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

Distribution of Cd, Pb, Cu and Zn total and dissolved concentrations in water columns, and sediment columns of two anchialine caves, Bjejajka and Lenga in the Mljet National park situated on the island of Mljet, during four years period (2006 - 2009) was studied. This has been the first report ever on both metals concentrations, total and dissolved in water column of the anchialine caves. Water samples for Cu, Cd, Pb and Zn determination were collected in pre-cleaned high-density polyethylene bottles (HDPE) (1 L), while sediment column samples were collected in hand-driven acrylic corers, both by speleo scuba-diver. 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. 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 four metals in water columns of both caves were significantly higher in comparison to metals amount found in seawater samples taken in near vicinity. 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, cadmium concentrations in water column of Bjejajka cave were greatly elevated in comparison to Cd amounts found in water column of Lenga cave. Substantial concentration of total Cd (300 ng L-1, January 2009.) were found in water bottom layer (9 m) of Bjejajka cave, while total Cd in Lenga cave water did not exceed 42 ng L-1 (bottom water layer, 8 m, May 2007). In surface seawater samples collected in the vicinity of the Bjejajka cave (about 100 m, May 2007), quite low total concentrations of 6 ng L-1 Cd, 33 ng L-1 Pb, 220 ng L-1 Cu and 190 ng L-1 Zn were detected. Namely, small colony of bats seasonally inhabits Bjejajka cave. Concentration of cadmium in guano of these mammals was found to be 1.8 mg kg-1 wet weight, which is somewhat above the range of Cd concentrations usually reported in sediments. Most probably, this natural factor was the main source of cadmium elevated concentrations in water column and sediment of Bjejajka cave. Unlike Bjejajka, bats were not spotted in Lenga cave, which is in accordance to its considerably lesser Cd concentrations. 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. Presented results express specific metals behaviour, particularly 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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