Lake Orta: a history of success, a challenge for the future

The deep subalpine Lake Orta was heavily and chronically polluted by industrial copper and ammonium sulphates discharges beginning in 1926. By 1930, the limnological pioneer, Rina Monti, declared that the Cu pollution alone led to a collapse of the planktonic food web precipitating a loss of most life in the lake, including fish. Meanwhile, bacterial oxydation of the ammonium depleted the lake's limited alkalinity, and the entire lake acidified, with pH levels of 3.9 to 4.2 lasting into the 1980s, despite dramatic reductions in external ammonia supply. Further, given this acidity, concentrations of many metals (Cu, Cr, Ni, Zn, Al) remained elevated and in their more toxic, ionic forms. Models produced at that time by the Italian CNR Institute of Hydrobiology indicated unassisted water quality recovery would take 20 years. To accelerate recovery, the lake was limed in the summer of 1989, and water quality improvements were rapid, matching predictions. Some anticipated biotic recovery quickly followed. First came the plankton. With their rapid dispersal, developmental and reproductive rates, the pelagic zone was soon populated with a progressively enriched and diversified flora and fauna, but hysteresis was evident, with the new communities being quite dissimilar from the original ones. Given that the most recent lake surveys indicate the communities are still in flux, ongoing monitoring and diagnostic research are both still warranted, and may indicate new interventions are warranted. In addition, new requirements under the European Water Framework Directive necessitate the consideration of fish, benthos and macrophytes, and these communities are more closely tied to the metal-contaminated sediments than are the plankton. They thus may well still be suffering from the pollution legacy in the lake. Hence, a broadened and new assessment of the Lake Orta ecosystem is clearly warranted.