50, 28922 Verbania Pallanza, Italy ABSTRACT In Lake Piramide Superiore (LPS) and Inferiore (LPI), situated in the Khumbu Valley (Nepalese Himalayas), organic particulate carbon (POC) forms a considerable part of the total seston (33% and 21% in LPI and LPS), never exceeding a concentration of 200 µgC l -1 . LPS has a higher seston concentration, of which 58% is made up of inorganic matter, proba- bly because it is closer to the glacier. Dissolved organic carbon (DOC) values are also very low, around 0.5 mg l -1 . The chlorophyll-a concentration is always lower than 1 µg l -1 and its percentage in the 0.2-1 µm size class represents 54% and 33% of the total phytoplankton chlorophyll in LPI and LPS. Autotrophic picoplankton was present in both Piramide Lakes, though in very low numbers, due to photoinhibition (underwater surface irradiance: 1200- 1400 µE m -2 s -1 ). The autotrophic cells are Synechococcus-type with phycoerythrin as acces- sory pigment. The heterotrophic cells exceed the autotrophic ones by three orders of magni- tude, suggesting a heterotrophy oriented food-web. The quality of the underwater light is characteristic of a clear high mountain lake. Key words: POC, DOC, picoplankton 1. INTRODUCTION In 1993 and 1994, as part of the Progetto Strategico "Ev-K2 CNR", a new line of research was planned in the two lakes of the Pyramid Laboratory: Lake Piramide Superiore and Inferiore (Khumbu Valley, Nepalese Himalayas) to investigate the pa- rameters related to the organic carbon cycle in the water column. The ultraoligotrophic conditions of both lakes (Tartari G.A. et al. 1998, this vol- ume), their clear water and their location in a remote area lead to the hypothesis that they have low concentrations of organic carbon and chlorophyll. It is likely that the microbial loop becomes more important in such nutrient recycling systems to im- prove the efficiency in exploiting the scarce organic substrate. The microbial loop is a component of the microbial food web and may be defined as the complete set of compartments through which energy and matter flow from dissolved organic matter to the higher trophic levels (zooplankton - fish) (Sherr & Sherr 1988; Stockner & Porter 1988). The non-living organic matter coming from phytoplankton exudation or leakage and degradation is taken up by bacteria which are then grazed by micro- flagellates and ciliates. This concept, recently reappraised by Azam et al. (1983), underlines the role of dissolved organic matter as a heap compartment of matter and energy in the microbial loop. To assess this, we measured the organic carbon con-