Zooplankton of Lake Orta after liming: an eleven years study

Abstract

Lake Orta (N. Italy) was severely polluted from 1927 by an effluent from a rayon factory, which discharged great amounts of ammonium nitrogen and copper into the lake. In the mid nineteen fifties, some plating factories also started dumping chromium and aluminum. As a result of ammonium oxidation, the lake became very acid and the concentration of metals in the waters reached very high values. Phytoplankton, zooplankton and fish disappeared suddenly from the lake which was by 1930 classified as “sterile”. Later on, about the fifties, a small population of Cyclops abyssorum re-colonised the lake together with some rotifers, in particular Hexarthra fennica. In mid eighties following the introduction of anti-pollution legislation, ammonium loads were greatly reduced and Daphnia obtusa was recorded. The lake waters however were still very acid, prompting the proposal of the Istituto Italiano di Idrobiologia to lime the lake with calcium carbonate to neutralise the excess acidity and reconstruct the alkaline reserve. This was done successfully from May 1989 to June 1990. pH values began to rise and in the same time the metal concentrations decreased, so that at present the lake waters are almost “normal”. In the meantime, due to the increased pH values, D. obtusa was replaced by D. longispina and, as toxic metal concentrations became lower, Megacyclops viridis, Bosmina longirostris, Diaphanosoma brachyurum, Keratella quadrata, Asplanchna priodonta. and other Brachionidae species appeared. Diaptomidae are still absent, except for some specimens of Arctodiaptomus wierzejskii

Evolution of the water chemistry of Lake Orta after liming

“…These values are indicative of a rather high production of algae, typical of mesotrophic lakes. In the case of Lake Orta, however, the lack of a pelagic fish population (Giussani 1990), and the peculiarity of a zooplankton population which is still small and poorly structured, are factors which, acting together, result in a less efficient control of algae (Ruggiu & Morabito 1990;Pizzolon et al 1992;Morabito & Curradi 1997;Bonacina & Pasteris 2001;Morabito et al 2001). The consumption of oxygen in the hypolimnion depends essentially on the mineralisation of the organic substance and on the oxidation of ammonium.…”

Lake Orta chemical status 25 years after liming: problems solved and emerging critical issues

“…4). This situation permitted a gradual biological recovery, with the reappearance of planktonic species, which have been excluded by the presence of toxic metals (Morabito et al, 2001;Bonacina and Pasteris, 2001).…”

Unexpected increases in rotifer resting egg abundances during the period of contamination of Lake Orta

“…between 1985 and 1990 zooplankton taxa number almost tripled (x 2.75), and 60% of the increase in richness was due to planktonic rotifers (Bonacina and Pasteris, 2001). …”