Ecohydrology of Karst Poljes and Their Vulnerability
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Ecohydrology of karst poljes and their vulnerability Prof. emeritus O. Bonacci Faculty of Civil Engineering, Architecture and Geodesy, Split University, 21000 Split, Matice hrvatske 15, Croatia E-mail: [email protected] Swallow-hole opened at the bottom of the Boljunčica reservoir through sediment deposit KARST POLJE Karst is estimated to cover about 25 % of the surface of all the continents. It represents a type of landscape with many specific surface and underground features, which facilitate and accelerate the exchange of surface water and groundwater. By this way karst strongly influenced on development of environment and its ecosystem. The total area covered by karst poljes represents approximately 2 % of the total karst area. Although they are relatively small in size, they are extremely significant from an ecological, social and economic standpoint. The importance of karst poljes is that they are the larger, fertile, and inhabited oases in karst, commonly providing the only conditions favourable for human beings as well as for development of reach but very vulnerable and mostly endemic karst flora and fauna. Karst poljes’ ecological role is extremely important but till now not enough recognized and investigated. Due to strong and uncontrolled anthropogenic pressure during last hundred years karst poljes belong between the most endangered environments on the Earth. Karst polje as the geomorphological term refers to large closed depressions with conspicuously flat bottoms developed on karst rocks. Their areas vary from less than 0.5 km2 to more than 500 km2 , and exhibit complex hydrological and hydrogeological features and characteristics. Geomorphological and hydrogeological features of poljes, mainly flat surface covered by impermeable soil, make possible formation of permanent as well as temporary streams and lakes, which enable development of reach biological forms in and around them. Poljes can be divided into the following groups according to the hydrological regime: (1) permanently flooded or lakes; (2) periodically, partly, or completely flooded; and (3) dry poljes. As a consequence of intensive tectonic activity the poljes in Dinaric karst have been formed as terraces from an altitude of more than 1000 m above sea level to the sea level. They represent more or less interconnected subsystems within the process of surface and groundwater flow through the karst spring catchment. From the hydrologic–hydrogeologic perspective, a polje is to be considered as part of a wider system. It cannot be treated as an independent system. System of 7 karst poljes in the Ljubljanica River springs catchment ECOHYDROLOGY !? Developing the research interface between hydrology and ecology has been recognised as a research frontier in geosciences. Despite a history of research that integrates insight from the two scientific disciplines, they still operate somewhat independently with different philosophies, conceptual frameworks, terminology and experimental approaches. Harte (2002) seeks a synthesis of what he calls the Newtonian and Darwinian approaches to science. He believes that such a synthesis offers opportunities for progress at the intersection of physics and ecology where many critical issues in earth system science reside. The pattern and intensity of hydrological variability significantly influences on biotic structure and activity. On the other hand, biotic structures can regulate abiotic ones. As a result of these interrelationships a new concept called ecohydrology has emerged. Its rapid development is a consequence of fact that complex scientific questions as well as environmental problems can be effectively solved only if several scientific disciplines are considered jointly. Ecohydrology was the coupling of landscape processes with hydrobiology. Due to particularities of water circulation in karst areas the coupling of surface water - groundwater processes is the most important prerequisite for understanding constraints on sustainable development. Karst ecosystem analysis should be focused on the flow of energy and the cycling of nutrients through biotic and abiotic components of the system. The absence of light in the karst underground, particularly in caves makes photosynthesis impossible. Plants, which are the major food source on the earth’s surface, are absent. Karst underground animals generally depend for food on what enters haphazardly from outside. In caves, there are two sources of organic matter: vertically percolating water from the epikarst and from sinking streams. A karst ecohydrological approach means integration karst studies into a more general ecological, biological, hydrological, hydrogeological, geomorphological, and geochemical context. The karst environment has very different characteristics than all other environments. Subterranean karst ecosystems are sensitive to environmental changes that occur on the surface. The importance of maintaining biological diversity goes far beyond mere protection of endangered species and beautiful landscape. Examples of few karst poljes Cres island AI = 404.3 km2 Lake catchment area AL = 25 km2 Lake water surface AW = 5 km2 Lake water volume VW = 200×106 m3 Permanently flooded or lake karst polje Vrana Lake on Island of Cres annual precipitation 1926-2012 mean annual water level 1929-2012 1800 P 16 P = 0.2831 × t + 508.5 (mm) Hav 2 (m a.s.l.) 1600 R = 0.001 15 14 1929-2012 1400 13 1200 12 11 1000 10 Hav = -0.0428 × t + 96.875 R2 = 0.4726 800 9 t (year) t (year) 8 600 1925 1930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010 2015 1925 1930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010 2015 16 H Hmax = 15.84 m a. s. l. (m a.s.l.) 1987-2012 15 14 13 Hav2 = 10.91 m a. s. l. 12 Hav1 = 13.41 m a. s. l. 11 1929-1986 ΔHav = 2.350 m 10 ΔHmax = 6.46 m Hmin = 9.38 m a.s.l. 9 ΔH = 4.37 m t (year) (1982-1990) 8 1925 1930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010 2015 Ecological impact on broader region !!???? The Trebišnjica River traverses through the Ombla Spring catchment. The mean annual discharge, after the canalization of over 60 km of its watercourse with spray concrete (from 1981), is 24.1 m3/s. In natural state the mean annual discharge was 28.4 m3/s. The Lika and Gacka Rivers (Dinaric karst of Croatia) are typical sinking streamflows : At the same time the Lika is losing river. Vrulja FLOOD IN KARST POLJE Floods are one of the most dramatic interactions between human beings and the environment. People look at floods as a catastrophe but in reality floods are integral part of nature, playing a critical role in ecosystem function. At the same time flooding brings many benefits particularly for ecological variability and soil fertility. Flooding promotes exchange of materials and organisms between habitats and plays a key role in determining the level of biological productivity and diversity. Those processes are especially important for karst environment. Poljes are regularly flooded in the cold and wet periods of the year. Because of this they can be treated as wetlands. Flooding of the poljes in the Dinaric karst in natural conditions lasts on average from 3 to 7 months per year, mostly between October and April, but there are cases when flooding can even persist for 10 months. With the objective of flood prevention in poljes, attempts have been made to increase the capacity of ponors. Such attempts have usually failed because the capacity of ponors depends on the conduit system to which they drain, and on the groundwater level. The most effective measure to prevent the flooding or to reduce duration of floods is the construction of a tunnel. Ecological consequences ??? Tunnels for karst polje flood control: An example of the Konavosko Polje Catastrophic flood on the Upper Dobra River 29 June 1999 3 Qmax = 254 m /s Maximum swallow capacity of the Đula Sink ≈ 80 m3/s. 80 m3/s << 254 m3/s FLOOD IN THE VRGORSKO POLJE The tunnel operation significantly reduced the durations of flooding, but did not eliminate them entirely. Cyprinid fish (Phoxinellus ghetaldii) Hawes (1939) gives an example of possible underground colonisation of the karst underground in the Popovo Polje. Cyprinid fishes spend most of their time underground. Floods wash them out in great quantities and regularly every year at the beginning of wet and cold period. Breeding occurs at this time, and the young fish are left to spend a year in the open, through their parents are carried back into the karst underground. After the next flood, the young fish in turn are swept into the underground. The eyes of Phoxinellus ghetaldii are normal but the fish exhibits a tendency to reduce scales, which is remarkably common among cave fishes. Maybe in this case we are witnesses of the early stages of colonisation of caves by an epigean fish. Because of extreme groundwater oscillations in karst, different hydrological connections are active depending on the groundwater level. The methodology of water tracing was developed especially to determine water interconnections in karst. In addition to tracing with salt, dye or temperature, the use of rich endemic fauna, their dispersion and population structure is lately considered. In this paper, population structure of the endemic fish species is used to confirm hydrological data as well as suggest new underground water connections in part of central Dinaric karst. Role of dry stonewalls The influence of drystone walls on ecological and geomorphological processes is important. Terracing is very effective measure against erosion. Stonewalls are of fundamental importance as a habitat for a very diverse Mediterranean flora and fauna. Changes of the biotic and abiotic factors above the ground lead to changes down through the massive stone clearing to deep karstic aquatic and terrestrial habitats. Massive stone clearing means entirely disappearance of stonewalls. This process could be very dangerous from ecological and hydrogeological point of view.