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|Authors: ||Caliro, S.*|
|Title: ||Geochemical and biochemical evidence of lake overturn and fish kill at Lake Averno, Italy|
|Title of journal: ||Journal of Volcanology and Geothermal Research|
|Series/Report no.: ||/178 (2008)|
|Issue Date: ||2008|
|Keywords: ||lake Averno|
|Abstract: ||Lake Averno is situated in the homonymous crater in the northwestern sector of the Campi Flegrei active
volcanic system in Campania region, Italy. In February 2005 a fish kill event was observed in the lake,
prompting a geochemical survey to ascertain the possible cause. In February 2005 a geochemical survey
revealed that the lake water was unstratified chemically and isotopically, presumably, as a result of lake
overturn. This fish kill phenomenon was recorded at least two other times in the past. In contrast to the
February 2005 results, data collected in October 2005, shows the Lake Averno to be stratified, with an oxic
epilimnion (surface to 6 m) and an anoxic hypolimnion (6 m to lake bottom at about 33 m).
Chemical and isotopic compositions of Lake Averno waters suggest an origin by mixing of shallow waters
with a Na–Cl hydrothermal component coupled with an active evaporation process. The isotopic composition
of Dissolved Inorganic Carbon, as well as the composition of the non-reactive dissolved gas species again
supports the occurrence of this mixing process.
Decreasing levels of SO4 and increasing levels of H2S and CH4 contents in lake water with depth, strongly
suggests anaerobic bacterial processes are occurring through decomposition of organic matter under anoxic
conditions in the sediment and in the water column. Sulfate reduction and methanogenesis processes coexist
and play a pivotal role in the anaerobic environment of the Lake Averno. The sulfate reducing bacterial
activity has been estimated in the range of 14–22 μmol m−2 day−1.
Total gas pressure of dissolved gases ranges between 800 and 1400 mbar, well below the hydrostatic pressure
throughout the water column, excluding the possibility, at least at the survey time, of a limnic eruption.
Vertical changes in the density of lake waters indicate that overturn may be triggered by cooling of
epilimnetic waters below 7 °C. This is a possible phenomenon in winter periods if atmospheric temperatures
remain frosty for enough time, as occurred in February 2005.
The bulk of these results strongly support the hypothesis that fish kill was caused by a series of events that
began with the cooling of the epilimnetic waters with breaking of the thermal stratification, followed by lake
overturn and the rise of toxic levels of H2S from the reduced waters near the lake bottom.|
|Appears in Collections:||04.08.06. Volcano monitoring|
03.04.06. Hydrothermal systems
04.04.12. Fluid Geochemistry
Papers Published / Papers in press
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