Ecological response of benthic foraminifera to the acid drainage from mine areas. An example from the Gromolo torrent mouth (Eastern Ligurian Sea, Italy)

Benthic foraminifera are used as indicators of environmental changes in coastal marine areas, affected by dismissed mines and dump areas. The Libiola Fe-Cu sulphide mine was intensively exploited in 19th and 20th centuries, and the activity ended in 1962. Waters circulating in the Libiola mine area, and discharging in the adjacent streams and creeks, are strongly polluted due to the diffuse occurrence of Acid Mine Drainage processes. The Gromolo torrent collects these acidic waters enriched of heavy metals which flow into Ligurian Sea. A total of 15 marine sediment samples (upper 2 cm) were collected by means of Van Veen grab in the coastal zone close to the Gromolo mouth and analyzed for living (rose Bengal stained) and dead benthic foraminifera, together with grain size, metals and trace elements, and metal fractioning. Quantitative foraminiferal parameters, like as abundance, species diversity, heterogeneity and assemblage composition, were determined and evaluated for environmental purpose. Additionally, possible increase above the natural background level of deformed specimens was considered as indicative of metal contamination. The foraminiferal study highlighted several samples with very low abundance, especially in the coastal belt between Gromolo and Entella torrents. Slightly higher abundance was recorded close to the Gromolo mouth, where sediments are strongly characterized by Ammonia. Both living and dead assemblages displayed low diversity and high dominance with Ammonia beccarii which accounts up to 66% of total abundance. Moreover, percentage of deformed specimens generally exceeded the natural background reaching 5%. All these aspects are clear evidence of an environmental stress.