Hydrographical properties of the of sea water »Lakes« on the island of Mljet and the adjoining sea in eastern south Adriatic Sea
Abstract
The hydrographical data, processed in the present paper, have resulted from investigations of the sea water, carried out at thirteen exploration stations situated off the north-western part of the Mljet Island in the eastern Adriatic Sea, i. e. in the open part of the Adriatic and in the area of the sea water inlets, popularly called Mljet Lakes.
The bulk of investigations had taken place from 1951 through 1955, but there were occasional ones in the course of later years.
The following physical parameters have been investigated in the said area: tmperature, sigma t, and transporency, but in part also the flow of sea water trough two straits, the Velika Vrata and the Mala Vrata, the two »lakes« with each other and with the open sea respectively. The investigations have further included the following chemical parameters, sea water salinity (Cl‰ or Sal‰), O2 content, saturation of sea water with oxygen (O2%), hydrogen sulfid (H2S) in the sea water, nutrient salts content, i. e. P-PO4, P-tot, Si-SiO2, Fe2+, N-salts; alkalinity, pH in the sea water, and the specific alkalinity.
As regards the main exploration stations, the paper contains a generalization of temperature behaviour in the cours of the investigation period. On the basis of temperature behaviour, the phenomenon of upwelling was established at an outer station (Gonoturska) which contrasted with the inner stations. The highest temperature fluctuation recorded through a number of years occurred in the Malo Jezero, while the lowest one was observe in open sea area.
The salinity values found at the outer stations showed a gradual increase from the sea bottom towards the surface. In the Veliko Jezero basin, however, the maximum salinity values were observed in the upper layers, which is the essential characteristic of the basin. This phenomenon is otherwise unknown in the Adriatic Sea. There is an explanation in the paper of the above mentioned characteristic of the Veliko Jezero basin. A normal distribution of salinity, i. e. when values increase with the depth, was found in the Malo Jezero basin where the highest value, exceeding the one observed in the Veliko Jezero basin, occurred in the bottom layer. This fact has led us to the conclusion that the Malo Jezero basin must be directly connected with the open sea by means of an underground passage. One of the chapters deals in detail with this assertion trying to substantiate it. The analysis of the collected data shows that here is a different annual fluctuation of salinity values in various areas. The values fluctuate within the following amplitudes during the period of investigations:
The open sea waters . . . . . . . . . . 2,0‰ Cl
The Veliko Jezero basin . . . . . . . . . . 2,0-4,5‰ Cl
The Malo Jezero basin . . . . . . . . . . >8,0‰ Cl
This largely results from the structure of the basins and from the part played by the submarine sills. In wintertime (with the exception of the Year 1954) a less saline water discharges from the Malo Jezero basin into the Veliko Jezero one, as well as into the Soline basin and Gonoturska cove. The average salinity values found in the Malo Jezero, Veliko Jezero, and Gonoturska in winter amounted to 34,03; 35,83 and 37,70‰ Sal. The discharging of water from the Malo Jezero basin into the Veliko Jezero one in summer does not considerably affect the salinity of the waters of the latter, but the waters of the open sea, showing higher salinity values in their surface layer, do considerably affect the salinity of the Veliko Jezero waters. The mean salinity values found in summer in the waters of the Malo Jezero and Veliko Jezero and in Gonoturska were 36,04; 36,31; and 38,32‰ Sal respectively.
The investigations included also the density of sea water. Fluctuation amplitudes of the σ t values found in the main basins are given in the paper. The highest σ t values occurred in the Malo Jezero basin. From the study of σ t isolines in seven cases has been drawn the conclusion that there is a fluctuation in the denser deep layers. It has been concluded from the situation found in June 1952 that the tg x (the angle batween the isopleths and the horizontal plane) is equal to 26 m/ 1250 m = 1° 10’. This has been attributed to periodical inflow of a more saline water into the Veliko Jezero basin.
The T-S curves have been used in order to give a numerical picture of fluctuation of the principal hydrographic properties of sea water in the basins. The authors’ own method has been devised for this purpose, by applying the described rectangle around the obtained polygon. The fluctuation observed at the Vrbovačka station was more than 8 times higher at the surface than at the depth of 42 metres. This difference was much greater in the Malo Jezero basin.
One subject of study was the replacement of waters at the locality called Velika Vrata where water was observed to flow in alternate directions with great intensity. The measuring of the very strong flow showed a sea water transport od 1,2 m3/sec. By considering the volume of the »lakes« as well as the nature and velocity of flow (alternating direction every six hours) it was possible to calculate the time needed for a complete replacement of water in the two basins: it amounts to 8,5 years. The statistical data covering the observed directions of flow at the Velika Vrata locality in 1953 and 1954 were highly unsymmetrical. The prevailing outflow from the »lakes« was attributed to abundant atmospheric precipitation in those years.
The Δ Cl/Δ t values for all levels throughout the year have enabled us to find the salt balance at the three principal exploration stations, i. e. it was possible to check the fluctuation of salt quantities at a station in the course of several years - to establish the quantities of salt now growing smaller and then larger again. The sum total of those quantities represents the total amount of salt circulation. These volues were 2,69 g of salts per m3 per year at the Gonoturska Station, 3,76 g of salts per m3 per year at the Vrbovačka Station, and 8,35 g of salts per m3 Gonoturska station, 3,76 g Sal m3 per year at the station, and 8,35 g Sal m3 per year in the Malo Jezero basin. The values may also be considered as a measure of water dynamics of the three adjoining basins.
By studying the transparency of the investigated water basins, we came to the conclusion that these basins may well serve as an excellent illustration of the observation that the transparency of their waters diminishes as their distance from the open sea grows larger. Grand averages of transparency were established for the Gonoturska and Vrbovačka stations and for the Malo Jezero basin by means of Secchi disks. The grand averages amounted to 21,8 m; 14,8 m and 7,8 m respectively. The artifical fertilization, carried out in the Veliko Jezero basin in 1954, reduced the transparency of its waters (1954, 1955). In the other two basins, on the contrary, there was a trend of increasing transparency in the course our five-year-investigations.
The fluctuation of oxygen content and the place of maxima and in all the three basins in the course of the said period were also objects of investigations. Some regularities were observed, but they were different in each of the three basins. In this respect, each basin shows its characteristics, e. g. the difference between the annual extreme oxygen content was 1,6 ml/l O2 at the Gonoturska station, 5.81/l O2 at the Vrbovačka one, and 6,68 ml/l O2 in the Malo Jezero basin.
Oxygen saturation of sea water, i. e. the distribution of this parameter in a year’s time reveals a particuliar stratification and a rhythmic repetition in two inner basins, with the exception of Gonoturska. The average O2% saturation values in the basins were the following:
Gonoturska . . . . . . . . . . 28,46%
Vrbovačka . . . . . . . . . . 76,64%
Malo Jezero . . . . . . . . . . 115,12%
A rare phenomenon in the sea water was observed in the Malo Jezero basin: hydrogen sulfid gas (H2S) was found in the free water of this basin, beginning at the depth of 20 m2 and continuing bottomwards. The phenomenon had lasted from, March 1951 until November 1953, but it appeared again in 1954, 1955, and 1956. The causes of this phenomenon are discussed. Stratifcation of free phosphate (P-PO4), one of the nutrient salts found in water, was established at the Vrbovačka station and in the Malo Jezero basin for a series of years, but none was found at Gonoturska station. The grand averages of phosphate content were the lowest at Gonoturska, higher in the Malo Jezero basin, and the highest at the Vrbovačka station. This difference can be explained as a consequence of artifical fertilization carried out in the Veliko Jezero basin in 1954.
The distribution of total phosphorus (P-tot) and silicate (Si-SiO2) showed - to some extent - similar characteristics. Waters containing H2S were particularly rich in silicate. The values were up to 2100 mg Si/t.
As regards free Fe2+, a developed gradient was found in the Malo jezero basin, while no vertical gradient was present in the two other aerated basins. The total gradient (Fe-tot) is less developed, but the bottomward increase of values is evident.
A special behaviour was obserwed in the distribution of nitrates in the basins. The waters relatively richest in nitrates proved to be at the Vrbovačka station (exposed to the influence of soil and woods): the (open) waters at the Gonoturska station were less rich in nitrates, while those in the Malo Jezero basin showed the lowest values of nitrates. The latter fact can be attributed to the reductive conditions of waters favouring denitrification. After the artificial fertilization experiment carried out by means of superphosphate, the two inner basins revealed a nitrate enrichement tendency. Starting from the Gonoturska station in the direction of the Malo Jezero basin, the waters grow richer in ammonium salts.
By considering the pH concentration, we found that this parameter, in contrast with the two inner basins, was not stratified at the Gonoturska station. As a consequence of fertilization, a pH decrease was observed in the sea water of the Veliko Jezero basin. The low pH values, permanently found in deeper waters of the Malo Jezero basin, ranging from 7,89 to 7,41 pH at a depth of 25 metres, from 1951 through 1955, were the lowest ever recorded in the Adriatic Sea.
No stratification of water in alkalinity was observed anywhere, with the exception of the Malo Jezero basin, where the highest values occurred on the surface. The values grew smaller in deeper waters. The causes producing this phenomenon and the behaviour of the vertical polarization of the specific alkalinity in some of the basins are discussed.
The annual rhythm of some properties which was another subject of study, produced the equation for the sinusoidal curve for temperature fluctuation in the surface waters of all the three basins. Different and ever larger time shifts of normal curves (phase shift) in comparison with the actually drawn for each station were found (1,0, 1,5 and 1,7 months). Some normal curves for deeper layers, as well as equations for the sinusoidal curve salinity fluctuation also in all the three basins have been computed and useful conclusions have been drawn from them.
The thus collected material has been very useful in drawing the well substantiated conclusion that a direct underground link must exist between the Malo Jezero basin and the open sea, most likely in the Pomina inlet area or not far from it, on the NW extremity of the Mljet Island. In orden to prove the above statement, we have availed ourselves of (a) the evidence resulting from the collected σ t data, and (b) the evidence obtained from the Sal ‰ data. The T-S diagram method was also employed. The underground tunnel, or cave with two openings providing connection between the Malo Jezero basin and the open sea is probably situated at a depth of 20-30 metres (at least in the Malo Jezero area.). This link considerably influences the hydrographic properties of the Malo Jezero basin. The investigation carried out for this purpose represents an example of employment of the hydrologic method in the exploration of the submarine relief.
