CHEMISTRY AND TROPHIC EVOLUTION OF LAKE LUGANO IN RELATION TO NUTRIENT BUDGET

Chemical studies performed in the last 50 years on the northern and southern basins of Lago di Lugano are reviewed and discussed in relation to the phosphorus and nitrogen loads from the drainage area. Both basins became eutrophic in the fifties as a consequence of the dense population and industrial activity in the drainage area; the effect of these was aggravated by the meromictic condition of the layer below 100 m in the northern basin. The reduction of phosphorus loads in progress since 1976 has resulted in a significant improvement in the chemical and biological condition of the southern basin and the upper 100 m of water in the northern basin. On the other hand, the balance between dissolved oxygen and reducing substances in the northern basin is shifting toward a decrease of the net oxygen concentration, partly as a consequence of the unusually dry conditions of the last three years. A reduction in phosphorus load down to 18 and 22 t a-1 in the northern and southern basins respectively is still necessary in order to reduce the lake phosphorus concentration to 30-35 mug O l-1.