-In order to determine the influence of lentic habitats and macrophyte stands on the plankton development and seston flux, an investigation of zooplankton was carried out in the karst Jankovac flowthrough system (Papuk Nature Park, Croatia). The system was characterized by low abundance (1-116 ind.L x1 ) and high diversity of identified zooplankton. Eighty-six taxa were recorded, comprising 57 rotifers, 15 cladocerans, 8 copepods and 6 members of other groups of organisms. The spatial oscillations of environmental parameters and biocoenosis assemblage revealed statistically significant differences between lotic and lentic habitats, as well as between vegetated and non-vegetated stations. These differences mainly respond to higher concentration of food resources and zooplankton/zooseston abundance and biomass in lentic, especially vegetated, habitats. This is also proved by results of principal component analysis (PCA), which suggested that the main drivers of development of the planktonic community were the food resources and the avoidance of flow velocity. Accordingly, shoreline areas with submerged macrophyte stands of Hippuris vulgaris L. were the most productive parts, represented by highest zooplankton abundance, biomass and biodiversity. Flow velocity significantly affected crustaceans assemblage, so that higher abundances of the larger cladocerans and copepods were achieved in vegetated stations with low flow velocity, while rotifers showed to be rheotolerance organisms. On the other hand, the longitudinal discontinuum of the stream channel by two man-made reservoirs could offer new habitats to enrich seston with organic particles and bioseston. The results of our study pronounce the need for further monitoring of this hydrosystem, especially considering biodiversity and microhabitats conservation.

The ai m of this study was to examine the combined effect of water transparency and narrow macrophyte belts on zooplankton assemblages in two oxbow lakes (Krapina River, Croatia). Samples were collected in open water and among helophytes in the littoral zone from April until September 2008. Rotifers were the most abundant group of zooplankton in both lakes, and dominated in the Krapina oxbow lake 1 (KO1). Lake KO1 had significantly lower transparency, lower percentage macrophyte cover and higher chlorophyll a concentration than Krapina oxbow lake 2 (KO2). In lake KO1, variation in the horizontal distribution of cladocerans and rotifers in terms of their abundance seemed to be determined by competition between Bosmina longirostris and Keratella cochlearis, initiated by oscillation in transparency and detritus availability. In lake KO2, with higher transparency and higher percentage macrophyte cover, the abundance of small-and large-bodied cladocerans increased in the littoral zone simultaneously with higher transparency, suggesting fish predation. Results of this study indicated that small differences in transparencies between the two lakes caused significant differences in horizontal distribution of the zooplankton assemblage. Even narrow helophyte belts offered a refuge to zooplankton, although lower transparencies reduced the effectiveness of macrophytes as a refuge from predators. IntroductionThe li ttoral zones of lakes and rivers often include macrophyte covered areas that provide a high diversity of microhabitats in comparison with the open water. Types of microhabitats also differ between emergent and submergent macrophyte species, and between the sections of rizosphere, stems and leaves (DUGGAN et al., 2001;KUCZYŃSKA-KIPPEN and KLIMASZYK, 2007). Percentage, density and biomass of macrophyte cover in freshwater communities have a significant influence on abiotic (temperature, oxygen concentration, light conditions) and biotic (competition, predation, food availability) factors (CAZZANELLI et al., 2008; EST-LANDER et al., 2009). Macrophyte biomass and cover area increase with lake size and decline as the littoral zone becomes steeper (KALF, 2002).

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Research into spatial and temporal variation in seston transport was carried out during the year 2000 on three reaches on the longitudinal profile of the karstic cascading system of the Plitvice Lakes in Croatia. The three investigated reaches were (i) a channel over a barrier with low gradient; (ii) flow through a deep lake; and (iii) a channel with cascades and a steep gradient. The aim of the study was to establish the influence of physiographical and hydrological differences of these reaches on the quality and quantity of seston transport and food resources in the seston. To calculate the seston transport, we measured: total suspended matter (TSM), particulate inorganic matter (PIM), particulate organic matter (POM), chlorophyll-a (chl-a), heterotrophic bacteria and discharge. The PIM contribution from TSM ranged between 60 and 90%, while the percentage of POM in TSM was the highest in summer and ranged from 33 to 46%. POM and discharge were significantly negatively correlated (r = -0.43, P < 0.05). For the transport of TSM, PIM, POM and chlorophyll-a statistically significant differences between the three reaches were established. In a principal component analysis, 86% of the variance was explained by the first two factors. The first factor corresponded well with net transport of TSM, PIM, POM and chl-a and distinguished investigated reaches in two groups: the 1 st group with increasing (reaches with low gradient and with high gradient), and the 2nd group with decreasing net seston transport (reach with flow through a deep lake). The second factor corresponded strongly with discharge and distinguished investigated reaches according to their temporal variability.

Impact of Environmental Variables and Food Availability on RotiferAssemblage in the Karstic Barrage Lake Visovac (Krka River, Croatia)key words: karstic riverine lake, rotifer density and diversity, macrofilter-feeder rotifer, microfilter-feeder rotifer AbstractWe evaluated the impact of 18 environmental variables (physico-chemical, nutrients, food resources) on rotifer assemblage in the sub-Mediterranean karstic barrage Lake Visovac. In terms of the spacetime distribution the highest density of rotifers was noted in the summer period (average 386 ind/l, relative annual abundance 62%), and in the epilimnetic layer (average 309 ind/l, relative annual abundance 58%). A total of 41 rotifer taxa belonging to 22 genera and 14 families were identified. Three rotifer species: Gastropus stylifer, Synchaeta tremula and Trichocerca birostris were dominant and perennial during the annual investigation. Pearson's product-moment correlations and canonical correlation analysis suggest that temperature, pH values, alkalinity, chemical oxygen demand and chlorophyll a concentration significantly influenced rotifer density. According to rotifer food collection and selection, in Visovac Lake macrofilter-feeders predominated, and microfilter-feeders were in a minority. IntroductionMany recent studies have established the relationship between the plankton community density and diversity and limnological characteristics of different water body types, for instance: floodplains (DE AZEVEDO and BONECKER, 2003), backwaters (ARORA and MEHRA, 2003), riverine lakes (ECKERT and WALZ, 1998;WALZ and WELKER, 1998;FERNÁNDEZ-ROSADO and LUCENA, 2001), karstic lakes (HABDIJA et al., 1993;MIRACLE and ALFONSO, 1993;RODRIGO et al., 2000). Lake Visovac is a lentic dilatation of the Krka River and is bounded by two barriers with impressive waterfalls, Roški slap to the north and Skradinski buk to the south. With respect to the origin of the lake basin, Visovac Lake belongs to a group of karstic barrage lakes (HUTCHINSON, 1957). Water flowing through the karstic area precipitates calcite (limestone), which creates different sedimentary forms, one of them being barriers.According to field investigations and experimental laboratory studies provided by many authors, rotifer distribution in lakes is related to both abiotic factors, such as temperature (BÇRZINš and PEJLER, 1989;WATANABE, 1992), oxygen content (MIRACLE and ARMENGOL-DÍAZ, 1995), pH values (B - ERZINš and PEJLER, 1987), light intensity (SAUNDERS-DAVIES, 1989), and to biotic factors, such as food availability (POURRIOT, 1977; GILBERT and BOG-DAN, 1981;BOGDAN and GILBERT, 1987;ARNDT, 1993;HABDIJA et al., 1993) (GILBERT, 1985;KIRK, 1991), interference competition (GILBERT and STEM- BERGER, 1985;BURNS and GILBERT, 1986) and predation . According to KIRK (2002), rotifers show a gleaner-opportunistic trade-off. With such characteristics, they are able to adapt quickly to changing and fluctuating physico-chemical and hydrological conditions, which exist in barrage lakes. Thus, in freshwater communities ...