Effects of building up a dam in a shallow mountain lake (Baciver, Central Pyrenees)

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 ye 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.