Population dynamics of Tubifex tubifex, first settler in the profundal of a copper and ammonia polluted, recovering lake (L. Orta, North Italy)

Abstract

Lake Orta is a large and deep warm monomictic lake in North Italy. Large scale chemical pollution began there in 1927, caused by copper and ammonia laden effluents from a rayon factory. All forms of life were immediately affected: in 1929 phytoplankton, Zooplankton and fish were virtually absent. The outstanding ammonia load and the consequent in-lake nitrification gradually caused nitrate and ammonia accumulation (ca. 5 mg N-NO₃/l and ca. 4 mg N-NH₄/l,respectively) and an extremely low pH (down to 3.8), which enhanced Cu toxicity.

In 1980–1982 the effluent from the factory was drastically reduced so that the ammonia loading is now less than 1% of the 1970 peak value (2.7 mg N-NH₄/1 year). The biological community has begun to recover and nowadays Tubifex tubifex makes up a new and sole macrobenthonic population in the profundal zone in all lake basins.

During 1984, at a fixed station (depth: 34 m) in the south basin, we took 12 replicate Ekman samples at monthly intervals. The sorted material was split into 5 compartments (eggs, embryos, young, mature and ovigerous individuals); a first attempt to analyse the T. tubifex population was made. The results show that the population displays a very clumped distribution (settling population), a high clutch size (eggs/cocoon) and a very remarkable mid-summer mortality.

An annual numerical balance for the 5 biological compartments shows that the “cost” of one ovigerous individual, in terms of mortality of the previous compartments, does not substantially differ from that estimated for other unpolluted lakes.