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

Abstract

Lake Orta, located in Piedmont, northwestern Italy, has been severely affected by industrial pollution since the 1930s. A successful liming intervention, performed in 1988-1990, returned

When and how? Freshwater mussel recolonization in Lake Orta

“…We could tentatively interpret the lower growth of the younger specimens with a reduction of the nutrients loads to the lake. The annual phosphorus load from the tributaries decreased from 6-10 t P yr -1 before 2004 to values close or below 2 t P yr -1 in very recent years (Rogora et al, 2016). Though admittedly speculative for the small number of individuals we sacrificed, this hypothesis is matched by the evidence that Unio elongatulus has higher growth rates and larger size in eutrophic than in oligotrophic water bodies (Ravera et al, 2003(Ravera et al, , 2007.…”

“…Though admittedly speculative for the small number of individuals we sacrificed, this hypothesis is matched by the evidence that Unio elongatulus has higher growth rates and larger size in eutrophic than in oligotrophic water bodies (Ravera et al, 2003(Ravera et al, , 2007. Lake Orta can be classified as oligotrophic or ultra-oligotrophic, with very low total phosphorus concentrations (Rogora et al, 2016). Under severe nutrient limitation even a single discharge of organic matter can modify the trophic conditions at local scale, and particularly in littoral shallow waters.…”

“…Indeed, these effects have been previously described for mussels from acidic waters (Preston et al, 2010). The increase in the epilimnetic pH to alkaline values during summer (Rogora et al, 2016) likely enabled mussels to grow, while pH lowering at winter mixing caused shell corrosion.…”

“…2)]: acid-base status of Lake Orta had completely recovered, ammonium levels had lowered, falling in the range of those observed in the other subalpine oligotrophic lakes, while nitrate was still >1 mg L -1 . Present concentrations of Fe, Mn, Al and Cu in the water column are as low as those of the nearby Lakes Maggiore and Mergozzo (Rogora et al 2016) while, Ni and Cr, although at low levels, still might pose a threat for biota (Piscia et al, 2016). No data is available on sediment chemistry after 1996 ) when the area colonized was highly enriched in Cu (~2000 mg kg -1 ), Cr (~1000 mg kg -1 ), Pb (~500 mg kg -1 ) and Mn (~1500 mg kg -1 ) with pore water concentrations of Cu and Cr respectively of 0.63 and 0.10 (Guilizzoni et al 2001) the layer deposited between 1996 and 2001 (about 1 cm) was probably not sufficient to isolate organisms from the toxic layers underneath.…”

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

“…In 1999, the concentration of copper was close to zero in the whole water mass (Calderoni and Tartari, 2001). The lake has had neutral pH and low concentrations of toxic metal since that time (Rogora et al, 2016).…”