Hydrological System of the Plitvice Lakes—Trends and Changes in Water Levels, Inflows, and Losses

The Plitvice Lakes National Park is inscribed on UNESCO’s World Heritage List. The lake system is composed of 16 cascading lakes of different sizes separated by tufa barriers, which are the park’s key phenomenon. The lakes are characterized by highly diverse trends of the characteristic hydrological indicators—mean annual water levels, discharges, and tufa barrier growth. The analyses carried out in this paper identified that in the period before the early 1990s, Kozjak Lake had a trend of decreasing discharges, together with a trend of increasing water levels and growing tufa barriers. In contrast to this, in the period after 2001, a trend of increasing discharges was recorded, as well as a trend of decreasing water levels and decreasing tufa barriers. A potential cause of the barriers decreasing in size were the extremely high discharges during the last decade, which resulted in increased erosion of the tufa barriers. Losses of water due to the sinking from the lake system as well as the upper Korana course were confirmed, and it was identified that during the analyzed period the losses had not changed significantly. It was determined that the losses of water from Kozjak Lake occurred during low-water periods; however, they depended not only on the quantity of water flowing through the lakes but also on the hydrological conditions underground. The analyses carried out and the methodological procedures used in the analyzed area of the Plitvice Lakes are useful examples for the performance of analyses at similar lakes in karst formed by tufa deposition processes.

APPLICATION OF 14C METHOD TO CHRONOLOGY OF THE CROATIAN DINARIC KARST—A CASE OF THE PLITVICE LAKES

ABSTRACT Karst environments preserve some of the best archives of past climate, vegetation, hydrology, anthropogenic impact, and landscape evolution providing that a reliable chronology can be established. Here we present an example of the system of the Plitvice Lakes (Dinaric karst, Croatia), which is characterized by intensive tufa and lake sediment formations. The radiocarbon dating method, combined with some other dating methods and various geochemical and isotope analyses, showed that the Plitvice Lakes system in the present form has existed for about 8000 years. Older tufa deposits were dated to warm interglacial periods. A long-term comprehensive multi-proxy study showed that all environmental compartments (atmosphere, various water bodies, soil, bedrock, DIC, terrestrial and aquatic biota, and of course various secondary carbonates) must be included in order to obtain trustworthy results.

Freshwater reefs as mesohabitats for the assessment of diel invertebrate drift patterns

Freshwater reefs (known as tufa barriers) are special karst features recognized for highly heterogeneous habitat structures, complex hydrogeological features, and unique macrozoobenthos drift (downstream dispersal) patterns. Our study objective was to investigate diel and seasonal drift patterns between barriers and pools, both composed of moss-rich and fishless mesohabitat types, aligned on a small spatial scale within the karst, tufa-precipitating Plitvice Lakes hydrosystem. We monthly sampled drift at the two mesohabitat types (barriers and pools) during midday and dusk and examined quantitative and qualitative drift compositions, including drifting invertebrates, moss, and associated particulate organic and inorganic matter (APOIM). Barriers displayed higher invertebrate drift densities than those of pools. The same pattern was observed for moss and APOIM. At both mesohabitat types, invertebrate drift showed peak but highly variable densities during late spring and summer (mean >100 individuals m-3), whereas during late winter and early spring the drift densities were 5-fold lower than those densities. The nonmetric multidimensional scaling analysis revealed that invertebrate drift seasonality was influenced by seasonal drift patterns of aquatic moss and moss-dwelling invertebrate taxa. Adult and/or larval Riolus spp. and larval Hemerodromia spp. were found to be the most significant for the separation of midday and dusk within the NMDS ordination of barriers and pools. At barriers, midday drift densities of invertebrates, moss, and APOIM were higher than the respective dusk records. Within pools, invertebrate drift was largely aperiodic. We suggest that increased midday and/or aperiodic drift are a consequence of the lack of fish between barrier- and pool-mesohabitats. Our results further indicated that aquatic invertebrates inhabiting fast-flowing barriers and slow-flowing pools mostly exhibit “passive drift” mediated by transport agents such as water flow and dislodged aquatic vegetation. The observed spatio-temporal drift patterns are also likely influenced by ontogenetic shifts in drift periodicity (i.e., shifts depending on the development stage and morphological characteristics of the individual taxa) as well as benthic distribution of moss-dwelling invertebrate taxa. We can conclude that biotic (vs. abiotic) controls of drift are likely minimized in the fishless case of the freshwater reefs and associated barrier–pool sequences within Plitvice Lakes hydrosystem.

Rediscovery of the terrestrial green alga Trentepohlia aurea (Linnaeus) C. Martius (Chlorophyta) in Croatia after 130 years

The first record of the terrestrial green alga Trentepohlia aurea for Croatia was made in 1890 in the area of the Krka River, and has remained until this day the only record of this globally widespread species in the country. During studies related to aquatic vegetation, the species was finally rediscovered from the area of Plitvice Lakes, in the Gorski kotar region and near Vukova Gorica village, after 130 years during which no records were made. All recorded populations formed round, woolly, orange-red patches on the surface of rocks and concrete walls, partly forming a community with bryophytes and lichens. In some cases in these mixed communities, Trentepohlia thalli overgrew bryophytes blocking their access to nutrients, water and light. This long period of lack of records of T. aurea in Croatia is mostly a consequence of the discontinuity in research on certain algal groups in Croatia.

Occurrence and Distribution of Technology-critical Elements (TCE) in Recent Freshwater and Marine Pristine Lake Sediments in Croatia: a Case Study

The appearance and vertical distribution of technology-critical elements (TCE) such as Li, Nb, Sc, Ga, Y, La, Sb, Ge, Te, and W were investigated in sediment cores taken in pristine freshwater and marine lakes located within protected areas of Croatia (Plitvice Lakes, Visovac Lake, and Mir Lake). These lake systems are isolated from the direct impact of human activities, and accordingly, the geochemical research in such environments may help to assess temporal trends in historical deposition of anthropogenic TCE. The analytical method of choice in determining the concentration of TCEs was High resolution inductively coupled plasma - mass spectrometry (HR ICP-MS). The results obtained have shown that the measured TCE spanned with wide ranges of concentrations, as a result of the different input of terrigenous material into the investigated lake systems which is further diluted by the formation of authigenic and biogenic carbonates. Evaluation of the anthropogenic impact on TCE levels showed low enrichment of Sb and Te in the upper sediment layers of the Mir Lake and the Plitvice Lakes. Despite that, concentrations of investigated TCE were low, compared to results described in the literature, indicating that the investigated remote lakes can be considered uncontaminated with these elements.