Biserka Primc-Habdija

Vegetation Cover and Substrate Type as Factors Influencing the Spatial Distribution of Trichopterans along a Karstic River

In the longitudinal continuum of the Kupa River the vegetation cover and substrate type were the important environmental factors influencing the spatial differences in the biomass and community composition. Of total macroinvertebrate biomass, a significantly greater percentage of trichopterans was found on boulder and cobble substrata covered with moss (54.3% on boulders, 55.8% on cobbles) than on substrata covered with periphyton (9.9% on boulders, 14.8% on cobbles). In the potamal, trichopterans were markedly reduced (<2.5% of total macroinvertebrate biomass) on gravel substrata. A comparison of the Shannon diversity index values suggested that for trichopteran species diversity the substrate type was a more influential factor than vegetation cover. On the other hand, multidimensional scaling analysis showed that trichopteran community composition was related more significantly to vegetation cover and river area than to substrate type. In the rhithral the vegetation cover was an important factor influencing the functional feeding group composition of trichopterans. The spatial distribution of scrapers and filtering collectors depended significantly on the vegetation cover associated with substrate type, and shredder trichopterans were related to vegetation cover only. Predatory trichopterans made up 17‐65% of total predator biomass, and in the rhithron area they were correlated significantly only with vegetation cover. On gravel substrata in the potamal, vegetation cover did not affect the spatial distribution of shredder and collector-filterer trichopterans significantly.

Transport of seston in the karstic hydrosystem of the Plitvice Lakes (Croatia)

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 1st 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.

Tufa deposition and periphyton overgrowth as factors affecting the ciliate community on travertine barriers in different current velocity conditions

A characteristic of aquatic systems in karstic region is the formation of tufa – the product of calcium carbonate precipitation. Artificial substrata (glass slides) were used to investigate the influence of tufa deposits at two different current velocities (5 cm s−1 and 50 cm s−1) on the ciliate assemblages in periphyton. After two-month exposure periods, periphyton biomass and tufa deposit were c. three times greater at 50 cm s−1 than at 5 cm s−1. Ciliate population density was also higher on artificial substrata exposed in a lotic than in a lentic microhabitat (the overall mean number of ciliates at 5 cm s−1 was 122 ind. cm−2, and at 50 cm s−1 497 ind. cm−2). At each of the two observed current velocities, a Principal Components (PCA) ordination of the colonized ciliate associations showed a spatial separation of the associations that developed under two different conditions of tufa deposition. During the period of greater tufa deposit, associated with greater periphyton overgrowth rate, the ciliate communities had higher species diversity (a higher number of species and a lower number of individuals). Species diversity of ciliates had a positive nonlinear relation to tufa deposition rate. These results suggest that artificial surfaces covered by a rough tufa layer associated with greater periphyton biomass offer diverse conditions for colonization by ciliates.

Seasonal and fine-scale spatial drift patterns in a tufadepositing barrage hydrosystem

We examined seasonal patterns of drifting macroinvertebrates, and transported moss and (in)organic particulate matter at a fine spatial scale within the tufa-depositing barrage hydrosystem of Plitvice Lakes, Croatia. Samples were collected at six sites along a stream reach during autumn, winter, spring and summer. The sampling sites differed in flow velocity, depending on their position within the study reach and distance from tufa barriers and associated waterfalls. Seasonality was pronounced for most physico-chemical measurements. The highest concentrations of transported (in)organic particulate matter, being related to increased amounts of moss in transport, were observed in autumn. Drift densities were highest at tufa barriers that had higher flow velocities. Drift abundance partially followed the seasonal pattern in transported (in)organic matter. Autumn peaks in drift of Oligochaeta, Coleoptera as well as some Diptera and Trichoptera were related to increased moss and particulate matter quantities, while observed increases in drifting Nematoda, Cladocera, Copepoda, Ephemeroptera, Simuliidae and Chironomidae in spring and summer were probably due to distribution shifts of larvae or to higher seston levels originating from the upstream lake. Although we found that faster flowing stream sites generally had higher drift densities, there typically was no significant correlation between flow velocity and the amount of drift. Therefore, our expectation that fine-scale spatial drift patterns primarily depend on flow differences between lotic habitats was not supported for this system. The positive correlation between transported moss and other measured drift parameters, both at the seasonal and fine spatial scale, suggests that drift patterns were influenced by trapping/refuge efficiency of moss-covered tufa substrate and respective seasonal changes in instream vegetation (moss) as linked to hydrochemistry and water temperature.

Simuliid silk pads enhance tufa deposition

Simuliid silk pads and their effect on tufa deposition and biofilm development with respect to current velocity and time of exposure have been studied during the winter on tufa barriers in Plitvice Lakes (Croatia). Analysis of silk pad types on artificial substrates showed that moderate velocity substrates were slightly more favourable for simuliid larvae as landing attachment site, while they strongly preferred high velocity substrates as wandering attachment sites and exclusively as long term attachment sites for feeding. Although silk covered only 0.78% of surface area on glass slides, it accumulated 17% of diatoms and 56% of individual calcite crystals under moderate velocity conditions and 13% of diatoms and 41% of individual calcite crystals under high velocity conditions, thus proving that silk influence periphyton development and serves as biomediator in early stages of tufa formation. Diatom and calcite densities were mainly functions of time, with higher densities of both under moderate velocity. Maximum dimension of calcite crystals wasn’ t affected by the two month time of exposure, with larger crystals found under moderate velocity. We conclude that low winter temperature inhibited intensive calcite precipitation so tufa deposition during winter time happened mainly by trapping and binding microcrystalline calcite. Increased current velocities resulted in a poorer periphyton development and a lesser accumulation of calcite crystals.

Composition of ciliate fauna and its seasonal changes in fluvial drift

Ciliate composition and its seasonal changes in seston depending on the discharge regime were analyzed in the lower rhithral area of the river Sava. Higher values for ciliate density, dry biomass, index of species diversity and concentration of particulate organic matter (POM) were associated with discharge peaks. Using the power model: y = axb ± c a significant positive correlation was found between POM and ciliate dry biomass (as dependent variables) and discharge (as independent variable). The ciliate drift constitutes 0.78% of the total annual POM transport. Depending on the discharge regime, the composition of ciliate drift reflects the temporal and structural changes in periphytic community.

Development of ciliate community on artificial substrates associated with vertical gradients of environmental conditions in a karstic lake

The composition of ciliate communities, their abundance and biomass were studied on glass slides exposed in the epilimnion (1 m), metalimnion (9 m) and hypolimnion (18 m) in the karstic barrage Visovac Lake (SW Dinarid, Croatia). After one-month exposure periods from May 1995 to February 1996, mean ciliate biomass decreased with depth (at 1 m 54 μ g cm-2, at 9 m 37 μ g cm-2 and at 18 m 28 μ g cm-2), whereas the mean population density was relatively uniform (675 Ind. cm-2 at 1 m, 592 Ind. cm-2 at 9 m, and 678 Ind. cm-2 at 18 m). Sessile forms (peritrichs and suctorians) were dominant during all exposure periods and at all three depth layers. Omnivorous ciliates were dominant in the epilimnion, whereas in the metalimnion and hypolimnion bacterivores and carnivores were most abundant. Ciliates inhabiting artificial substrates in the epilimnion had two temporal peaks of density and biomass: in June and in October. In the deeper layers these maxima appeared one or two months later. Seasonal changes in ciliate biomass and community and trophic composition were associated with changes in thermal stratification and vertical oxygen gradients as important abiotic parameters, periphyton biomass as food source, and the tufa deposit determining the properties of substrate.

Trophic role of rotifers in the plankton of lake Kozjak (Plitvice Lakes)

The trophic role of rotifers in the zooplankton community of dimictic, oligotrophic lake Kozjak, the largest lake of the Plitvice Lakes, NW Dinarid Mountains, is analyzed. Their spatial and temporal biomass distribution in relation to that of protozoans, cladocerans and copepods shows that they form a significant part of the non-predatory zooplankton of this karstic standing water.

Trophic structure of ciliate associations in periphytic communities in karstic waters

Current speed, temperature, and the amount of dissolved and suspended organic matter as food source are the factors most responsible for the dynamics of ciliated protozoans living in different types of periphyton communities. In lotic and lentic habitats in the Plitvice cascade lake system located in the karstic Dinaric area we observed that the community and the trophic structure of ciliate associations depend upon the physical structure of the vegetation cover. These differences in trophic ciliate composition were explained by the degree of the retention and accumulation of organic detritus in periphytic habitats.

Overflow effect on the dynamics of the rotifer community in cascade lake systems

Many recent studies have been concerned with rotifer life cycles in relation to changes of abiotic environmental conditions (PAULI 1990), predatory pressure (CHRISTOFFERSEN et al. 1993, DuMONT et al. 1994), competition (GILBERT 1985, SARMA et al. 1996) and availability of food (HESSEN 1992, RoTHHAUPT 1995). In karstic standing waters, especially in cascade hydrosystems, these ecological interactions are very complex and insufficiently understood.