The Trophic status of Bidighinzu Reservoir (Sardinia) before and after the diversion of waste waters

The purpose of this paper was to assess the effect of the diversion of wastewater on the trophic status of Lake Bidighinzu, a hypertrophic
man-made lake in Northern Sardinia, used as a drinking water reservoir. There have been problems with potabilization
since the early years of the diversion operation, particularly in the summer-autumn period. Data available (August 1978, February
1979 and March 1985) before the reservoir (1987) were compared with those collected during a study carried out in the annual cycle
immediately after (1988-1989) and after some years (1994 and 1996-1997). The study examined the dynamics of temperature,
main nutrients (total phosphorus, nitrate and ammonia), chlorophyll-a and phytoplanktonic biomass. No particular variation in the
water nutrient availability emerged from the comparison between these two situations - especially for total phosphorus, whose annual
mean concentrations were similar in the two annual cycles (386 mg P m-3in 1988-1989 and 305 mg P m-3in 1996-1997). Chlorophyll-
a and biomass were high during each period of study (annual mean values were 17 mg m-3and 3.7 mg l-1in 1988-1989 and
11 mg m-3and 4.6 mg l-1in 1996-1997). However, peaks were never higher than values recorded in August 1978 (112 mg m-3and
133 mg l-1) due to an extraordinary bloom ofCeratium hirundinella(O.F. Müller). Species composition of phytoplankton was typical
of highly trophic conditions and was frequently characterised by the presence of Cyanophyceae and Bacillariophyceae. Results demonstrated
that, ten years after construction of the by-pass, the lake had shown no improvement in its trophic status. This might depend on many factors, among which the most likely might be the non-use or partial use of the diversion system, which allows the
sewage to continue to flow into the lake and keep its trophic status almost unchanged.