UNDERWATER WORLD OF THE PLITVICE LAKES - part 2

03.08.2009

 
 
Astacus

… continued from part 1

AN ICHTHYOLOGICAL MYSTERY

Whilst in terms of the Arctic char, the chub, the pearl roach and the Californian trout it is perfectly clear that those species were added to lake system by humans relatively recently, scientists still do not know the answer to the question: “Is there any native species, one that naturally lives in this area, in the Plitvice Lakes or were they all introduced during the twentieth century?” Most probably the research into the genetic origins of these fish will resolve this mystery.

By careful observation of the aquatic environment in the park it is possible to see shoals of minnows (Phoxinus phoxinus) who are members of the carp family. This fish reaches a maximum length of fifteen centimetres. It most often lives in fast flowing water and lakes with poor supply of food, so it has found an ideal habitat just here. Minnows are the favourite food on the menu of adult lake trout, which also includes insects and plankton fresh water crustaceans. The later are interesting catch for young trout and are most numerous right at the beginning of the summer when the surface layer of the lake water become rich in nutrients, which encourages the development of plant and animal plankton.

Visitors who only spend a short time at the Plitvice Lakes probably will not notice species of crayfish barely visible to human eye, but it is probable that river crayfish (Astacus astacus), may be seen at dawn or twilight on the bottom of a clear stream or lake. Although at the beginning of the seventies a crayfish plague almost wiped out this ten-legged creature, it still inhabits the waters of the national park. In the calm of the summer day, the park warden Zdravko Luketić led me with great precision to its richest habitat on the river Korana and Prijeboj. And as the day slowly melted into night, the males and females, with fertilized eggs attached beneath their fins, were getting ready for a night hunt on insect larvae, snails and baby fish. Thanks to the chromatophora in their skin, with multi-coloured grainy pigment, crayfish that I was observing completely merged with the brownish-white river bed. At the moment they passed over the lighter or greenish coloured river bed, they were able to change the colour of the surface of their body armour relatively quickly adapting to the new background. Camouflaged in this way, they become almost invisible to the untrained eye.

PRESERVING BIODIVERSITY

Later in the night I studied photographs showing clearly the anatomy of the crayfish. On the head linked to the chest in cephalothorax, eyes shone out, raised on stalks, below which there were one pair of antenna and one pair of antennules waving. Eight pairs of legs could be clearly seen. Four rear pairs serve for walking along the river bed, three pairs are joined to the jaw, and the front pair, on the head (cheliped), ends in oversized defensive claws, which keep most of the natural enemies away. However, these claws do not protect them from negative human influence. This reminded me of the enormous importance of national parks in preserving the biodiversity of our planet. People such as the head warden of the park, Ante Bionde, devoted their lives to protecting the world’s natural heritage. Regardless whether you visit the park as a tourist or on a scientific mission, it is vital to cooperate with the trained members of the ranger service, since Plitvice is not just their place of work, but also their home.

During our first early morning meeting in the Ivo Pavelek scientific centre, the leader of the Plitvice rangers showed me clearly on a map the areas we were about to visit and pointed out the problems they come across every day as they protect the park. So one of our first dives at Plitvice took place precisely in Prošćansko Jezero, which suffered most in the past by the inappropriate watercourse management and the construction of roads and hotels. Today the lake has been partially returned to its original natural condition, but we encountered new problems on the shore. We found the fresh remains of an illegal camp fire left behind by poachers. Some reckless group had thrown tin cans and glass bottles into the lake, and the variety of bottles were scattered throughout the nearby forest. The scene in the water was no better. We came across old stoves, metal barbecues, and various forms of waste, thrown away in here over several decades. The warden service constantly struggles with this problem. Few irresponsible people have left their mark, and the damage is accumulating. Every bottle or even umbrellas, blown by the wind into a lake from the careless tourists’ hands, settle on the bottom, which in some places looks more like a waste dump than protected body of water. On our way back, after the successful underwater photography session, we symbolically carried out the rubbish we had found and began to plan a campaign of more comprehensive cleaning.

DIVING IN ICY COLD WATER

One of the places where we definitely wanted to take some photographs of the natural environment was the overflow area between Kozjak and Gradinsko Jezero, known as Burgeti. One morning we arrived at the nearby wooden pier. Beautiful waterfalls fell over the cataracts, the limestone thresholds that had grown higher than the surface of Kozjak Lake. I sat right by the waterfall allowing the icy clear water to soak my diving suit and examined the constructions that had resulted from the magnificent biodynamic process of the travertine buildup. The white foamy water poured over the short neuron moss Cratoneuron communtatum, carrying with it the dissolved limestone which settled on the travertine covered parts of the moss, immediately below the living green shoots. I made use of the exceptional moment and put my head under the waterfall, allowing the naturally clear water to flow down my throat, quenching my thirst, just as our ancestors did in past millennia. Few moments latter I heard “Sssshhh, shush”. Something was rustling in the nearby heap of dried leaves. Intrigued, I looked carefully and quickly realized that the rustling sound was coming from a variety of brown and yellow lizards rushing to find cover. At that moment a slightly louder “shush” sound could be heard. I looked in the direction where the sound was coming from and saw a common green lizard (Lacerta viridis), Croatia’s largest lizard hiding in the shadow of the sun-drenched undergrowth. It was an adult male, with a large head and a beautiful turquoise blue underbelly. The lizard looked at me for a few moments, and then disappeared into the brush. A while later I saw a female viviparous lizard (Lacerta vivipara) resting on the mossy limestone rock.

AN ADDER DOUBLE WITH A SCARRED EYE

Those days in the park I did not notice a single viper, who probably stay further away from the water, in the rocks of the surrounding mountains, but in the water plants bellow the Veliki Prštavac falls I saw a snake looking like a poisonous adder, which I knew sometimes likes to sun itself in the greenery and on old tree trunks surrounded by water. I then examined the head of the snake through my camera lens and instead of elliptical vertical pupils, I saw round ones. This was not a poisonous adder which is anyway rare in these parts, but the much more common dice snake (Natrix tesselata), its harmless double. It was an adult snake with a scar on its eye, a reminder of one of the daily battles for survival and now it was lying in the sun with its body winding around, not taking much notice of its inquisitive audience. We watched it for almost an hour, and then it decided to dive into the clear water pocket and, keeping to the bottom. Finally it disappeared into the reeds on the other bank.

Sightings of animals at Plitvice Lakes are never isolated or individual, but flow into each other in a constant kaleidoscope of colours. And so I could go on listing the hundreds of species in the water of Plitvice Lakes, and still not name all the creatures that live there. On one of my future visits I hope to meet with an otter (Lutra Lutra), which they say, having disappeared from this area, has now returned to the waters of the streams that feed the unique phenomenon of Plitvice Lakes. The world’s natural heritage has once more generously opened its doors to visitors, but has also shown its fragility, which demands not only our admiration, but also unambiguous and continuous protection projects, which every inhabitant of the Earth should support in his/her own way. Goodbye, Plitvice, see you soon.