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

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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

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Cohorts of Branchiura sowerbyi were reared at different temperatures and initial population densities in order to obtain data suitable for the interpretation of population dynamics in field populations. Percent hatching from cocoons reaches its maximum at 25 "C and decreases towards lower and higher temperatures. Embryonic development time, T,, was measured and the relative threshold temperature, 10 "C, calculated by extrapolation. The degree day requirement for embryo development is 195 "C d. The time of first cocoon laying (T,,) was observed and the ratio T,/T,, was seen to fit with that of other tubificid species cultured so far. Embryo mortality is rather high, while worm mortality is low or very low. Fecundity increases from 15 to 20 "C but decreases at 25 "C. A mathematical model for the simdation of population densities with four size-stage compartments is suggested. It could be used for the optimization of worm uptake (simulated as stage specific mortality) in mass cultures reared for the production of Branchiura, to be used as food for fish fingerlings.