engleski

Natural causes of changes in marine environment: mercury speciation and distribution in anchialine caves

Mercury speciation and distribution were assessed, for the first time, from the water, sediment, rock, soil and air of anchialine caves. We evaluated the origin and distribution of four mercury species (total (THg), reactive (RHg), dissolved gaseous mercury (DGM) and monomethylmercury (MeHg)) in water from Bjejajka cave (Bjejajka) and Lenga Pit (Lenga) in the Croatian Adriatic Sea from 2006 to 2011. Concentrations of all mercury species were elevated at both sites compared to adjacent seawater, which had concentration range of 0.5-2.4 ng L-1 for THg, 0.06-0.2 ng L-1 for RHg, 0.02-0.07 ng L-1 for DGM, and 0.01-0.02 ng L-1 for MeHg. In Bjejajka, the maximum THg concentration (350 ng L-1) in the middle layer (3-7 m) of the 12 m deep stratified water column was up to 3 orders of magnitude greater than in adjacent seawater. During and after the rainy sampling periods in January 2009 and 2010, mixing of Bjejajka water column resulted in elevated concentrations, up to 3700 ng L-1 of THg (4 orders of magnitude higher compared to values in nearby seawater) in the bottom water layer. MeHg concentrations in the Bjejajka water column were also considerably elevated (0.7-6.3 ng L-1) compared to the surrounding seawater (0.01-0.02 ng L-1). The vertical distribution of MeHg concentrations followed that of THg, however the ratio of MeHg/THg above the Bjejajka halocline was drastically higher (up to 57 %) compared to MeHg proportion (1-2 %) below the halocline, which was similar to that of surface seawater. In sediment of Bjejajka, THg concentrations were up to 3.3 mg kg-1, considerably above concentrations in unpolluted Adriatic marine sediment (0.1-0.3 mg kg-1). The highest THg amounts found in soil and air were inside and in close proximity to Bjejajka, while THg in rock (≤0.01 mg kg-1) were below reported values for unaltered carbonates. Laboratory experiments indicate that bat guano was the major source of elevated mercury concentrations in the water column and sediment of Bjejajka. Concentrations of THg in bat guano were up to 0.45 mg kg-1, above the range of THg concentrations found in unpolluted marine sediment. Seventy-two days after the addition of bat guano in solution, a 100-fold higher Hg concentration was observed, from 2.5 up to 252 ng L-1. In Lenga bat guano was not spotted and THg water and sediment concentrations were drastically lower, mostly below 7 ng L-1 and 0.8 mg kg-1, respectively.

Mercury speciation; Anchialine caves; Stratified water column; Croatia; Adriatic Sea

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