engleski

Salinisation of Vrana Lake in Dalmatia Within the Context of Anthropogenic Influences and Climate Changes (Situation in 2008)

Vrana lake, a coastal lake in Dalmatia (Croatia), is of great value and has been declared a Nature park. Although Vrana lake is by its surface of about 30 km2 the largest lake in Croatia and although its maximum level depth is only – 3 m of altitude, its volume is about 82.5 mil m3. The lake is a cryptodepression separated from the sea by a 0.8 – 2.5 km wide limestone ridge within which the coastal karst aquifer functions and dynamically levels with both the sea and the lake and through which the lake also interacts with the sea. Therefore it is the matter of a very sensitive karst coastal system with a close interaction with the sea - a direct interaction occurs through the 800 m long Prosika canal and the indirect interaction occurs through the karst aquifer. The period of a few recent years, particularly the two year period 2007-2008, has been characterized by pronounced draughty hydrological conditions. Consequently, during the summer of 2008, the water level of Vrana lake fell under the sill level of Prosika canal and furthermore in specific conditions of daily sea-level fluctuations the water level of the lake was even below the sea level. That caused the sea to flow directly into the lake and also through numerous springs on the lake shore, especially on its southern part. The result was an enormous increase in the salinity at several measurement stations in the lake as compared to the previously analyzed period (2000 – 2006). The increase was several times larger than mean values of the mentioned period. In the northern part of the lake, during the year 2007-2008 the observed conductivities were from 2030 to 12790 μScm-1 (the average being 6643 μScm-1 as compared to the average of 2347 μScm-1 of the previously analyzed period). At the adjacent locality at the lake surface by the campsite Crkvine, during the year 2007-2008, the measured conductivities were form 1659 to 12720 μScm-1 (the average being 5437 μScm-1 as compared to the average of 2004 μScm-1 of the previously analyzed period). In the southern part of the lake where the Prosika canal is situated and where there is the highest concentration of inshore springs through which salty water flows into the lake, even larger concentrations of the salinized waters were observed. In the year 2007-2008 the electrical conductivity at this measurement station was between 1635 and 18830 μScm-1 (the average being 7191 μScm-1 as compared to the average of 2122 μScm-1 of the previously analyzed period 2000-2006). Evidence of the lake critical condition is the total volume of water flowing out into the sea through the Prosika canal of only about 0, 19 mil. m3 in the year 2008 which is minor as compared to the average of about 31, 8 mil.m3 for the analyzed period 2000-2008. Within the context, the sill level of the constructed Prosika canal, which was once sufficient for the stabilization of Vrana lake, can be a problem which generates an increase of lake salinization.

water salinization; karst; Vrana lake; Dalmatia

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