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.