Lake Orta: the undermining of an ecosystem

Abstract

From 1927 to 1986 Lake Orta (North-western Italy) received loadings coming from a rayon factory, rich in copper and ammonium sulphate. Immediately after the onset of pollution, the food web was destroyed: first phytoplankton, then zooplankton, and finally fish and ultraplankton disappeared from the lake. Twenty years later a rise in nitrate nitrogen and copper ions was recorded and in the early sixties ammonium nitrogen started to accumulate and pH to fall. In the meanwhile, a poorly structured biological community appeared, mainly composed of Cyclops abyssorum (copepod), Hexarthra fennica (rotifer) and Coccomyxa minor (green alga). Fish and benthonic animals were still absent. In 1986 the ammonium loading was substantially reduced (copper had been partially recovered since 1956), so that the in-lake ammonium concentration began to decrease. Some phyto- and zooplankton as well as benthic species were recorded in this period. Nevertheless, as pH was still very acid and alkaline reserve absent: a proposal to lime the lake was accepted and funded by regional authorities. Liming was performed in May 1989-June 1990 and resulted in a real improvement of the environment.