Relation between hydrological characteristics and anthropogenic influence in the context of lake protection – case studies of Plitvice Lakes and Vrana Lake in Dalmatia (Croatia) Ana Katalinic 1, Gordana Zwicker 2, Andrijana Brozincevic 3, Danijela Peros-Pucar 4, Josip Rubinic 5 1Public Institution Nature Park Vransko jezero, Biograd na moru, Croatia, e-mail: [email protected] 2 State Institute for Nature Protection, Zagreb, Croatia, e-mail: [email protected] 3 Public Institution Plitvice Lakes National Park, Croatia, e-mail: [email protected] 4 Public Health Institute, Zadar, Croatia, e-mail: [email protected] 5 Faculty of Civil Engineering University of Rijeka, Rijeka, Croatia, e-mail: [email protected] Abstract Lakes represent especially fragile and valuable aquatic systems, therefore often protected by law in the category of a national park, nature park, special reserve, natural monument or important landscape. Protection of lake's ecosystems is mostly based on unfavourable anthropogenic impact assessment and finding a way of diminishing it. What is often neglected are the hydrological mechanisms of lakes' systems and the consequent changes in hydrological conditions which can significantly increase the risk of potential unfavourable impact, hence making an important issue to consider in the complex integrative protection approach. This paper analyzes the problematics on the case study of Plitvice Lakes National Park, more specifically its largest lake Kozjak, and of Vrana Lake near Biograd, protected with its immediate surroundings in the status of a nature park. Both lakes have originated from karst area, but retained their specificities. While the lake Kozjak, with the surface of 0.83 km2, makes an integral part of a 16 lakes cascade with flowing water, Vrana Lake is the largest natural stagnant waterbody in Croatia, comprising 30.2 km2 of surface, with depth so small defining it partly as a wetland. Different geographical positions and different depths condition the water dynamics of these two lakes. The given paper analyzes interrelations of characteristic water quality parameters and yearly flow variations in the water systems of each of the mentioned lakes, and also, on a larger time scale, defines relation trends between the two lakes themselves. Special attention has been given to salification mechanism analysis of Vrana Lake, and the consequences of this process on the lake's system. The importance of integral observation of natural and anthropogenic influences is determined, pointing out the impact of water quality and hydrological conditions on biological characteristics of the lakes' systems, and providing the guidelines for improving their protection. Key words: lakes, nature protection, hydrological systems, water quality, Plitvice Lakes, Vrana Lake (Dalmatia) BALWOIS 2008 – Ohrid, Republic of Macedonia – 27, 31 May 2008 1/14 Introduction This paper analyzes the influence of hydrologic changes and environmental influences on functioning of fragile water ecosystems. The analysis include seasonal and long-term oscillations of water quality chemical parameters and available indicators of trophic state in two natural lakes. Kozjak Lake as a part of Plitvice Lakes, and Vrana Lake in Dalmatia were chosen as case studies (Figure 1). Although different in origin, morphology and the way of functioning, the two lakes share the fact that their complete ecosystems depend on fine changes of basic hydrological and physical-chemical parameters. Multi-annual analysis of these variables are conducted to improve understanding of basic conditions for maintaining equilibrium in the two systems, and to point to the need of continuous monitoring of these variables in protected areas management issues. Figure 1. Location of Vrana Lake Nature Park and Plitvice Lakes National Park Plitvice Lakes are situated in the hilly area at the beginning of Lika region, between the massif of Mala Kapela and Licka Pljesivica. First visitors used to refer to them as The Devil's Garden (Franic, 1910). Later their beauty was appreciated and with the merit of Ivo Pevalek, a researcher who pointed to the need of their protection (Pevalek, 1924, 1935, 1938), in 1949 the lakes were pronounced a national park. Their uniqueness is recognized on a global level, and in 1979 Plitvice Lakes were included in UNESCO World Natural Heritage List. The central and most attractive part of the National park are cascadely lined up lakes, created by travertine barriers rising in the river valley. Karst water overflows from lake to lake over the barriers, until the last waterfalls where river Korana starts its course. The lakes occupy less than 1% of total park area, but attract a large number of visitors, getting even larger each year, especially in the summer months. The lake zone also includes hotels, parking areas, water pumping unit and wastewater drainage system. Basic characteristic of Plitvice Lakes National Park is the process of creation and up-levelling of travertine barriers. This process is very complex (Pevalek, 1935; Pavletic, 1957; Matonickin et al, 1971; Srdoc et al, 1985) and demands the existence of equilibrium of many factors, including physical-chemical parameters, biological conditions and optimal waterflow. Although the water in Plitvice Lakes generally shows good quality, there is a regional trend of waterflow decrease especially accentuated in Plitvice Lakes (Zwicker et al, 2006). Strong anthropogenic influences in the past century, as well as the large number of visitors today, have a negative influence on equilibrium maintenance. Overgrowth of travertine barriers and lake coastal line with vegetation indicating increased eutrophication has been noticed already in the 1980-ties (Habdija & Stilinovic, 1986, 1987, 2005.) Cryptodepression Vrana Lake in Dalmatia is the largest natural lake in Croatia by surface (30 km2). Because of its potential for fishing, irrigation and agriculture on the adjacent fertile land it has been in the focus of human interest since first settlements, causing strong anthropogenic influences. At the end of 20th century the value of Vrana Lake as an important waterbirds habitat was recognized and it was proclaimed an Important Bird Area in 1983, and its northwestern part with vast reedbed was proclaimed a Special Ornithological Reserve in the same year. Different from Kozjak Lake, which is susceptible to continental climatic conditions, Vrana Lake is a shallow Mediterranean lake situated in the lowest part of Ravni kotari, only a kilometer away from the sea. Additional factor not to be neglected in the analysis of Vrana Lake water regime is the BALWOIS 2008 – Ohrid, Republic of Macedonia – 27, 31 May 2008 2/14 salification of lake's water. This is caused by intrusion of sea water through permeable karst barrier and through Prosika canal, dug in the 18th century for irrigation purposes. Significant variations in salinity and strong influence of human settlements and agriculture land in the catchment’s area indicate the fragility of this ecosystem. Described dynamics of these two systems have indicated the need to conduct more complex anaysis concerning a longer time period. Most research conducted up to date was static observance of lakes' processes, limited to a short period of time, missing the quantification of changes at a larger time-scale. Intent of this work was to make progress in the sense of conducting a comparative anaysis of hydrological and chemical parameters on a longer time-scale, to allow drawing conclusions and guidelines for future protected areas management strategies. The goal was to analyze data on environmental conditions and present them in a form that is acceptable for time-space comparisons and indicative for decision making in protected areas. Study area Kozjak Lake at Plitvice Series of 16 cascadely lined-up lakes, with altitude difference of 133 m in total, is carved partly in dolomites (Upper Lakes) and partly in limestone rocks (Lower Lakes). The Upper Lakes coastline is covered in wood, while the Lower Lakes are carved in a 70 meters deep canyon. Total lake surface is 1.9 km2, and the volume is around 23x106 m3 (Babinka, 2007). Lakes are separated by travertine barriers overflown by water forming waterfalls. Water splashing on the waterfalls, supported by biogenic processes, stimulates extraction and sedimentation of calcium carbonate from water and subsequent rising of travertine barriers. Concurrently, a much slower process of sedimentation is happening at the bottom of the lake. The overall result is the growth of the travertine barriers, causing the rising of lakes' water level, in spite of the present long-term trend of decrease in waterflow (Zwicker&Rubinic, 2005). The largest lake of Plitvice – Kozjak Lake (Figure 2) is situated at the lowest part of the Upper Lakes, where the poorly permeable dolomites of Upper Triassic and the permeable limestone rocks of Upper Cretaceous meet. Surface of Kozjak Lake is 0.83 km2 and the volume around 12.7x106 m3 (Babinka, 2007). An island rises in the northeastern part of the lake. A submerged travertine barrier divides the lake in two parts that used to function as two lakes 400 years before (Srdoc et al, 1985). Maximum depth is 27 m in the up-stream part of the lake, and 46 m in the down-stream part of the lake. Main water inflows from the Upper Lakes, as well as Rjecica stream are situated in the southwestern part of the lake. Average annual amount of precipitation is estimated around 1550 mm (Berakovic, 2005). Average annual flow for the period 1953 – 1990, according to data from hydrological station Kozjak- bridge (KH), amounts 3.44 m3s-1 (Zwicker et al, 2006). Average water retention time for Kozjak Lake is 43 days. During long-term dry periods the discharge, which is according to Srdoc et al (1985) on of the most dominant factors in travertine barriers growth, can decrease to even 0.5 m3s-1 (0.516 m3s-1 registered in November 1983). Additional stress factor for Kozjak Lake, and therefore the entire Plitvice Lakes system, is the pumping of around 0.060 m3s-1 of water for watersupply, especially in the periods of minimum water inflows.