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)

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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.

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Figure 2. Position of Kozjak Lake and its bathimetry (Babinka, 2007)

Lake's coastal shallows and barriers are covered with marsh vegetation and demonstrate visible signs of eutrophication. Frequent question in scientific circles is whether this is the result of lake's natural aging process or the cause is more anthropogenic. Numerous anthropogenic influences in the past have affected the level of eutrophication: watercourse regulations, introduction of allochthonous fish species, saw-mills, agricultural activities and permeable septic tanks in the catchment area, leaking wastewater systems, drainage of polluted rainwater into the lakes. The lake's sediment contains traces of anthropogenic pressures from earlier times (Obelic et al, 2006). The influence of human activities is strongest in the hotel zone and in the lake's source area. Eastern side of the lake contains a visitors' complex of three hotels, one restaurant, parking area and sanitaries. Quality of water in the lake is also influenced by inflowing waters that in their source area pass through the settlement Plitvicki Ljeskovac, burdened with permeable septic tanks and cattle holding by the watercourse. In the post-war period, wastewater sanitation has been conducted gradually. Sanitation of sewage system down-stream from the pupming station at Kozjak Lake till the karst sinkhole in Rastovaca village where wastwater is released in the underground was done in 2001. The part from the hotel till the pumping station was sanitated in 2005. With these actions the entire wastewater system of the Mukinje settlement and of the tourist contents at the eastern side of the lake was sanitated and distanced from the lakes' system. But, considering the yearly growing number of visitors in the lake area which already amounts up to 900.000 visitors per year, the negative anthropogenic influence is inevitable, already due to washing out of lake's gravitating surfaces with rainwater. According to past investigations (Petrik, 1958, Habdija, 1983) Kozjak is a termally stratified dimictic lake with clear distinction between epilimnium, metalimnium with termocline and hypolimnium. Mixing of layers which leads to isothermy of the entire water column happens twice a year, in late autumn and early spring. Surface layer of the lake is fluent due to overflowing of water from lake to lake.

Vrana Lake in Dalmatia Vrana Lake in Dalmatia (Figure 3) is actually a submerged karstic field, created by the uplevelling of sea level after the last glacial period. It spreads in the northwest-southeast direction between the ridge of Crnogorka hill (h = 305 m) and the 0.8-2.5 km wide ridge (h = 71 m) that separates the lake from the sea. With the area of 30 km2 and depth of 2-5 m it represents the largest waterbody in the area of Ravni kotari, with the volume of around 82.5x106 m3 (Faculty of Civil Engineering, 1994). Vrana Lake collects rainwater and groundwater from the 470 km2 large catchment area, and the average water retention time is 1.28 years. Lake water is brackish because of the altitude relations between the lake water level and the sea water level which allow the intrusion of sea water into the lake, through underground and through the narrow barrier separating it from the sea. This is also possible due to composition of lake's substrate, made of impermeable sediments of Eocene flysch, partly permeable dolomites from Cretaceous Age and permeable limestone rocks from Eocene and Cretaceous Age (Fritz, 1984). Sea water intrusion was facilitated in the 18th century by digging through of the Posika canal at the southeastern part of the lake, which therefore demonstrates most frequent rises in salinity, at times

BALWOIS 2008 – Ohrid, Republic of Macedonia – 27, 31 May 2008 4/14 reaching conductivity values up to 14500 µS/cm (Romic & Tomic, 1997). The lake is actually a cryptodepression with the bottom at -3 msl, and surface above sea level during most of the year (lake’s average annual water level for the period 1948-2005 was + 0.81 msl), which results in overflow of water from the lake into the sea. Only in extraordinary situations, usually following low autumn levels of lake’s water and strong sea tides, the situation reverses. Balancing of favourable hydrological conditions is crucial to maintenance of fragile lake’s equilibrium, which can easily be disturbed by excessive salification (Katalinic et al, 2007). Water level for the period 1986-2006 recorded at the station Prosika varied from 0.05 m to 2.23 m. Lake’s volume is estimated around 82.5 x 106 m3. Average annual precipitation in the catchment of Vrana Lake is estimated at 1000 mm (Berakovic, 1983), and average annual evaporation from the lake amounts around 1660 mm. Besides the occasional inflows of sea water, the lake regularly receives rainwater from several surface water courses (Kotarka, Mirosnica, Klicevica, Skorobic) collected in two main irrigation canals, as well as underground water rising through several springs in the lake’s catchment or in the very lake. The most significant spring in the catchment is Kakma with the average minimum affluency of 0.08 m3s-1, intensively exploited for purposes of irrigation and watersupply (Kapelj et al, 2003). Quality of lake’s water is greatly influenced by wastewater washed out from agricultural areas in the lake’s flood zone and the whole catchment area, communal waters reaching the lake through underground from Benkovac area, and draining waters from the irregular landfill of the city of Biograd situated northeast of the lake. Over 40 % (193.12 km2) of land in the Vrana Lake’s catchment is used for agriculture (Faculty of Civil Engineering, 1994), and the population list from year 2001 counts 26102 inhabitants in closer surroundings of the Park, with 45 % of them living in the lake’s hinterland (Institute for tourism, 2006). There are no available data on the quality and quantity of these inflows into the lake, so the antropogenic influence on the lake system can only be analyzed through data on the water quality of the lake itself, keeping in mind that this is a polymictic lake with vertical mixing of water column throughout the year causing regeneration of nutrients from the sediment (Peros-Pucar, 2006).

Figure 3. Position of Vrana Lake and locations of its sampling and gauge stations

Methodology

This paper is based on analysis of elementary hydrological data provided by Meteorological and Hydrological Service of Croatia, data on water quality provided by Croatian Waters (legal entity for water management of Croatia), and from the study of Faculty of Science (2004) and Master thesis on the trophy of Vrana Lake (Peros-Pucar, 2006). The idea was to analyze longest available sequence of homogenous data, to perceive the long-term realistic trends of certain parameters, and make comparison of oscillations of hydrological and chemical parameters in ecosystems of the two analyzed lakes, Kozjak at Plitvice and Vrana Lake in Dalmatia.

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For hydrological analysis of Vrana Lake, data on relative water level values for the period 1996-2006 at the station Pakostane Bridge (VH) were used (Figure 3). Sampling for water quality is conducted at three different locations, but Prosika station was not convenient because of short history of available data and also significant impact of sea water. The stations used were the Crkvine Camp (V1), and Kotarka Estuary (V2) (Figure 3), with the longest available sequence of continuous and homogenous data. Station V2 is situtated at the estuary of irrigation canals into the lake, receiving most of the surface waters from Vrana Lake’s catchmnet.This makes it indicative for monitoring of acute outlets of polluted waters from surface water courses into the lake. Station V1, situtated next to the Crkvine Camp, is more illustrative for estimation of the general condition of the lake, but keeping in mind the impact of potential outlets of wastewater from the camp. Available continuous data on water quality comprehend an 11-years period from 1996 to 2006. The measurements have been conducted already from 1982, but not regularly, and with a large pause during the war in the 1990-ties. For the purposes of comparison with the conditions on Kozjak Lake, the biggest attention was given to the data sequence from 2000 to 2006. For hydrological analysis of Kozjak Lake, data on relative water level values for the period 2000-2006 at the station Kozjak Bridge (KH) were used (Figure 2). The station is situatued at the down-stream end of the lake with the altitude of 534.6 msl. Samples for water quality analysis were taken in the vicinity, at the station K1 (Figure 2). Because of complexity of data, in this work only the data reffering to epliminum were considered. Available data on water quality for Kozjak Lake comprehend a short period from 2000 to 2006, as all earlier measurements were done for needs of individual projects, lacking continuity. All data are presented in the form of average monthly values. Values below detection level for water quality parameters were approximated to 0.01 mg/L for total phosphorus and ammonium, and 0.1 mg/L for nitrates. This approximation is realistic enough for the needs of analysis in this paper, as it does not change the long-terms trends of data. Out of numerous available parameters of water quality we choose for analysis only chemical parameters, more specifically the main nutrients: phosphorus and nitrogen. Phosphorus was analyzed in the form of total phosphorus (P), which is considered to be one of the four main trophy indicators, according to the National Water Classification Directive (NN 77/98). Total phosphorus is often described as the limiting factor for phytoplankton development (Carlson & Simpson, 1996), which is confirmed for Vrana Lake in prior investigations of nitrogen – phospohorus ratio (N:P) (Faculty of Civil Engineering, 1994). Acute pollution was indicated by data on nitrogen amount, in the form of + ammonium (NH4 ) and nitrates (NO3-). Water was classified in categories of quality according to the National Water Classification Directive (NN 77/98) (Appendix 1). Statistical analysis were done by Pearson’s linear correlation analysis, using 95% confidence interval (P < 0.05) to determine significance of the results and correlation coefficient (r) to determine the strength of correlation.

Results

Main hypothesis of this paper is that in dynamic hydrological systems, such as Vrana Lake and Kozjak Lake, all changes through time must be observed in the context of oscillations in hydrological conditions. Due to short history of available data, and the complexity of the factors influencing the trophhic state of the lakes, it was not possible to draw conclusions on direct correlation of hydrology and trophy of the lake in this paper. Nevertheless, basic hydrological conditions and chemical parameters are presented as the basis for further research of this issue, to facilitate better understanding of the lakes’ aging process and timely reaction in the sense of management of these valuable protected areas. Hydrological conditions at the studied lakes (Figure 4) are presented through overview of average monthly water level values. Both lakes show seasonal oscillations with maximum water levels in the period from January till April. In the short analyzed period the increase of water level is visible for Kozjak Lake, which corresponds with the long-term recorded increment trend (Zwicker and Rubinić, 2006). Water level oscillations at Vrana Lake do not exhibit significant changing trend in the period 2000-2006, while the long-term monitorings record an increasing trend in water level, most probably due to up-levelling of the sea (Katalinić et al, 2007). The exception from the normal hydrological conditions at Vrana Lake in the analyzed period is observed in the winter period of 2001- 2002, when the long-term dry conditions caused the absence of the winter water level maximum.

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Vrana Lake hydrological system shows bigger inertion compared to Kozjak Lake, due to bigger volume of the lake and its karstic aquifer.

Kozjak Lake Vrana Lake 140 250

130 200 120

110 150

100

100 90 H (cm) - Vrana Lake - Vrana (cm) H H H (cm) - Kozjak Lake 80 50 70

60 0 Jan-00 Jan-01 Jan-02 Jan-03 Jan-04 Jan-05 Jan-06

Figure 4. Comparison of average monthly water level oscillations at the stations VH and KH for the period 2000-2006

Overview of maximum, minimum and average values of the analyzed chemical parameters for both lakes is given in the Table 1. The comparison shows similar values of total phosphorus in both lakes, but significantly higher values of ammonium and nitrate for Vrana Lake.

Table 1. Overview of characteristic parameters of water quality monitoring at Kozjak Lake and Vrana Lake (2000-2006) Kozjak Lake Vrana Lake – V1 MIN MAX AVE MIN MAX AVE Ammonium <0,01 0,24 0,04 <0,01 1,08 0,12 (mgN/L) Nitrates <0,10 1,21 0,57 <0,10 6,60 1,29 (mgN/L) Total phosphorus <0,01 0,12 0,02 <0,01 0,12 0,03 (mgP/L) Temperature 2,9 25 12 3,7 27,2 16,4 (ºC) pH 7,2 8,6 8,2 8,1 8,4 8,3 Samples 84 82

Time and space dynamics of abiotic factors (phosphorus and nitrogen) is an important criterium in lake quality classification. As can be read from Figure 5, showing oscillations of phosphorus concentrations through period 2000-2006, water of Kozjak Lake is mainly in the I and II category. These are oligotrophic and mesotrophic waters (NN 77/98), with individual late-spring and early-winter peaks belonging to III category (eutorphic waters). Research of Kozjak Lake in the 1980-ties have confirmed two maximums of primary production in spring and early autumn (Habdija, 1983; Habdija & Stilinovic, 1986; Habdija et al, 1991), followed by corresponding changes in phosphorus concentrations. Comparison of recent data with individual results of past measurements show significant difference in maximum recorded values of phosphorus: among past short-term data the maximum recorded surface concentration of phosphorus was 0.8 mgP/L in December 1981 (Habdija, 1983), while maximum recorded concentration in the period 2000-2006 was only 0.12 mgP/L (December 2005). Nevertheless, in the scope of the analyzed period of 2000-2006, a general increment trend of phosphorus concentration is noticed. Elevated phosphorus concentrations in Kozjak Lake in the past century as a result of significant anthropogenic influences, left trace on nowadays phosphorus values, even though these influences have been largely diminished. Namely, the process of eutrophication continues with almost the same intensity

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Kozjak Lake Vrana Lake 0,140

0,120

0,100

0,080

P P (mg/L) 0,060

0,040

0,020

0,000 Jan-00 Jan-01 Jan-02 Jan-03 Jan-04 Jan-05 Jan-06

Figure 5. Comparative overview of total phosphorus concentration oscillations (mgP/L) at Vrana Lake (V1) and Kozjak Lake (K1) for the period 2000-2006

Statistical analysis showed correlation (r = 0.2994, P = 0.0057) between oscillations of phosphorus concentrations at Vrana Lake and Kozjak Lake, implying that the seasonal changes of this nutrient can be largely assigned to the natural metabolic processes of the two lakes.

Water of Vrana Lake is, according to phosphorus values, mainly in III category, excluding the mentioned extremely dry period when the water can be in IV (station V1) and even in V category (station V2) (NN 77/98). Because of large influence of inflowing waters on the stations at which the water quality was analyzed, these data were compared with the values obtained from samples taken at the center of the lake, which describe the general condition of lake water more objectively. According to data from 2004 (Peroš-Pucar, 2006) phosphorus concentrations at the center of the lake indicate to water of II and III category. Unlike Kozjak Lake, phosphorus concentrations at Vrana Lake demonstrate significant trend of decrement. This trend is most visible at the station V2 (Figure 6), and it is mostly justified by high levels of phosphorus at the beginning of the analyzed period (during the second half of 2001 and the entire 2002), characterized by scarce hydrological conditions on the lake and the absence of the winter water level maximum. Therefore, one can not draw conclusions about a significant general decrement trend of phosphorus values, but rather notice the increase in phosphorus concentration (especially the spring maximums) during the long-term dry period. This can be partly explained by bigger productivity of waters flowing into the lake and partly by more effective mixing of the water column, resulting in improved regeneration of nutrients from the sediment.

phosphorus - V1 phosphorus - V2 waterlevel - VH

0,180 400 0,160 350 0,140 300

0,120 250 0,100 200 0,080 H(cm)

P(mg/L) 150 0,060 100 0,040 0,020 50 0,000 0 Jan-00 Jan-01 Jan-02 Jan-03 Jan-04 Jan-05 Jan-06

Figure 6. Comparative analysis of water level oscillations (H (cm)) and total phosphorus concentrations (mgP/L) at the stations V1 and V2 at Vrana Lake, for the period 2000-2006

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Second group of nutrients analyzed in this paper are nitrogen compounds – nitrates and ammonium (Figure 7 to 10). According to nitrate values Kozjak Lake water belongs mainly to I and II category (NN 77/98). Flexible interseasonal variations are mostly accented with the nitrate concentration peak in the early spring, corresponding to water level maximums (Figure 7).

nitrate waterlevel 1,4 160

1,2 140 120 1 100 0,8 80

(mg/L) 0,6 3 60 H (cm) H

NO 0,4 40

0,2 20 0 0 Jan-00 Jan-01 Jan-02 Jan-03 Jan-04 Jan-05 Jan-06

Figure 7. Comparative overview of water level oscillations (H (cm)) and nitrate concentrations (mgN/L) at Kozjak Lake for the period 2000-2006

Elevated nitrate concentrations point to intensive nitrification processes in the metabolism of planktonic community (Habdija & Stilinovic, 1986). Waters of Plitvice Lakes are burdened with elevated nitrate concentrations already at the sources of the main inflows (Bijela rijeka, Crna rijeka, Rijecica) (Pavlus et al, 2007), so the high concentrations are probably, at least partly, of natural origin. But the distinctive increment trend of nitrate values recorded in the observed period is not to be neglected. Although the nitrate maximums are far below the values recorded for Vrana Lake (Figure 8), the complexity of travertine forming process at Kozjak Lake makes it extremely sensitive to nutrient concentration increase. Kozjak Lake Vrana Lake 1,4 7,00

1,2 6,00

1 5,00

0,8 4,00

0,6 3,00

0,4 2,00 (mg/L) - Vrana Lake -Vrana (mg/L) (mg/L) - Kozjak Lake -Kozjak (mg/L) 3 3 0,2 1,00 NO NO

0 0,00 Jan-00 Jan-01 Jan-02 Jan-03 Jan-04 Jan-05 Jan-06

Figure 8. Comparative overview of nitrate concentration oscillations (mgN/L) at Vrana Lake (station V1) and Kozjak Lake (station K1), for the period 2000-2006

As determined from available data on nitrate concentrations, water of Vrana Lake belongs to II and III category at the station V1, and III to IV category at the station V2, with seasonal early-winter peaks that reach up to V category. These peaks can be explained by washing nitrates out from the land into the lake with the first rainfalls in the winter period. Unlike phosphorus, nitrate values do not show significant increase in the long-term dry period (2001-2002), but correlation between water level oscillations and nitrate concentrations can be noticed, with the mild delay of water level peaks after the nitrate peaks, especially at the station V2 (Figure 9). Although nitrate concentration oscillations did not show significant correlation between the two lakes, of all the analyzed nutrients only nitrate oscillations in both lakes demonstrated statistically significant correlation with water level changes (for Vrana Lake (V1) r = 0.3982, P < 0.0001; for Kozjak Lake (K1) r = 0.3427, P=0.0313).

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nitrate - V1 nitrate - V2 waterlevel 10,00 250

8,00 200

6,00 150 (mg/L) 3

4,00 100 (cm) H NO 2,00 50

0,00 0 Jan-96 Jan-97 Jan-98 Jan-99 Jan-00 Jan-01 Jan-02 Jan-03 Jan-04 Jan-05 Jan-06

Figure 9. Comparative overview of water level oscillations (H (cm)) and nitrate concentrations (mgN/L) at the station V1 and V2 at Vrana Lake, for the period 1996-2006

Nitrate values for Vrana Lake in the available 11-year period demonstrate a slight decrease. This trend must be observed in the context of nitrogen cycling in the metabolism of planktonic community, for which the only existing data refer to 2003 (Faculty of Science, 2004) and 2004 (Peros- Pucar, 2006). Another major factor is the change in the intensity of anthropogenic pollution: in the last 10 years there has been a decrease in the intensity of agricultural production in the catchment, and a transfer from mostly vegetable cultures to mostly arable crops, which demand lesser quantities of fertilizers.

Ammonium concentration oscillations do not show correlation with water level changes, neither with changes in nitrate and total phosphorus values. Ammonium shows stabile values for Kozjak Lake in the period 2000-2006 (Figure 10), with no significant increment trend, and based on this parameter the water normally belongs to I category. Considering the fact that ammonium points to the first steps of organic compounds mineralization, its relatively low values in the upper layer of epilimnium are expected. This is especially true after the sanitation of the communal water system, which was a constant source of pollution in the 1980-ties.

Water of Vrana Lake belongs to the I and II category (V2), and II and III category (V1), according to ammonium values. There is a trend of increase, but mainly due to individual maximums reaching up to IV and V category. Very high values are recorded at the station V1 in the summer months of 1998 and 1999, and can be assigned to influence of wastewaters of the nearby camp. This camp is less visited in the recent years, which could explain the absence of such summer peaks in the following years. Since the start of active protection of the area in 2001, large bird flocks are not to be neglected as a potential impact factor on winter ammonium peaks (number of coots - Fulica atra reaches up to 100 000 individuals in some winters). The weight of this problem is yet to be explored.

Kozjak Lake Vrana Lake - V1 0,3 1,20

1,00 0,2

0,80 0,1 0,60

0 0,40 (mg/L) - Vrana Lake (mg/L) - Kozjak Lake 4 4 -0,1

0,20 NH NH

-0,2 0,00 Jan-00 Jan-01 Jan-02 Jan-03 Jan-04 Jan-05 Jan-06

Figure 10. Comparative overview of ammonium concentration oscillations (mgN/L) at Vrana Lake (station V1) and Kozjak Lake (station K1), for the period 2000-2006

The given results show that values of nitrate and total phosphorus in the two analyzed lakes follow opposite trends. Constant increment trend in nitrate and phosphorus values in the water of Kozjak Lake indicates to continuous increase of pressure on the water system. When explaining the

BALWOIS 2008 – Ohrid, Republic of Macedonia – 27, 31 May 2008 10/14 dynamics of nutrients, important factor is the inflow of water from the epilimnium of productive Proscansko Lake and other Upper Lakes, as well as from coastal areas of Kozjak described as highly productive because of small depth. Although Kozjak Lake is by volume and surface quite an inert lake, it is also a flowing system. Vrana Lake catchment does not suffer from a significantly increased pressure of land usage in the observed period, partly due to prior war happenings in the area. Protection of the area in the form of a nature park has even brought alleviation due to regulation of activities in the Park’s area. Because of this, the lake shows no continuous increase of pressure on the water system in nutrient concentration changes, but rather shows high individual peaks of nitrates and ammonium, as results of seasonal or acute pollution events. Unfortunately, the protected area includes only a narrow coastal zone around the lake, which makes it realistic to expect future increase of unfavourable activities in the larger part of the catchment that is outside the protection area. Next to this, the most concerning is the correlation between the water level changes and total phosphorus concentrations. This suggests that the phosphorus level in the water rises significantly during long-term dry periods, influencing the total trophic potential of the lake, which in such shallow lakes can cause a shift from a clear – vegetated state to a turbid – phytoplankton dominated state (Scheffer, 2001).

Conclusion

Given paper presents analysis of long-term data and comparison of dynamics of certain parameters on a seasonal and annual level. The results of the analysis set grounds for further investigations of chemical parameters relevant for the trophic state of the lakes Vrana and Kozjak. Oscillations of nutrients as indicators of water quality are presented for a longer time period. Comparison with oscillations in hydrological parameters did not allow conclusions on firm interrelations, except for nitrates which show certain correlation with hydrological conditions. Vrana Lake and Kozjak Lake show different dynamics, with correlation of seasonal oscillations noticed only for total phosphorus. On a longer time-scale Vrana Lake demonstrates improvement of water quality according to total phosphorus and nitrate values, but these nutrients are not realistic indicators of actual system pollution. Ammonium, as a more objective indicator of recent pollution and direct anthropogenic influence, shows distinctive maximums values in irregular terms. Unfavourable impact on the trophic state of the lake is noticed during long-term dry periods, in the sense of total phophorus values increment. Unlike Vrana Lake, Kozjak Lake demonstrates an increase in nitrate and total phosphorus values, present probably due to natural cycling of biogenic elements, as the main pollution sources have been sanitated. Ammonium dynamics is stabile through the period 2000-2006, not indicating recent anthropogenic impact. This paper shows the importance of long-term and continuous monitoring of defined parameters, because only in that way it is possible to make comparative analysis, bring conclusions and gudielines for management of protected areas, and estimate the efficiency of undertaken sanitation and protective measures in the impact area.

BALWOIS 2008 – Ohrid, Republic of Macedonia – 27, 31 May 2008 11/14

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Appendix 1. Extract from table showing limiting values of physical-chemical parameters for different water quality categories (NN 77/98) www.nn.hr

Moderately Parameter Oligotrophic Mesotrophic eutrophic Eutrophic Hipertrophic I category II category III category IV category V category 0 .025 - < 0.01 0.01 - 0.025 0.06 – 0.15 >0.15 Total phosphorus (mgP/L) 0.06 Ammonium (mgN/L) <0.1 0.1-0.25 0.25-0.6 0.6-1.5 >1.5 Nitrates (mgN/L) <0.5 0.5-1.5 1.5-4.0 4.0-10.0 >10.0

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