Six years’ (1992–1997) evolution of phytoplankton communities after recovery by liming in Lake Orta, northern Italy
Lake Orta became heavily polluted in 1926 by ammonium sulphate and copper. After a couple of years the biological communities became extremely simplified and were composed of a few resistant species. This acute copper pollution was followed by a chronic acidification of the water, brought about by in-lake biochemical oxidation of ammonium. Since the 1960s, heavy metals discharged from some plating factories became an additional source of pollution. After the reduction of ammonium discharge in 1982 and from May 1989 to June 1990 the lake was limed: the pH at overturn increased from pH 4.4 to 5.9, the metals’ concentration decreased and the number of planktonic taxa rapidly increased.
The present paper deals with the recent (1992–1997) evolution of the phytoplankton communities analysed by sample clustering: the cluster analysis indicates the separation of two large groups of samples (1992–1993 and 1995–1997). Such clustering is because of the gradual decline of the chlorophytes (for a long period the most important group in Lake Orta) between 1992 and 1994, followed by their marked decrease between 1995 and 1997, during which they were replaced by blue–green algae.
Also noticeable was the increase in diatoms, which had disappeared from the phytoplankton community from the 1930s until 1994, but during spring and summer 1997 they amounted to 40% of community abundance. Although it is now possible to describe a phytoplanktonic seasonal succession in Lake Orta, the dissimilarities shown by the cluster analysis seem to indicate the existence of a certain year-to-year variability inside the dominant species assemblage, meaning that the phytoplankton community is still evolving.
In spite of the phytoplankton’s fast recovery after liming, it is only since 1994 that the algal community started to change toward a phytoplankton assemblage more typical of deep subalpine lakes.