Hydrobiologia 207: 161-166, 1990. D. J. Bonin & H. L. Golterman (eds), Fluxes Between Trophic Levels and Through the Water-Sediment Interjhce. 0 1990 Kluwer Academic Publishers. Printed in Belgium. 161 Variation of nutrient stocks in the superficial sediments of Lake Geneva from 1978 to 1988 Daniel Span, Didier Arbouille, Helene Howa & Jean-Pierre Vernet Institute F.-A. Forel, 10 route de Suisse, CH-1290 Versoix, Switzerland Key words: nutrients, speciation of phosphorus, superficial sediments, Lake Geneva Abstract Two sediment sampling campaigns were conducted in 1978 and 1988 in Lake Geneva (Switzerland). Organic carbon, total nitrogen, total phosphorus and its various forms were analyzed. Results indicate a stability of organic carbon and nitrogen mass, and a significant increase of phosphorus. The variation of phosphorus mass is related to the increase of nonapatite inorganic phosphorus. This study attempts to quantify the phosphorus exchanges at the water sediment interface. The dissolved oxygen level in the bottom water determines the exchange direction. In aerobic conditions, sediments accumulate the excess of phosphorus, while in anaerobic conditions, they constitute an internal source. Introduction The eutrophication of lakes is caused largely by excessive loading of phosphorus into the lakes (Vollenweider et al., 1979). At the beginning of this century, Lake Geneva showed the typical characteristics of an oligo- trophic lake. By 1976, Lake Geneva was identi- tied as phosphorus-limited and classified as mesotrophic. The total phosphorus concentration in the lake water was then about 90 pg l- ’ but, in recent years (1986- 1987), that concentration has decreased to approximately 60 pg l- ’ (Blanc et al., 1986). A large fraction of this nutrient load is retained by the lake sediments. Since 1950, the vertical distribution of phosphorus and nitrogen concen- trations has increased exponentially towards the sediment surface (Ravera & Pelletier, 1980). Under appropriate circumstances, the sedi- mented phosphorus can be released as biologi- cally available phosphorus. Since 1957, CIPEL (Commission Internatio- nale pour la Protection des Eaux du Leman) has conducted systematic studies on the lake and its watershed. The aim of this contribution is to analyse the recent trends of nutrient loading in superficial sediments of Lake Geneva (Arbouille et al., 1989). Description of the site studied Lake Geneva is the largest of the alpine lakes with a surface area of about 582 km2, a volume of 89 km3 and a maximum depth of 310 m (Fig. 1). By differences in bathymetry, morphology and geochemical facies of superficial sediments, Lake Geneva can be divided into four distinct areas (Jaquet et al., 1982):