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Distribution of Hg, Cd, Pb, Cu and Zn in water columns and sediments of two anchialine caves in Mljet National Park - Croatia

Distribution of Hg total, and Cd, Pb, Cu and Zn total and dissolved concentrations in water columns, and their total amounts in sediment columns of two anchialine caves Bjejajka and Lenga, in the remote Mljet National park in Croatian Southern Adriatic during four years period (2006 - 2009) was studied. Water samples for Cu, Cd, Pb and Zn determination were collected in pre-cleaned high-density polyethylene bottles (HDPE) (1 L), while for Hg water was collected in 1 L borosilicate glass bottles. Sediment column samples for all metals were collected in hand-driven acrylic corers, both by speleo scuba-diver. Cu, Cd, Pb and Zn measurements were performed by differential pulse anodic stripping voltammetry (DPASV). Obtained trace metals concentrations are presented as dissolved and total fractions. Total concentrations were measured in acidified (pH < 2), unfiltered and UV irradiated samples, while dissolved concentrations were obtained from filtered (0.45 µm cellulose nitrate filter), acidified and UV irradiated samples. Hg measurements were performed by cold vapour atomic absorption spectrometry (CVAAS). Mercury concentrations found in unfiltered, acidified and UV irradiated samples are presented as total (Hgtot). Additionally, temperature, salinity, pH and dissolved oxygen concentration were determined directly in water columns of both anchialine caves. Salinity increased constantly with depth in the range from 3 ‰ at the surface to 38 ‰ in bottom water layer, showing water column stratification. In general, concentrations of all metals in water columns of both caves were significantly higher in comparison to metals amount found in seawater samples taken in near vicinity (~ 100 m). Moreover, total Cu (up to 28 µg L-1) and Zn (up to 10 µg L-1) in Lenga cave water column were found to be considerably higher comparing to Bjejajka cave (Cu < 5 µg L-1 and Zn < 4.5 µg L-1), while Pb exhibited similar abundance in both caves. Most intriguingly, mercury and cadmium concentrations in water column of Bjejajka cave were greatly elevated in comparison to their amounts found in water column of Lenga cave. Substantial concentrations of total Hg (920 ng L-1 at 7 m) and Cd (300 ng L-1 at 9 m) were found in water column of Bjejajka cave in January 2009, while in Lenga cave water total Hg and Cd did not exceed 15 and 42 ng L-1, respectively. In surface seawater samples collected in the vicinity of the Bjejajka cave (~ 100 m), quite low total concentrations of 0.5 ng L-1 Hg, 6 ng L-1 Cd, 33 ng L-1 Pb, 220 ng L-1 Cu and 190 ng L-1 Zn were detected. In Bjejajka cave surface sediment samples, mercury concentrations were up to 3.3 mg kg-1 and cadmium up to 1.34 mg kg-1 dry weight. On the other hand, in Lenga cave sediment, Hg and Cd concentrations were considerably smaller, 0.8 mg kg-1 and 0.7 mg kg-1 dry weight, respectively. Namely, small colony of bats seasonally inhabits Bjejajka cave. Concentrations of mercury and cadmium in guano of these mammals were found to be 0.31 and 1.8 mg kg-1 wet weight, respectively. These values are in the range of Hg concentrations in sediment, and above the range of Cd concentrations usually reported in surface sediments. Unlike Bjejajka, bats were not spotted in Lenga cave, which is in accordance to its considerably lesser Hg and Cd concentrations. Most probably, bats were the main natural source of mercury and cadmium elevated concentrations in water column and sediment of Bjejajka cave. Higher Zn and Pb content in surface sediment in Lenga cave were detected, comparing to those found in Bjejajka cave sediment (Lenga cave: Zn up to 700 mg kg-1, Pb up to 65 mg kg-1 dry weight ; Bjejajka cave: Zn up to 160 mg kg-1, Pb up to 18.5 mg kg-1 dry weight). All metals concentrations in Bjejajka cave increased with sediment column depth. This has been the first report ever on Cu, Cd, Pb and Zn both concentrations, total and dissolved in water column of the anchialine caves. 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 express specific metals behaviour, particularly mercury and cadmium in aquatic system of Bjejajka and Lenga anchialine caves, specific natural objects in Mljet NP.

trace metals; water column; sediment; anchialine cave

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