LAKES IN THE CROATIAN LIMESTONE REGION

Milivoj PETRIK

SUMMARY A discussion is presented on the permanent natural accumulations of water on earth’s surface in the limestone region of Croatia. Such accumulations are divided into two groups: lakes proper, and small accumulations called “Oka”. The discussion covers all lakes of that region. They are: 1. the lake Vrana on the island of Cres, 2. the lake Jezero on the island of Krk, 3. the lakes of Plitvice, i.e. the 14 larger basins, 4.the lake Vrana at Biograd in Dalmatia, 5. the lakes along the river Krka, 6. six lakes in the environs of Imotski, in Dalmatia, 7. the lakes of BaCina, in Dalmatia. Because the smaller accumulations, often called “Oka”, are too numerous to be discussed in their totality, only a representative series of the more interesting ones are discussed, viz. those at Sv. Stjepan in Istria, at Rijeka, Slunj, Ogulin, in the region of the lower Krka river, at Imotski and those in the region of the lower Neretva rive?. The discussion includes-as far as they are at present known-data on geographic position, altitude, general morphology, geology and hydrology, genesis, quality of water, temperature, dissolved oxygen, free carbon dioxide, total, carbonate and non- carbonate hardness, alkalinity, calcium, magnesium and chloride, transparency and productivity. Seasonal stratification of water is also discussed. Finally, an attempt is made to arrive at the common characteristics of the lakes in that region.

RESUME

Les lacs de la region cnlcnire de Croatie Cette étude porte sur les accumulations naturelles permanentes d’eaux super- ficielles dans la région calcaire de Croatie. On distingue les lacs proprement dits et les accumulations d’eau moins importantes appelées «Oka ». Tous les lacs de cette région sont étudiés, à savoir : (1) le lac Vrana, dans l’île de Cres; (2) la lac Jezero, dans l’île de Krk; (3) les lacs de Plitvice, c’est-à-direles 14 bassins principaux; (4) le lac Vrana A Biograd en Dalmatie; (5) les lacs situés sur le cours de la Krka; (6) six lacs des environs d’hotski, en Dalmatie; (7) les lacs de BaCina, en Dalmatie. Les N Oka )) étant trop nombreux pour etre étudiés en totalité, l’auteur ne considère qu’un ensemble représentatif des plus intéressants d’entre eux : ceux de Sv. Stjepan en Istrie, de Rijeka, Slunj, Ogulin, dans la région du cours inférieur de la Krka, d’Imotski et du bassin inférieur de la Neretva. L’auteur fait en outre le bilan des connaissances actuelles sur les points. suivants : situation géographique, altitude,morphologie générale, géologie et hydrologie,origine, qualité de l’eau, température, oxygène dissous, quantité totale de gaz carbonique libre, dureté en carbonates et non carbonates, alcalinité, calcium, magnésium et chlorure, transparence et productivité. La stratification saisonnière des eaux est égale- ment examinée. L’étude se termine par un essai de définition des caractéristiques communes aux lacs de la région calcaire de Croatie.

Lakes are defined as large or extensive bodies of water entirely surrounded by land, occupying a depression in the land’s surface. The definition is vague because the terms “large”, “extensive” and “depression” can include various orders of magnitude. The line between lakes and marshes, and between large and small bodies of water is not sufficently sharp. In this paper, we shall deal with such bodies of water which are deep enough to exclude development of aquatic vegetation except along their shores and sufficiently wide to be forced to the use of boats for their exploration. The number of such bodies of water is large in the limestone region of Croatia.

565 For this reason, we shall consider only non-artificial and perennial bodies of sweet water; of these, all easily accessible, major bodies of water will be included, but of the smaller bodies, called ‘‘Oka” and “jama”, only some characteristic examples will be given. The geographic position of them is given in figure 1 to 4 of our paper on “Charac- teristics of water from sources in the limestone region of Croatia” P).

THELAKES OF PLITVICE The lakes of Plitvice make a series of 14 larger and a great number of smaller lakes, arranged in single or multiple cascades. For the eight largest lakes, the principal data are given in table 1. The lakes lie in the uppermost part of the original canyon of the river Korana, as is shown on figure 1. Their orjgin is due to the growth of barriers built by carbonate

Fig. 1 -The lakes of Plitvice: situation and longitudinal section along the line of greatest depths. depositing mosses, as was shown by I. Pevalek (2* 3). In the opinion of J. Roglié their growth started after glaciation, on places favourable for their development (‘9,and is still in progress; it produces, sometimes interrupted by breaks of barriers, slow but constant changes in the altitudes and dimensions of the lakes. In figure 1 is visible how the bottom of the lakes follows the bottom of the original canyon; also are visible the imposing measures of the barriers, both those which are still active and also some of the submerged barriers. On figure 2 is shown a dead lower part of a still active, huge barrier. AI1 lakes on the upstream side of the largest lake, Kozjak, are situated in dolomite, all lakes on the down-streamside in limestone, and the lake Kozjak itself is in dolomite along its western and southern shores, and in limestone on its eastern side. According to A. Pollak (5$ 6~ 72), upper-triassic dolomite makes the most imper- meable sediment of the dolomitic region, causing water from the south and west to circulate towards the lakes. It is concordantly covered by jurassic dolomite and lime- stone, and these by upper-cretaceous, permeable, dolomites or limestones, bare of ail springs. The Iakes are fed by water from a number of springs,ali of them in dolomites (1, points 46-49).The circulation of water before its appearance in springs is governed by tectonic factors, especially faults.

566 Fig. 2 -The lakes of Plitvice: the dead lower part of a phytogenic barrier. The lakes below Kozjak are arranged in a single row and their banks are high, almost perpendicular, and in large areas eroded smoothly. Remnants of subterranean water courses still exist in the shape of caves and tunnel trunks on various elevations on both banks; one of the caves is shown on figure 3. These phenomena brought J. Poljak to interpret the genesis of the lower lakes by assuming an original flow of water in their region partly on the surface and partly underground, until erosive and corrosive processes caused destruction of the ceiling of subterranean courses and successively established the present open canyon which was later, by the growth of phytogenic barriers, divided into four lakes. The bottom of the lakes follows the slope of the canyon but is across the lakes nearly horizontal and consists of precipitated carbonate mainly. The total dry weather flow through the lakes is around 600 l/sec. and forms, with the water from the river Plitvica,the river Korana (i, points 50-51). The lakes are oligotrophic. Their main macroorganisms are trout and crayfish. The transparency of the lakes, as measured by the Secchi disc, is between 3.60 and 12.45 m;the lowest values were obtained in the lakes ProSCe and Galovac,the highest, in the deepest region of Kozjak. it was lowest in February-May,highest in October- November.

567 The lakes show thermal stratification. It is most clearly expressed in early autumn. A weil defined epi-,meta- and hypolimnion is established in the lower Kozjak, with the temperature at bottom varying between 4.0 and 4.5"C throughout the year, and at the top between O and 22°C. Less well established is the stratification in the lake ProSCe, the water of which is cooled in summer and heated in winter by the springs and creeks feeding the lake; its summer temperatures at the bottom are around 6°C.

Fig. 3 - The lakes of Plitvice: remnants of a former subterranean water course in the region of the lower lakes.

For this reason, the lake is covered by ice in winter-if at all-for a shorter period than the lake Kozjak, the altitude of which is 100 m lower than that of ProsCe. The lakes of 10 m of depth do not show any stratification. As is evident, the lakes are holomictic, with two overturns. In examinations carried out in all seasons in the period 1952-1960,in the deepest regions of the various lakes, from top to bottom, the concentration of oxygen varied between 0.4 and 14.7 mg/l, but were commonly between 8.6 and 1.8 mg/l, the satu- ration varying between 82.5and 125.8%. Graphic presentation of oxygen concentrations is of the clinograde type.

568 As a rule, no carbon dioxide was present. The mineral content varied as shown in table 2. The percentage of magnesium hardness in total hardness discloses water from dolomite with some limestone which is in full agreement with the geological conditions. During the flow from the springs through the lakes into the Korana, the water loses some carbonate hardness through precipitation and utilization by barrier-building mosses and algae. The values of carbonate hardness were in the springs between 221 and 291 mg/l,in the lake ProlCe on the surface 230-240,in the lake Kozjak 204-218,

in the river Korana below the lakes 200, and at Slunj 196 mg/l ('J 10). No nitrogen at all was found in the lakes, and the consumption of oxygen from KMn04 was below 2 mg/l.

Fig. 4 - The lake Vrana on the island of Cres: View along the longitudinal axis.

THELAKE VRANAON THE ISLAND OF CRES

Among the Adriatic islands, there are several with accumulations of water in the form of lakes, as on Cres and Krk, or marshes, as on Krk, Pag, KorEula and Mljet. Of these accumulations,the lake Vrana on Cres is by far the largest one;it belongs also to the largest lakes in Croatia. Its geographic position is 44"51' N and 17"23' E. According to the results of comprehensive geodetic and other measurements organized by the present author in 1953-1956(li, 12, 13), the older data on its elevation and depth which still persist in literature, should be corrected. The elevation of its surface oscillates around 12.25 m above the sea and may be up to 2.5 m higher or lower, according to meteorologic conditions (li, 13). The form of the lake is shown on figure 5. Its length is about 5200 m,its greatest width 1500 m,and the surface around 5.75 km2;its volume of water is about 0.28 km3. The slope of the bottom is steep on the eastern and western flank and mild on both ends. At 40 m below the sea level the slope goes over into a flat bottom, nearly horizon- tal; its lowest point is 42.8 m below the sea level. At the south-easternend of the lake,

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1 I I lI .I Il I_ I I I I I I l I I I there is a funnel-shaped depression in the bottom; its lowest point is 61.5 m below the sea. Along the flanks, the bottom is rocky, but on mild slopes and in its flat part it is muddy and consists mainly of precipitated carbonate; it is covered by a Chara vegetation (I4). Another characteristic organism of the lake is Ceratium (Is). The lake is situated in a cryptodepression which constitutes a submerged "polje" without sufficient drainage capacity which was, in its lower part, transformed into a lake when the rise of the sea level caused a decrease of the seepage of water. The relatively high mean elevation of the lake surface is determined by low permeability of the lake's flanks. As shown by A. Magas, the area of the lake's watershed is built of three main geologic members: limestones of upper-cretaceous and lower-cretaceous age, with transitional dolomites between them. Originally, they were arranged in normal sequence, forming a surface warping in a series of alternating synclines and anticlines of dinaric strike. After eocene, that system was disturbed by a series of parallel thrust faults, of the same strike. The outcome was the present statea sequence of series, all dipping to the east, each having lower-cretaceouslimestone covered by dolomite, and dolomite by upper-cretaceous limestone, with insignificant areas of eocene and quaternairy. One of the faults runs along the western border of the lake and is inter- sected by another fault striking along the longitudinal axis of the lake. These two faults seem to have predisposed the present lake, at a time when the sea was much lower with respect to land, and caused disintegration of rocks in the area of the lake. They also provided the way for subterranean drainage of that area. Subsequent erosion cleaned away the debris and established the present basin of the lake. It would seem that the entrance to subterranean drainage channels was in the depression of the bot- tom of the lake which successively became clogged, thus causing accumulation of water in the basin (16). The drainage area of the lake is partly forest, macchia and bare rocks, it is very sparsely inhabited, with an insignificant cultivated area. The only water which enters the lake is derived from rains within the watershed which has an area of some 33 km2. The water washes away the salt deposited on the surface with sea spray carried inland by winds, especially by the strong north-easterly,the "bura". The lake can lose water-besides by evaporation-only by subterranean flow. That flow may be either concentrated on one or several points, or more or less uniformly distributed along the flanks, but not through the bottom which consists of thick deposits of extremely fine precipitated carbonate. Since there are no submarine sources of greater discharge known up till now around the northern part of the island, it does not seem impossible that the lake loses water through its steep lateral banks, the seepage being distributed along them. It has been shown that the rate of the seepage is in linear relation to the elevation of the lake surface, as is the case with the flow of water through fine-grained media, but that the coefficient of permeability is much lower; the average rate of seepage was shown to be around 570 l/sec. (I1). The transparency of the lake water was found to vary between 17.65 and 23.85 m in March, and between 10.15 and 15.1 min summer. The temperature on the surface of the lake, as measured by Casagrandi (15), Nümann (I7) and the present author (12), was at the end of July and in August, on the surface between 22.2 and 24.8"C, and at the bottom between 6.1 and 9.0"C;in March it was at the surface between 4.7 and 7.4"C, and at the bottom between 4.5 and 6.5 "C.The temperatures at the end of winter show the lake to be holomictic and monomictic; the mixing takes place at the lowest temperature to which the lake is cooled in the course of the winter. In summer, there is a very well defined thermal stratification of the water, with a thoroughly homogeneous epilimnion 9 to 10m deep at the end of July. A perfect thermal and chemical homogeneity of the lake is established at the end of the winter.

571 As to oxygen in the lake,the whole lake is, at the end of winter, practically saturated. In high summer, the epilimnion is a little supersaturated,the metalimnion markedly supersaturated,and the hypolimnion still between 80 and 100 per cent of saturation. The curve of the oxygen concentrations is orthograde. The oxygen disolved from the atmosphere in the period of mixing is used up very slowly in summer, and the pro- duction of oxygen by phytoplankton is very low. Total hardness varies in July between 130 and 150 mg/l, the lowest values being found in epilimnion. In March 1956 it was 139 mg/l in ali determinations and at ali depths. Non-carbonatehardness is very low,between 25 and 29 mg/l,and magnesium hardness 21 mg/l,or 15% of total hardness,which indicates a drainage area of dolomite and limestone in exact correspondence to the geology of the drainage area. Chloride was found in July 1954 at all points and depths between 69.2 and 72.2mg/l, and in March 1956 in 20 determinations on 3 points and at all depths at 69mg/l. Since it was present at all depths in the same concentration, it cannot be taken as a

Fig. 6 -The lake of Visovac: view across the lake at the point of the greatest width. sign of the intrusion of sea-water but must be considered as brought in by winds, with the spray, deposited on the vegetation and rocks, and washed into the lake by rain water. Nümann found on five occasions between 63 and 66 mg/l, also at all depths (17). No nitrogen was found; the consumption of KMn04 was below 2.5 mg 02/1. Nümann found in the lake between 1.8 and 20.4 gramatoms Si!m3, and phosphorus and iron only in traces. All characteristics of the lake show it to be highly oligotrophic.

THELAKE JEZERO, ON THE ISLAND OF KRK

In a shallow depression in the background of Njivice, there is a lake which once covered a much larger area but was, for agricultural reasons, in part drained by a

572 tunnel into the sea. The elevation of the lake surface was by that measure brought down to 1.1 m above the level of the sea, in dry weather. The geographic co-ordinatesof the lake are 45" 10' N and 14" 34' E. The shape of the lake is very roughly rectangular. At the elevation of 1.1 m it measures in length 740 m and in width about 600 m. The slope of the bottom and of the surrounding area is so small that the borderline of water is ill recognizable,on account of reed. The basin consists, in fact, of two basins divided by a submerged reef, also marked by vegetation. The lake covers an area of about 25 ha. Its greatest depth is 8.3 m,at the gauge reading of 1.1 m;the deepest point is thus 7.2 m below the level of the sea. The lake lies, therefore,in a cryptodepression covered by alluvial sediments. The substratum is an anticline the core of which consists of cretaceous breccias and limbs of upper-cretaceous limestones.The lake is in the eastern limb of the anticline which, further on,dips under eocenic limestones; the western limb dips under the sea. The drainage area of the lake extends further on to the south and is built of cretaceous limestones and breccias, while eocenic layers of shale probably act as impermeable barriers (la). Onthe very border of the lake, towards the west, a sinkhole is coming into action if the lake surface reaches the elevation of 2.4 m above the sea. The lake is fed by surface drainage from the east, and by underground drainage from the south. Close to its southern border are two springs, in alluvium, which always deliver water to the lake (I, points 97-98;the lake itself is point 96). Surplus water is evacuated from the lake through the tunnel into the sea; at elevations of its surface above 2.4 m the sinkhole also comes into action. Because of the swampy character of the lake borders, it is difficult to put a workable boat on the lake, which is probably the reason why it has been examined only partially. At an examination conducted by the present author in August 1964, the results of table 3 were obtained. Oxygen and carbon dioxide show concentrations usual in swampy waters. Alkalinity and hardness are much lower than in springs on the lake border, and so is salinity. The latter was, however, in observations made in October 1959 on many points and at depths from O to 7 m found to vary between 90.0 and 95.6 mg/l. That is not very much higher than in the lake Vrana; the origin of the salinity is in both cases probably the same. The percentage of magnesium hardness to total hardness is in accordance with the nature of rocks in the possible drainage areas. The essential difference between the hardness of the lake and of the springs on its border may be caused by the r6le of surface drainage from the east which is periodical and could not be examined in August 1964.

THALAKE VRANAIN DALMATIA

Northern Dalmatia consists, in a great measure, of a series of synclines and anti- clines which strike parallel to the coast. The bottoms of synclines are covered by more or less impermeable eocenic sediments. Between Biograd and the Bay of Sibenik, in the first syncline from the coast, the lake of Vrana is situated, at 43" 53.5' N and 15"35' E. In the past, it was very much larger until an outlet into the sea was built in 1897, for agricultural reasons, which caused a lowering of the lake surface. It is fed by water from a number of springs situated beneath the north-eastern limb of the syncline; they deliver water from the hinterland. It loses water not only through its artificial outlet, but also-according to Gavaui (19)-through sinkholes on its bottom. It drains a vast agricultural area and at present it is used for raising fish. So it lost its natural state of purity a long time ago and in its present state it is of no great interest, in spite of its still great surface area. By Gavazzi's measurement, its depth is 3.9 m.

573 THELAKES OF THE RIVER KRKA

Onseveral points along the river Krka barriers of phytogenic origin can be observed (20), some of them of imposing dimensions, which divide the course of the river into several lake-like sectors. In its lower course, above Skradin, there is such a barrier, the greatest of all, having a total height, between the lower and upper water surface, of 45 m.It is called Skradinski Buk-the falls of Skradin. The rise of that barrier caused a rising of the water level on its upstream side in such a measure that two lakes were created: the lake Visovac, on the river Krka, and the lake of Cikola, a river tributary of the Krka which joins the Krka just above the falls of Skradin (i, points 126-127 on fig. 3). The two lakes are connected at their lower ends and are on the same elevation of 45 m above the sea. They are, in fact, parts of the two river courses, but because of their relatively great depth the velocities and the turbulence of flow are very low, which causes stratification of the water as it occurs in lakes.

THELAKE OF VISOVAC

It is situated in the canyon of the lower Krka,between the falls of Skradin and those of Rogovo (RoSki Slap), which latter are on another travertine barrier, on the upstream side, 23.5 m high (20). The lake is a little longer than 12 krn; its width and depth in various sectors are given in table 4. The greatest depth is in the most narrow stretch of the lake, the smallest ili the widest stretch. B. Petrik, S. Sirca and I. Degkovik found in August 1957 at various points and depths of the lake the main characteristics of its water as shown in table 5. Both ternper- ature and dissolved oxygen showed stratification. Temperature below 19 m of depth was in general essentially different than above that depth; for oxygen, the criticai depth was around 12 m.

THELAKE OF THE CIKOLA

That part of the canyon of the river Cikola which is below the elevation of the barrier crest of Skradinski Buk has become the lake of the Cikola. Its length is about 3.5 km,and the maximum width about 650 m. The greatest depth at the confluence with the lake Visovac is 28 m. Just beside the uppermost part of the lake is Torak, a strong, perennial spring of the peculiar shape of an “oko” (i, point 126). Cikola itself. on the upstream side of the lake, may run dry in rainless periods. The deepest point of the cone of Torak is at 2.6 m below the level of the sea, and the deepest point of the lake Cikola is i7 m above the sea at its mouth into the lake of Visovac, and 34.3 m above the sea at the distance of 750 m downstream of Torak. The deepest point of the “oko” of Torak is, therefore, nearly 20m below the deepest point of the lake which is fed by it and nearly 37 m below the lake in its upstream part. Both Torak and Cikola were examined in August 1957 by the same investigators as the lake Visovac. The water of Torak originates in pure limestone, that of the Cikola corresponds to dolomitic limestone.

THELAKE PROKLJAN

In the course of time, the eastern coast of the Adriatic came partly under the level of the sea, by the rise of the sea or by the lowering of the continent, or by both (21).

574 In that process, the valleys of the Dalmatian rivers Zrmanja, Krka and Neretva were partly submerged. The subterranean circulation of water in fissured carbonate rocks was at that time already well developed, as is witnessed by many submarine springs which still function, and-among others-also by the “oko” of Torak. The valley of the river Krka below the Skradinski Buk also came under the sea; in this way the lake Prokljan was formed in a flat part of the Krka valley. It is called a lake but is, in fact, an enclosed part of the sea, filled by sea water except in the upper 2 metres, as was found by the above-mentionedinvestigators in 1957 in that part of the Krka which is between the falls of Skradin and the lake of Prokljan. The variations of temperature and oxygen dissolved on two points of that sector, from top to bottom, are shown in table 6, together with the variations of salinity. The greatest depth of the lake is 21 m (22).

THELAKES OF IMOTSKI From the standpoint of the hydrography of limestone regions, the lakes of Imotski are very interesting and instructive indeed. A long and important line of tectonic disturbance seems to extend from LjubuSki over Grude and along the north-eastern border of the “polje” of Imotski towards BuSko Blato. Along that line, there is a number of hydrogeologic phenomena: springs, sinkholes, “ponikvas” (23, 24). Among them are the springs of the river Vrlika (i, points 150-152) at the altitude of 268 m above the sea, and a number of young “ponikvas”, not yet filled up and containing water. These are known as the lakes of Imotski. All of them are in Dalmatia, close to the border of Hercegovina, and along the border of the “polje” of Imotski (see 1, fig. 4). For the most important of them, the main data are given in table 7. Except the run-off from their geographic drainage areas, which are extremely small, all lakes receive and lose water only subterraneously with the exception of the lake of Proloiac (ProloSko Jezero, also called Imotsko Blato) which also receives the discharge of the river Suvaja,and loses water partly into the river Vrlika. According to J. Poljak (26), sinkholes in the north-eastern hinterland of the polje, within the watershed of the Suvaja river, may lose water into these lakes, but other sources are not excluded. Since the Modro Jezero (the Blue Lake) reaches an altitude of 342 m, it must receive water from sources higher than that, but could be fed, in addition, also by lower sources. On some bodies of water in this region, colour was observed after it was introduced into the water at the sinkholes of BuSko Blato (27). The relations between the levels of the various lakes at different times and in simultaneous observa- tions can be seen, for some characteristic periods, jn table 8. In July 1955 was the water level in the greatest lake,that of Proloiac,just on the level of the springs in the “polje” but in the three next greatest lakes it was very much lower. In September of 1955 Modro Jezero was nearly 30 m below them,if not more, because at that time the lake had run dry. In September 1956 were the levels of all the lakes observed also below the level of the springs;in March 1957 were, however,all lakes above the spring level. The differences in the levels of individuai lakes at various times were also great, and so were those between different lakes at the same time: Galipovac was in 1955 higher than Modro Jezero and lower than the lake of Proloiac,but in 1957 it was lower than Modro Jezero and higher than the lake of Proloiac. The observed levels of Crveno Jezero show fluctuations of 20 m,and in the Modro Jezero of more than 100 m. The hydrography of Imotski was A. Grund’s argument for his conception of the “Karstwasser” (28). One may wonder what modifications his theory might have undergone, if such data had been accessible at his time. The “ponikva” of Crveno Jezero (fig. 7) is of cylindrical form. This fact makes us believe that it is the youngest among them. It is 519 m deep, the highest point of the

575 circumference being at 522.9 m and the deepest point of the bottom at 4.1 m above the level of the sea. The surface of the water oscillates around the middle, that is about 260 m above sea. Its rocky wails are on three sides nearly perpendicular in their entire height, and on the fourth side in the lower half. The bottom is rocky and slopes to the west. The earthquake of 1942 caused a large portion of the western wall to drop into the lake. The surface of the lake has an area of about 33,000 m2;the water volume is more than 6,000.000m3.

Fig. 7 -The Red Lake at imotski: view from the rim of the “ponikva”.

The “ponikvas” of the other lakes are older; the older the lake,the more advanced is the process of filling up the cavity of the “ponikva” by disintegration of its walls. All other lakes have the form of inversed truncated cones, with horizontal and flat bottom built up of precipitated carbonate, and steep slopes. The largest of them is the Modro Jezero (the Blue Lake), in which the observed level varies between less than 239 and at least 342 m above the sea level. The KnezoviCa Jezero has very steep and unstable sloping walls and is difficult of access which is the reason why it could not have been measured and investigated.The lake LokviTiC has less steep walls; the water has an observed depth between 30 and 40 m. Galipovac has between 40 and 50 m of water. The lake of Proloiac has been filled up with rocks and sediment above the lower part of its brim and presents now a wide expanse of water the depth of which varies between 25 and 30 m.it has a perfectly flat bottom. All of these lakes are in upper-cretaceous limestones which lie on dolomite. In the opinion of J. RogliC, so far the most thorough investigator of this region, the subterranean water courses, at the contact of limestone and dolomite, corroded the limestone and eroded the dolomite so far that the ceilings of the large cavities succes- sively collapsed. That process is still active, as witnessed by a sinking of the ground, caused by the earthquake of 29 December 1942, and also by two minor “ponikvas” which appeared a few years ago in the plain of the Jezero of Proloiac (29).

576 As the deepest point of the Crveno Jezero is at 4.1 m above the sea, the true, orig- inal bottom must be still deeper, because the ground is covered by the material of the ceiling and walls, and also by that rolling down the slope below the eastern part of the brim. It should lie, therefore, below the present level of the sea.

3 3' 4 12.VII 1955. 1

/ 4.1 I' v

Fig. 8 -_The Red Lake at Imotski: cross-section.

The contact of water in the lakes with both limestone and dolomite is confirmed by the percentage of magnesium hardness in total hardness, especially for Crveno Jezero, as can be seen in table 9. From the values for total hardness, in the same table, it is discernible that the lakes Crveno, Galipovac and LokviEiC receive water from greater depth than Modro and ProloSko Jezero, or-possibly-from differentsources;

577 for such conclusion speak the great differences in hardness both at the surface and at bottom, as also variations in hardness at the bottom. All lakes show homogeneous water from top to bottom, its hardness increasing with depth, as is usual, but with all characteristics nearly constant in hypolimnion. Only the lake of Proloiac displays heterogeneous water in various depth regjons. All show thermal stratification with well expressed epi-, meta- and hypolimnion. The overturn may occur at temperatures higher than 4.0°C,as is shown by bottom temperatures of all lakes; in that case, the lakes would be monomictic. Ali of them are holomictic. With the exception of the lake of Proloiac all lakes are oligotrophic, with transparency varying between 8.6 and 13.05 m while in the lake of Proloiac it varied from 0.3 to 1.35 m (table 9). A comparison of the characteristics of water from lakes and from springs shows about the same hardness in springs as in Crveno Jezero, Galipovac and LokviEiC, with a smaller percentage of magnesium hardness, but a higher hardness than in Modro Jezero and ProloSko Jezero.

&INSMA JEZERA

Fig. 9 -The lakes of BaCina: situation with isobates.

It should be mentioned here that some of the dye put in one of the sinkholes of the BuSko Blato on 31 December 1960 appeared on three points at Imotski-in the lake of Proloiac, in the spring Opatac and the periodic spring at Grude (i, points 150 and 158) which fact discloses somehydrographic connexion between the two regions (30). Since there is always oxygen present at all depths of all lakes, with the saturation at bottom above at least 35%, it can be taken that parts of the underground channels are in contact with outer atmosphere.

578 All observations are in harmony with the assumption of the sole, or principal, entrance of water at the bottom of the lakes, with the exception of the lake of Proloiac, but that lake receives also surface water and-seemingly-underground water, above the bottom. In fact, on the bottom of Modro Jezero are clearly visible holes which perform the double function of feeding and draining the lake. In Crveno Jezero, Modero Jezero and the springs in the “polje” there were found only few microorganisms and bacteria. The density of bacteria in Crveno Jezero increased with depth. Fishes are present in that lake.

THELAKES OF BACINA

The lakes of Bafina are situated in the immediate hinterland of the port of Plote, at the southern end of the Biokovo range;they are separated from the port by a saddle in the coastal hills and from their main drainage area, the periodic lake of Vrgorac, by another ridge of the Biokovo. They consist of five basins, four of which are inter- connected and the fifth isolated (fig. 9). Their geographic position and main measures are given in table 10, for the surface at the elevation of 0.8 m above the sea.

Fig. IO -The lakes of Batina: view across the two middle lakes taken from south towards north.

579 The lakes fill a string of cryptodepressions of irregular form. Their bottoms are built mostly of precipitated carbonate and some alluvial material and are in wide areas flat and nearly horizontal, especially in the basins OhSaand Podgora;the lake CrniSevo shows in its southern part a depression some 5 m deeper than its ñat surroundings. The sides of the hills around the lakes are rocky and steep; they are built of jurassic limestones and constitute a quite small geographic drainage area (31). The hydrologic drainage area is much wider: the lakes receive water from the periodic lake of Vrgorac, called Jezero, which is situated in a deep “polje” south of Vrgorac, with sinkhoes at its southern end, nearest Badina, 25m above the sea, and springs on the northern side; these springs, again, receive water underground from a stili farther “polje” to the north of Vrgorac, called Rastok (23). Geologic structure of these regions is of upper-cretaceous age. The present hydrologic régime of the Batina lakes differs from the past one because, in 1912, a tunnel was constructed from the basin Sladinac into the port of Plote in order to lower the level of the lakes, and in 1940 another tunnel was made from the Jezero at Vrgorac into the basin Podgora,for agricultural reasons. Consequently,the present measures of the lakes, especially depth, and even the number of basins, greatly differ from the data in older literature (32). In the present, as in the past, the level of the lakes is governed on one side by meteorologic conditions and on the other side by the head necessary to overcome the friction and other losses of head in the incoming and outgoing channels of underground flow. Beside the artificial incom- ing and outgoing channels,the old natural subterranean ones are still in function and are, in dry periods, the only active channels. Even with the two tunnels in function, the surface of the lakes varies between 0.8 and 2.94 m above the sea, the lower value corresponding to dry weather conditions and the higher to rainy periods in late autumn (33, 34). The old, natural subterranean flow into the lakes appears in the springs around the lakes, in first line in the spring Klokun, but also in two springs on the shore of the basin Ohla, and finally in a number of saline springs in the basin of CrniSevo (i, points 188-191). The main characteristics of the water in the various basins and springs are shown in table 11. The springs show the saturation with oxygen to vary between 37 and 99%. The perennial and strong spring Klokun has the highest hardness, excepting the saline springs on CrniSevo. All non-saline bodies of water show-by the percentage of their magnesium hardness-an origin in dolomitic limestone, or limestone with dolomite. The basin of CrniSevo contains water the character of which is altered by its saline springs;in a smaller measure they alter also the character of water in Sladinac,through which basin the water of all other basins flows to the sea. The isolated, small basin of Vrbnik also shows higher salinity which finds, so far, no explanation in observed facts. The deepest part of CrniSevo, below -23 m,shows presence of sea water; at one occasion, there was below -27 m about 50% of sea water in the lake water. There is a very clear thermaI stratification of water in the three deepest lakes, but a proper hypolimnion is found only in CrniSevo. The epilimnion was in CrniSevo around 7 m deep in the beginning of August and about 11 at the end of September; in OCuSa and Sladinac it varied between 9 and 13 m.In the lower part of the epilimnion, and in metalimnion of CrniSevo, there was supersaturation with oxygen with a maxi- mum,at the end of September in 1964, of 18.4 mg/l or 202% of saturation. In OCuSa and Sladinac, the curves of oxygen concentrations took approximately the shape of the curves of temperature. In CrniSevo, there was usually also some chemical strati- fication of water, not congruent with the thermal one. Thus, on 28 August 1959, there was. an increase of chloride between 8 and 16 m of depth and again on 5 August 1962 between 7 and 13 m and between 15 and 23 m,and on 27 September 1964 between 22 and 28 m.The most saline water was, therefore, not always at the bottom, but on the first occasion at 11 and on the second at 12 m of depth. This was probably caused by

580 the varying combinations of salinity and temperature of the water from saline springs and of the receiving lake water. The transparency was in CrniSevo around 9 m,in OCuSa between 4.7 and 6.6 m, in Sladinac around 5.5 and in Vrbnik 3.5 m. No data on winter conditions are known. It is, therefore,not possible to confirm the opinion that the lakes were holomictic and monornictic, which was formed on basis of observations made on the lakes of Imotski which are on a higher elevation and farther from the mediterranean influence.

THE“OKA”

Besides proper lakes, there are in our region many smaller depressions permanently filled with water which cannot be called lakes very well. They are, on account of their circular shape,often called “Oka” (which is plural of “oko”,and this means eye). Most of them receive water at the deepest point of their bottom, but some,in addition,from springs in their vicinity. Some of them lose water by overflow on their brim, others do not. Since they are too numerous to be treated here all, only a few representative examples will be shown.They are arranged in table I3 with their main characteristics. The best and largest example of an “oko” with constant discharge of water at an overflow is Torak. the already mentioned spring on the border of the lake of Cikola (i, point 126). It is shown on fig. 6. It measures 200 m in diameter and nearly 50 rn in depth. It has the shape of an inverted cone,has a circular surface and receives water

Fig. 11 -The “oko” of Torak. at the deepest point, from subterranean channels. Similar, but smaller, are two basins called Jezerine, at Imotski, close to each other, one of them active, with a conical shape,another inactive,in the shape of a truncated cone,with flat bottom (1, point 152). Wider than Torak, but shallower, is the periodically active “oko” Krenica, near Grude,300 m wide and 41 m deep, also of conical form.The most picturesque of them

58 1 seems to be Modro Oko (the blue eye) in the marshy region of the lower Neretva river, 130 m wide and 24.5 m deep (35), which is also a perennial spring. (1, point 184). Of similar shape are the springs: Sv. Stjepan in Istria, 12.3 m deep, the main spring of the river Cetina, the spring Rude, also a spring of the Cetina, Bili Vir, a spring in the lower Neretva Valley, Bud,another spring in the same valley, some springs of the Gacka river (i, points 35-37,69, 122, 136, 142, 181, 185) and many others. A general characteristic of the true Oka is their position-they are usually in alluvial ground, at the foot of a hilly background, that is: in shallow alluvium. Similar to the “Oka” are basins of irregular shape, of which again some yield water always and others do it periodically or not at all. The last three examples of table 13 are of this type, the first two being active and the third non-active.The spring of the SluSnica river is 72 m long and nearly 27 m deep; the spring of Zvir (fig. 10) is smaller, but in a cryptodepression,like Bubica Jama, at PIomin (1, points 55 m and 84) (38, 37). As most of the “Oka” are perennial or periodical springs, and are situated in alluvium at the foot of a hilly background, it is easy to imagine them in the far past as springs on the side of hills, delivering water into their valleys; as the latter were being constantly filled up, the alluvium reached at a time the level of the spring and was still growing. With strong springs, the jet of water could prevent the alluvial material to cover the spring and gradually caused the cone in the alluvium, around the mouth of the spring. On the other hand, one could imagine them to be ancient “ponikve” filled up initially with rocks from the walls and later by alluvial material. Also, new “Oka” may appear in consequence of the collapse of the ceiling of a cavity; the present author had occasion to visit such a collapse which, however, did not result in the hole being filled with water. Finally, pressure of water underneath an ancient clogged sinkhole may blow away the clogging material, in which case water will fill the new hole and eventually start a constadt flow. Three blow-outs of this kind have also been visited by the author. Irregular “Oka”, and those situated in bare rock, can be imagined as true “Oka” in the initial phase oï their development. The ‘‘Oka”and “jame” which constantly discharge water do not show any thermal nor chemical stratification of water, but inactive “Oka” do, if they are sufficiently deep. The latter also show rich algal population and very small transparency, while the first display opposite conditions.

SUMMARY In the region considered, all of the larger natural accumulations of water which merit the name of lakes contain clear water of low productivity, except those which receive water from inhabited or cultivated areas. So do also smaller accumulations, “Oka”, if they are active,but not those without constant overflow. All of them show water of the hydrocarbonate type with a mineral content charac- teristic îor spring waters of their region. In all of them can be observed the process of precipitation of part of their carbonate hardness in warm periods of the year. If they are of sufficient depth, about 15 m or more, they show clear thermal strati- fitation in summer and are holomictic, with one or two overturns, according to the lowest winter temperature. On one lake basin chemical stratification was observed. In the coastal belt, many accumulations are in cryptodepressions;it seems that their genesis took place at an essentially lower level of the sea than the present one. The genesis of the accumuIations was variable. Some developed in normal river canyons where conditions favoured the growth of phytogenic barriers. Their growth still goes on where it has not been interrupted by human interference. Such lakes receive and lose their water mainly on the surface. In other cases, the origin was caused pri-

5 82 marily by tectonic processes of which extensive faults seem to have been the initial ones, with subsequent erosion or destruction of cavity ceilings. Synclines may also offer conditions favourable to the development of lakes. Lakes of tectonic origin in our region usually receive and lose water by subterranean channels. Whether a lake will develop and subsist depends upon several factors, the main of which are-beside meteorologic conditions-the capacity of incoming and especially outgoing subterranean channels. In the coastal belt, the capacity of outgoing channels may be diminished by a rising of the sea level. That factor probably caused the develop- ment of some lakes and "Oka" in the valley of the lower Neretva river, and in the region of BaCina.

TABLE I Main geographic data on the lakes of Plituice

Geographic Area of Maximum Altitude Lake coordinates the surface depth m ha m N E

ProSCe 636,5 44" 51,5' 15'36' 68,3 37,4 Ciginovac 625,5 44" 52' 15"36' 735 16,l Okrugljak 613,6 44" 52' 15" 36' 41 15,3 Galovac 584,6 44"52,5' 15"36,5' 12,5 24,4 Gradinsko jezero 553,2 44" 52,5' 15"36' 8,1 10,o Kozjak 535,O 44" 53 15"36' 81,5 46,4 Milanovac 523,3 440535' 15"3?' 3,3 18,4 Kaluderovac 505,2 44"53.5' 15"37' 291 13,4

TABLE II Main analytical data on the lakes of Plituice

Hardness of Mg-hardness Lake Depth % in total hardness total carbonate magnesium

ProSCe surface 230-240 228-235 79-197 34,4-44,9 bottom 224-227 218-220 61-73 30,4-48,3 Kozjal surface 204-218 200-205 81-93 38,7-45,2 bottom 215-226 215-226 69-91 3 1,7-40,3

583 TABLE III Main analytical data on the lake Jezero on the island of Krk

Hardness Depth 'C O2 COZ % Of Chloride total carb. Mg Mg-hardness

0,O 23,6 7,8 4 172 150 32 18,6 54 4,7 19,6 0,6 i6 178 165 36 20,2 50 7,s 17,5 0,O 30 234 230 50 21,4 65

TABLE 1V Main geographical data on the lake of Visorac

Sector Distance from Width Maximum Elevation of Temperature the falls of m depth the deepest at bottom in Rogovo m point August 1957 km m

Lake of 0,70 267 17,6 27,4 15,O Visovac 2,73 128 32,2 12,8 14,8 above the 5,90 493 24,3 20,7 14,5 falls of 7,90 243 29,O 16,O - Skradin 11,oo 362 25,4 19,6 14,5

TABLE V Main analytical data on the lake of Visovac

Distance from Hardness the falls of Depth "C Oz COZ Chloride Rogovo, km total curb. Mg

0,7 O 21,l 9,2 O 258 175 50 17,s 15,O 2,0 12 258 190 46 2,7 O 21,l 9,4 O 256 165 28 30 14,s 3,8 ô 244 188 40 599 O 22,4 9,4 O 244 170 58 24 14,5 3,8 10 252 195 42 ll,o O 22,6 9,2 O 240 158 52 24 14,5 4,l 5 257 183 49

5 84 TABLE VI Teinperariire and oxygen dissolued in lake Prokijnn

Oxygen dissolved Point Depth Temperature Chloride "C mg/l of satur. mg Cl/i

1 0,5 22,23 9,OS 103,7 480 13 22,48 11,80 161,O 17.000 2,4 29,80 10,04 164,3 20.500 390 29,47 5,94 96,6 20.500 43 26,92 5,20 81,3 21.250 2 0s 22,92 834 104,4 2.675 16 25,68 9,?6 138,8 15.375 292 27,35 10,12 155,7 19.650 2,s 27-60 8,80 136,6 19.900 52 25,69 7,OO 107,O 21.900

TABLE VI1 Main geographical datn on the lukes of Imotski

Geographic Observed eleva- Elevation coordinates tion above sea Maximum of the Lake deepest point NE max. min. length width

Modro jez. 43"27' i7"13' 283,6 239 0-250 0-100 239,O Crveno lez. 43"27,5' 17"12' 274,5 252,s 220 190 4,1 Galipovac 43"28' 17"06' 270,s 261.9 170 120 210,7 LokviEiC 43"28' 17"06' 271,4 260,9 116 90 229,3 KnezoviC. 43"27,5' 17"06' 272,O 270,5 not measured unknown Jezero Proloiac 43"28,5' 17"07' 271,2 266,2 not well 241,7 defined, great

TABLE VI11 Coniparatioe elecutions of the lakes of Imtoski

Date Modro Crveno Galipo- Prolo& Lokvi- Knezo- Springs in jezero jezero vac ko jez. ëic viC the polje

21 7 1955 250,O 252,s 264,7 268,2 - - 267,7-268,2 26 9 1955 239,O ------2 9 1956 - - 261,9 266,2 260,9 - around 268 20 3 1957 278,s 275,O 270,5 270,2 269,s 272,O 267,9-268,5

585 TABLE IX Main analytical data on the lakes of Imofski

Hardness Lake Depth "C Oz CO2 %ofMg- C1 total carb. Mg hardness

Crveno O 19,s 9,7 O 160 155 57 23,l 6,O 10 7 55 200 9,4 7,4 2 212 200 49 23,l 7,O Galipovac O 24,l 8,7 O 132 130 18 13,6 5,0 20 7 55 42 9,O 5,6 5 190 188 26 13,7 5,O 1 9 56 O 22,l 8,O o 148 130 16 10,8 5,6 45 6,5 43 3 21 3 175 22 10,3 6,5 LokviCii: O 26,O 8,7 O 112 108 18 16,l 6,5 3 9 56 30 935 5,9 1 220 180 30 13,6 6,5 Modro jez. O 18,O 10,l o 163 145 26 16,l - 7 6 58 34 7,2 8,7 4,4 178 160 30 16,9 - ProloSko j. O 23,7 7,4 O 128 120 48 37,5 7,O 11 7 55 20 7,7 7,6 O 124 124 24 19,3 6,5 28 8 56 O 28,3 9,2 - 118 118 30 25,4 7,O 24 5,6 6,O O 180 158 26 14,4 6,5 30 8 56 O 25,5 9,0 O 124 123 28 22,6 6,O 23 5,8 7,O 5 176 170 32 18,2 5,6 12 2 58 O 7,7 12,6 O 138 135 16 11,s 5,6 27 4,6 12,8 O 140 135 16 11,4 5,6 9 9 59 O 22,3 8,6 O 120 120 20 16,7 8,6 24,5 8,O 4,4 6 172 165 20 11,6 73

TABLE X Main geographical data on the lakes of Badina

Basin of Geographic coordinates Maximum Elevation of the lakes the deepest N E length width depth point

Crniievo 43"05' 17"25' 1110 680 30,6 -29,s OCuSa 43"05' 17"26' 1250 540 19,6 -18,8 Sladinac 43"05' 17"26' 560 345 16,9 -16,l Pogora 43"04' 17"26' 390 325 10,1 - 9,3 Plitko jez. 43"05' 17"25' 1160 170 595 - 4,7 Vrbnik 43"05' 17"25' 275 160 8,7 - 7,9

586 o”

587 TABLE XII Main geographical data of some "Oka" and "jarne"

Geographic coordinates Maximum Elevation Name Elevation of deepest N E m length width depth point

Torak 43"49' 16"Ol' 45 202 171 41,5 -2,6 Jezerine N 43"28' 17"lO' 268,6 43,6 43,l 13,2 255,4 Jezerine S 43"28' 17"lO' 268,6 49,l 44,l 6,O 262,6 Modro oko 43"03' 17"31' 0,9 130,6 95,7 24,5 -23,6 Krenica 42"52,5' i 7"20' 252,6 300 275 41,3 211,3 SluSnica, spring 45"05' 15'34' 240 72 40 26,9 213? Zvir, spring 45"25' 14"25' 5,5 33 24 17,8 -12,3 BubiCa jama 45"09' 14"lO' 095 23 14,5 16,4 -15,9

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589