ECOLOGY AND MORPHOLOGICAL VARIABILITY OF AULACOSEIRA-GRANULATA (BACILLARIOPHYCEAE) IN SPANISH RESERVOIRS

The ecology of Aulacoseira (formerly Melosira) granulata and its different forms in similar to 30 reservoirs scattered throughout Spain has been analysed. Nutrient concentration explained a significant proportion of the variation in cell characteristics. In general, there is a tendency to find large cells with less densely areolated valves in nutrient-rich waters. Ecological differences between the type and its variety angustissima mainly concern calcium concentrations. Aulacoseira granulata showed a unimodal response to the concentration of calcium, and preferred lower concentrations of calcium than the variety angustissima. The ecological differences between the alpha and gamma forms (statuses) of A.granulata were also the object of our investigation. The gamma status occurred in all the reservoirs studied in summer, but in only 3 of 28 reservoirs studies in the overturn period. Light and turbulence explained the ecological differences observed between the two forms. The gamma status showed a certain preference for less turbulent waters compared with the alpha status, which is a more cosmopolitan form in its requirements.