Benthos of Lake Orta in the year 1996

Abstract

Lake Orta has been interested since 1926 by severe industrial pollution, due to the effluents of a rayon (artificial silk) factory that discharged huge amounts of copper and ammonia. As a consequence, the benthic fauna faced noticeable changes, becoming extremely poor in species. However, after extensive liming (in 1989-90), the biotic communities of the lake showed many signs of recovery, and in 1996 it was decided to study the benthos composition and abundance in detail, to assess whether the lake restoration resulted in a complete recovery of this biotic compartment. Actually, both the small number of taxa, and the low abundance of the benthos, indicate that the sedimentary environment is still very stressed. In fact, albeit this study shows that benthos fauna is actively recolonising Lake Orta bottom, spreading from river mouths, in 1996 the benthic community cannot be considered fully organised

When and how? Freshwater mussel recolonization in Lake Orta

“…Indeed, in a very small scale the water quality could have been maintained by inflow of underground or surface water. However, data on the composition and abundance of benthos in 1996 do not seem to indicate that the conditions for survival were better in the area currently colonized than in the rest of the littoral zone (Baudo et al, 2001a). In spite of the adaptation of Lake Orta benthos to high metal concentrations (Baudo et al, 2001a), no freshwater mussels were found in the area that is now colonized neither in 1993-1994 (Nocen- (Baudo et al, 2001a) and 1998 (Bielli and Tesauro, 2001).…”

“…However, data on the composition and abundance of benthos in 1996 do not seem to indicate that the conditions for survival were better in the area currently colonized than in the rest of the littoral zone (Baudo et al, 2001a). In spite of the adaptation of Lake Orta benthos to high metal concentrations (Baudo et al, 2001a), no freshwater mussels were found in the area that is now colonized neither in 1993-1994 (Nocen- (Baudo et al, 2001a) and 1998 (Bielli and Tesauro, 2001). Therefore, the most likely hypothesis, also supported by genetic analysis, is a recolonization from a broodstock in a neighboring population, most probably that of Lake Maggiore or Lake Mergozzo, which are the closest sites with genetically similar populations.…”

“…Several toxicity tests ran in the years following liming (1992)(1993)(1994)(1995)(1996)(1997)(1998) confirmed that the sediments were contaminated enough to induce either a direct short term mortality or sublethal effects on crustacean species (Rossi and Beltrami, 1998;Beltrami et al, 1999;Barbero et al, 2001;Baudo et al, 2001b;Burton et al, 2001). This likely hindered the re-colonization by most benthic taxa resulting in a benthic community that was both poorly structured and scanty and with a small number of taxa, and whole groups of organisms, such as Mollusca, missing (Tesauro et al, 1995;Baudo et al, 2001a;Nocentini et al, 2001;Bielli and Tesauro, 2001). …”

“…Severe acidification and increase of metal concentrations (Bonacina et al, 1986;Bonacina, 2001) prevented their recovery over the following decades, as testified by many studies (Moretti, 1954a(Moretti, , 1954bOioli, 1969), which revealed a littoral fauna reduced to a tiny, monotonous community composed of highly adaptable species completely lacking entire groups, including molluscs. Even after the improvement of Lake Orta conditions and the neutralization of pH values accomplished by liming (1989)(1990), studies documenting the progressive recovery of both pelagic and benthic populations (Baudo et al, 2001a;Bielli and Tesauro, 2001;Nocentini et al, 2001) failed to reveal the presence of mussels or any other molluscs. Since mussels are highly sensitive to acidification (Økland and Økland, 1986;Taskinen et al, 2011), ammonia (Augspurger et al, 2003;Wang et al, 2008) and heavy metals (Naimo, 1995;Keller et al, 2006), particularly at the earlier life stages (Wang et al, 2007a(Wang et al, , 2007bCope et al, 2008;Cleawater et al, 2014), the high sediment toxicity, that did not improved but was rather worsened by liming, likely hindered recolonization.…”

“…Conversely, the post-liming ten-year long monitoring failed to detect any improvement of the benthic community (Nocentini et al, 2001). Whole groups of organisms such as Mollusca were missing (Tesauro et al, 1995;Baudo et al, 2001a;Nocentini et al, 2001) at least until 1998, when the last survey of the benthic littoral community was performed (Bielli and Tesauro, 2001). There are no data from the lake after that date, but the lack of mussels has been taken for granted based on the legacy effects of the accumulated toxics that resulted in long-term effects on populations (Strayer et al, 2004).…”