engleski

Mercury distribution in water column and sediment of two anchialine caves in Mljet National Park - Croatia

Mercury distribution in water columns and sediment columns of two stratified anchialine caves Bjejajka and Lenga in the remote Mljet National park in Croatian Southern Adriatic during four years period (2006 - 2009) was studied. Also, temperature, salinity, pH and dissolved oxygen concentration of the stratified water column in both anchialine caves were obtained. Salinity constantly increased with depth in the range from 3 ‰ at the surface to 38 ‰ in bottom water. Water samples for mercury analysis were collected in 1 L borosilicate glass bottles, while sediment columns were collected in hand-driven acrylic corers, both by speleo scuba diver. Measurements were performed by cold vapour atomic absorption spectrometry (CVAAS). Mercury concentrations found in unfiltered samples are presented as total (Hgtot). Extremely high concentration of total Hg (920 ng L-1) in the middle layer (7 m) of the water column of Bjejajka cave were found in January 2009., comparing to low 5 ng L-1 in the surface water sample of the cave, which is 2 order of magnitude difference. Moreover, in the vicinity of the Bjejajka cave (about 100 m), in the seawater sample taken at the surface, only 0.5 ng L-1 of total mercury was detected. In surface sediment samples in Bjejajka cave, mercury concentrations were up to 3.3 mg kg-1 dry weight. On the other hand, in Lenga cave, Hg water and sediment concentrations were drastically smaller (mostly below 7 ng L-1 and 0.8 mg kg-1 dry weight, respectively). Seasonally, small colony of bats inhabits Bjejajka cave. Concentration of total Hg in guano of these mammals was found to be 0.31 mg kg-1 wet weight, which is in the range of total Hg concentrations usually reported in sediment. This natural factor was the source of mercury high concentrations in water column and sediment in Bjejajka cave. Unlike Bjejajka, bats were not spotted in Lenga cave, which is in accordance to its considerably lesser Hg concentrations. This is the first report on mercury distribution in anchialine cave water and sediment. Also, mercury speciation in water columns was assessed. Five different mercury species were characterized: total, reactive, monomethylmercury, total gaseous mercury and labile ionic mercury. Presented results suggest complex behaviour of mercury species investigated in water column and sediment of Bjejajka and Lenga anchialine caves.

Mercury; water column; sediment; anchialine cave

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