POPULATION AND INDIVIDUAL VARIABILITY OF ISOETES-LACUSTRIS L WITH DEPTH IN A PYRENEAN LAKE

Structural (density and biomass), morphological and phenological chara cteristics of a population of Isoetes lacustris L. in a high mountain lake in the Pyrenees were studied. Changes in these parameters along a depth gradient can be used to distinguish a shallow water population (0.3-1.3 m depth) from a deep water population (1.8-2.3 m depth). Dens ity of individuals decreased with depth, while biomass, size, size div ersity and fertility showed the opposite pattern. The effect of ice-sc our during winter together with a lower organic matter content in shal low areas are suggested as the main ecological factors responsible for these differences. Extremely high biomass values, when compared with North European populations of I. lacustris, seem to be related to a hi gher light availability in Pyrenean lakes. High rates of sedimentation causing burying of individuals and light depletion can explain the sh arp, shallow lower boundary of the distribution of I. lacustris in Lak e Baciver.