Oecologia aquatica, 11: 55-66, 1998 55 Effects of building up a dam in a shallow mountain lake (Baciver, Central Pyrenees) E. GACIA' & E. BALLESTEROS1,2 ' Centre d'Estudis Avancats de Blanes (CSIC). C. Santa Bárbara s/n. 17300 Blanes. Girona. Spain. Centre de Recerca d'Alta Muntanya. C.N. 230, km 152. 25555 Vielha. Spain. Received: June 1994 Accepted: February 1995 SUMMARY Changes in the water, the sediment, and the structure and dynamics of the submerged macrophyte populations were studied after the building up of a dam in a high mountain Pyrenean lake in late summer 1990. The most outstanding variations in the physical and chemical features were: (a) a 5.5 m rise of the shore level, (b) a decrease in light transmittance into the water column, (c) an oxygen depletion in deep waters, coupled to a pH decrease and a rise in conductivity and cation concentration, (d) an increase in the total dissolved nitrogen (mainly ammonium) and phosphorus concentration in the water column, and (e) an increase in the reduced N-compounds (nitrite and ammonium) in the sediment pore-water of the Isoetes lacustris community, coupled to a decrease in nitrate. Changes in the chemical features seem to be related to the decomposition of organic matter provided by the flooded terrestrial subalpine meadows and shrubs, and the lake's submersed vegetation. Both the area occupied by the submersed vegetation and the number of macrophyte species suffered an important reduction with the daming of Lake Baciver. All the small, non dominant species (Eleocharis acicularis, Subularia aquatica and Isoetes setacea) dissappeared along the summer 1991. Sparganium angustifolium experienced a degradation (low density of shoots, short leaves) in 1991 which lead to its complete die off in spring 1992. The strong attenuation of the light regime could account for these losses. Isoetes lacustris individuals originally living below 0.6 m depth were completely dead in summer 1991, probably due to an oxygen depletion in winter time. The only surviving Isoetes plants (those originally living abo y e 0.6 m depth which were between 5.8 and 6.1 m depth after the dam) increased its leaf length as a response to the decrease in available irradiance. Although the number of produced leaves was slightly lower in 1991 than it was before the daming, the production of biomass increased, since leaves were longer. However, the lost of leaves was very high, resulting in a global decay of the population. In late spring 1992 surviving Isoetes lacustris had the remaining leaves very damaged, probably because of the winter anoxy, and no leaves were produced in summer 1992. This brought a complete dissapearence of the community in the lake. KEY WORDS: high mountain lakes, reservoirs, benthos, environmental impact. RESUMEN Efectos del represamiento de un lago somero de montaña (Baciver, Pirineos centrales). Se estudiaron los cam- bios en el agua, el sedimento y la estructura y dinámica de las poblaciones de macrófitos sumergidos después del represamiento de un lago de alta montaña de los Pirineos a finales del verano de 1990. Las variaciones más notables de las características físicas y químicas fueron: a) elevación de 5,5 m del nivel del litoral; b) disminución de la transmitancia de la luz en la columna de agua; c) depleción de oxígeno en las aguas profundas, junto a la reducción del pH y al aumento de la conductividad y de la concentración de cationes; d) aumento de la concentración de nitrógeno disuelto total (principalmente amonio) y de fósforo en la columna de agua, y e) aumento de los compuestos de N reducidos (nitrito y amonio) en el agua intersticial de la comunidad de Isoetes lacustris, ligada a una disminución del nitrato. Los cambios en las características químicas parecen estar relacionados con la descomposición de la materia orgánica aportada por los prados y matorrales subalpinos terrestres, así como por la vegetación sumergida del lago. Tanto el área ocupada por la vegetación sumergida como el número de especies de macrófitos experimentaron una importante reducción con el represamiento del lago Baciver. Todas las especies pequeñas, no dominantes (Eleocharis