the thermocline is the transition layer between the mixed layer at the surface and the deep water layer. The definitions of these layers are based on temperature. The mixed layer is near the surface where the temperature is roughly that of surface water. In the thermocline, the temperature decreases rapidly from the mixed layer temperature to the much colder deep water temperature. The mixed layer and the deep water layer are relatively uniform in temperature, while the thermocline represents the transition zone between the two.


Essay by Robert Kakarigi

The oar splashed once again and the wooden boat glided further over the smooth surface of the lake. Three pairs of eyes strained in an attempt to penetrate through the turquoise satin curtain of phytoplankton. For a while no one spoke, and then the park warden said in a hushed voice, “I believe we are right above it. Can you see how the colour of the lake is turning from light green to dark green?” We agreed with him. Only a sudden change in depth could cause this change in the light. We were floating right above Cousteau’s barrier in the middle of Kozjak Lake. I last saw this submerged travertine barrier fifteen years ago, whilst Captain Jacques Yves Cousteau was making a documentary about the natural park Plitvice Lakes. I longed to see it close up.

I allowed the watery surface to close over my head and looked towards the underwater photographer Vedran Frka. Judging by his widely open eyes, I knew that the scene I would see in the coming moments would take my breath away. At only five meters below the surface of the lake, the long slim barrier shone white. We dived to its edge, from where like the wall of a twenty-storey building it fell away into the darkness below. Above our heads the electric boats filled with visitors were just passing by on the longest tourist route in the park. It is a very rare traveller, mostly scientists and occasional photographers, amongst the several hundred thousands who annually travel on this unique voyage who can also witness this beautiful underwater scenery.  


On this dive, in conjunction with the wardens of the national park, we were attempting to compile a photographic record of Plitvice Lakes motifs not yet recorded, mostly related to the water. We dived during the summer solstice, so the surface of the water in the lake was still relatively cold. Our thick wet diving suits managed to protect us for a while. But, as soon as we crossed the thermocline, which divides the warm surface water from the much colder depths, the unpleasant cold penetrated to our bones. For a while I floated weightless parallel to the deepest side of the barrier, which looked like a kind of underwater Taj Mahal, whose builders this time, instead of human hands, were travertine moss and blue green algae. The crumby limestone, mixed with plant and animal debris, created stunning decorations in the places where waterfalls once fell. I examined the elegant travertine curtains, wrinkles, beards, and smooth kidney shaped travertine accumulations. Controlling my balance with the air from my diving jacket, I came closer to the wall and carefully touched it with the tip of my index finger. To my surprise my finger did not encounter any resistance, as you would expect from the limestone, but sank completely as though it were sinking into a heap of goose feathers. Although all the decorations and forms where shown perfectly, the entire surface of the barrier was covered in soft detritus. The covering layer, mainly organic in origin, had settled down over many decades. The careful way we were diving was shown to be justified and absolutely necessary, since any careless move with our fins could disturb the detritus. After we had photographed the underwater landscape surrounding Cousteau’s barriers, we set off back to the surface where the warden of the national park and our irreplaceable guide, Zdravko Luketić, was waiting for us in the boat.


Noticeably impressed by the close examination of the submerged barrier, we boarded Plitvice’s wooden rowing boat, which took us to new and beautiful underwater destinations. On all our dives we noticed freshwater fish in large numbers. They were mostly large shoals of pearl roach (Scardinius erythrophtalmus) ans chub (Leuciscus cephalus). At first sight it seems that this large quantity of fish would demonstrate the original nature and health of the water in the national park, but actually the opposite was true. Plitvice Lakes were never the natural habitat of chub and pearl roach. Both these species of fish belong to the carp family, who as a rule live in slow moving lowland running water, lakes and marshes full of aquatic plants. The presence of these fish in all sixteen lakes of the national park is the result of the negative local and global anthropogenic influence on the biotopes of the entire national park. Carp, who amongst other things feed on the fish roe, pose a significant threat to the lake trout and have overtaken their habitat. It is now almost impossible to remove the carp from the biological make up of Plitvice Lakes, but the staff of the “Ivo Pevalek” scientific centre is at least trying to control their numbers.

While diving in the small lakes, in the area of Burgeti and later below the Mali and Veliki (translates as little and large, respectively) Prštavac falls it seemed that the trout had found refuge just there. In these relatively small pockets of water it is impossible and unadvisable to use diving fins, as the uncontrolled jet of water could cause harm beyond repair to the sensitive environment. Moreover, a diver’s entire body has to be in a horizontal position as walking on the fragile travertine and upsetting the mud would be completely inappropriate in a national park where the central phenomenon is the biodynamic process of the formation of the travertine barriers. And while I was quietly floating in the clear water between the branches of submerged trees I saw the outline of a trout. I drew closer at once, kept still and waited patiently. I was quickly reworded as trout passed by about two feet away. Their sides were adorned with large red freckles, typical for river trout, (Salmo truttua fario). This species prefers clear, clean and fast running highland water, so the tributaries of the Plitvice and the freshwater pools, from which the waterfalls flow, seem like the ideal habitat for it. For ichthyologists, natural scientist who study fish, the Plitvice Lakes are an ideal natural laboratory for studying the differences between the trout. Every lake or water pool within the national park offers these fish from the salmon family, known for the fact that they do not like leaving their habitat, desired isolation and a variety of conditions for their life. As a result trout in Kozjak Lake may have different anatomical features than those who live in Galovac or Prošćansko Lake. Apart from river trout there are also lake trout (Salmo trutta lacustris) in the park waters, which are characteristic for deep Alpine and Scandinavian lakes. This species is recognizable by its shiny silver sides and round or angular black spots. This species can grow up to one metre in length, which is almost twice the maximum length of river trout, and they can reach a weight up to 15 kg. Additional confusion, while attempting to differentiate between species, may be caused by Californian trout, as, like all other members of the salomonide family, it has a characteristic spindly appearance. However in relation to the River trout, Californian trout have a slightly smaller head and mouth and a slightly slimmer body. This species was brought to Europe from its original home, North American water habitats, in 1880. At one time it was bred artificially in Plitvice Lakes, but it now breeds by itself. In the doubtful desire to enrich the waters of the national park, people in the past brought another alien predator originally from North America, the arctic char (Salvelinus alpinus). This species likes to remain on the bottom of the deep lake water and it is interesting that during spawning season the males take on reddish colour.

to be continued...