The effect of increased water level on Isoetes lacustris L. in Lake Baciver, Spain

Ten months after flooding lake Baciver, water levels were 5.5 m higher than the previous normal and the area formally occupied by the Isoetes population was reduced. All Isoetes originally present below 0.6 m died, and only plants standing between 5.8 and 6.1 m (new depth) survived. As irradiance decreased, remaining individuals produced fewer leaves, but their biomass production was higher, as leaf length increased in response to the diminished available irradiance. Because leaf mortality was excessive, regeneration capacity, population performance and dynamics were adversely affected. In 1992, no new production occurred in Isoetes lacustris individuals surviving after the winter. We suggest that low surface oxygen levels at the water column under the ice, and total anoxia below 6 m depth, caused the demise of the I. lacustris population in Lake Baciver.