Hydrobiologia 457: 87–96, 2001.© 2001 Kluwer Academic Publishers. Printed in the Netherlands.

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

Biserka Primc-Habdija, Ivan Habdija & And–elka Plenkovic-MorajDepartment of Zoology, Faculty of Science, University of Zagreb, Rooseveltov trg 6, HR-10.000 Zagreb, CroatiaTel.: +385 1 4877724. Fax.: +385 1 4826260. E-mail: bprimc@zg.biol.pmf.hr

Received 15 June 2000; in revised form 11 April 2001; accepted 20 April 2001

Key words: ciliates, periphyton, tufa deposition, current velocity

Abstract

A characteristic of aquatic systems in karstic region is the formation of tufa – the product of calcium carbonateprecipitation. Artificial substrata (glass slides) were used to investigate the influence of tufa deposits at two differentcurrent velocities (5 cm s−1 and 50 cm s−1) on the ciliate assemblages in periphyton. After two-month exposureperiods, periphyton biomass and tufa deposit were c. three times greater at 50 cm s−1 than at 5 cm s−1. Ciliatepopulation density was also higher on artificial substrata exposed in a lotic than in a lentic microhabitat (the overallmean 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 twoobserved current velocities, a Principal Components (PCA) ordination of the colonized ciliate associations showeda spatial separation of the associations that developed under two different conditions of tufa deposition. Duringthe period of greater tufa deposit, associated with greater periphyton overgrowth rate, the ciliate communities hadhigher species diversity (a higher number of species and a lower number of individuals). Species diversity of ciliateshad a positive nonlinear relation to tufa deposition rate. These results suggest that artificial surfaces covered by arough tufa layer associated with greater periphyton biomass offer diverse conditions for colonization by ciliates.

Introduction

A characteristic of the aquatic systems in the karsticregion of the NW Dinarid (Croatia) is the formationof tufa and lake sediments – the products of calciumcarbonate precipitation. Tufas are found in a largespectrum of forms, the most famous being travertinebarriers coming waterfalls (e.g. in Plitvice NationalPark and Krka National Park). The mechanisms ofcalcite deposition are not completely explained, butit seems that tufa is a product of physico-chemicaland biogenic precipitation associated with periphytoncolonization (Pedley, 1992, 1994; Ford & Pedley,1996). Tufa has high porosity, and typically con-tains the remains of micro- and macrophytes, bacteria,protozoa, fungi, invertebrates and detritus. Calcite de-position depends on environmental factors (Srdoc etal., 1985) such as: concentration of calcium carbonateand dissolved CO2, pH, the concentration of dissolvedorganic carbon (DOC), water temperature, and current

velocity. Tufa deposition is faster at higher water tem-peratures, in lotic areas and in places with water sprayfrom waterfalls (Srdoc et al., 1985).

The periphyton community plays a role in calci-fication, particularly cyanobacteria and certain algae(Freytet & Verrecchia, 1998). Periphyton also con-tains a large number of protozoa and small inver-tebrates that feed on biological mats, selectively orentirely (Sterner, 1986). Apart from having this dir-ect impact on food chains, heterotrophs with theirmetabolic activity change the microenvironment of acommunity. This living layer with crystalline crustresults from a complex dynamic equilibrium betweenorganisms during seasonal changes, with alternatinggrowth - grazing, and secretions accelerating or in-hibiting the growth of organisms among themselves(Meffert, 1993). Periphyton communities are oftendestroyed because most karstic rivers in Croatia areunderground in part of their flow and have variable dis-charge regimes (in summer months some parts could

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run dry). For this reason, knowledge of the recolon-ization processes of the periphyton is needed for theevaluation of the biological processes on travertinebarriers.

This work describes the changes in the communitystructure of ciliates in response to the tufa depos-ition rate associated with periphyton overgrowth on atravertine barrier at different current velocities. Artifi-cial substrata (glass slides) were used to investigate thegrowth potential of periphytic ciliates after 2 monthsof exposure in lentic (5 cm s−1) and lotic (50 cm s−1)areas of a karstic river. The main questions were howtufa deposition and periphyton biomass influence thenumber of ciliates, their composition and diversity.

Our hypothesis was that on travertine barrier mi-crohabitats in karstic running waters the community ofperiphytic ciliates would depend on seasonal changesof chemical variables and current velocity, but also onthe formation of tufa. We supposed that a recent tufadeposit is an important variable, which modifies thephysical structure of the substrate. The different lay-ers of tufa formed at different current velocities willmodify the roughness of a substrate, and influence thepossibilities of colonization by ciliates. In addition,we presumed that a higher degree of periphyton over-growth and greater tufa deposition offer numerous mi-crotopographical features and refuges for colonizationby ciliates and the development of their community.

Methods

The investigations were conducted on the biggesttravertine barrier of the karstic river Krka in Dalmatia(Croatia). Using artificial substrata (glass slides placedin PVC frames horizontally and parallel to the current)we analyzed the ciliate composition, tufa depositionand periphyton overgrowth, after bi-monthly coloniz-ation periods. We supposed that 2 months of exposurewould be sufficient for colonization by a periphytoncommunity, especially for ciliated protozoa, whichhave high dispersal ability. Two to six weeks havebeen the most common exposure periods in studiesof periphyton assemblages (e.g. Cairns et al., 1979;Cairns & Henebry, 1982; Primc & Habdija, 1987).

These experimental investigations were conductedin two types of microhabitats with different current ve-locity. Two frames with 10 glass slides (76 × 26 mm)were placed in a microhabitat with slow water flow(3–7 cm s−1, mean 5 cm s−1) and two frames in a mi-crohabitat with a medium current velocity (40–60 cm

s−1, mean 50 cm s−1). After 2 months, nine glassslides (3 for periphyton overgrowth, 3 for tufa depositand 3 for ciliate composition analysis) from one frameat each current velocity were taken for examination,and renewed.

The first series, exposed in April, in this studydenoted as A1 for the slow current and B1 for themedium current velocity, were examined in June. Thesecond series (A2 and B2), from the second frame,were examined in July. Renewed slides in June on thefirst frame (A3 and B3) were examined in August etc.The last series (A7 and B7) were exposed in October,and analyzed in December 1995 (Fig. 1).

Removed slides were taken to the laboratory insmall containers (one each for glass slide) of riverwater. In the laboratory, periphyton samples werescraped from the slides and suspended in the river wa-ter. Three replicate slides were used for the analysisof community composition and the other slides forthe analysis of periphyton standing crop biomass andcalcite deposition rate.

For the community analysis from each of threereplicates, ten 0.1 ml subsamples of suspension weretaken for cell count under an inverted microscope.Using a phase contrast microscope, ciliates were iden-tified in vivo or with the use of various cytologicaltechniques. In the calculation of population density,3 replicates × 10 0.1-ml subsamples were used todetermine average values.

Standing crop of periphyton was determined as ashfree dry weight after 24 h drying at 104 ◦C and 6 hashing at 400 ◦C. With a view to determining the tufadeposition, three replicate slides were dried at 104 ◦C,ashed at 400 ◦C, weighed and then treated with dilutedHCl to dissolve CaCO3and weighed again. The differ-ence was taken as tufa deposition after an exposure of60 days. Tufa deposit and periphyton rate per day wascalculated linearly so that each average value of threereplicate samples was divided by 60. Seasonal changesof environmental conditions: temperature, pH, alka-linity, O2 concentration, nitrates and total phosphoruswere measured according to APHA (1985).

The statistical significance of differences in tufadeposition rate after a 2-month exposure of the glassslides was designed by planned comparisons usingANOVA (Lindman, 1974).

Brillouin’s index was used to determine the speciesdiversity of ciliates in the colonized periphyton com-munities (Margalef, 1958; cited by Krebs, 1989). Witha view to analyzing ciliate composition in relationto various tufa deposition rates Principal Component

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Figure 1. Sampling program of glass slide exposure at two current velocities.

Analysis (PCA), a linear model, was used (StatSoftInc., 1995) to determine the spatial ordination ofcolonizing ciliate associations.

Study sites

The River Krka is an about 75 km long river in thekarstic region of the NW Dinarid Mountains (Croa-tia). In the lower course of the river, there is a lake,Visovac, which originated in the Quaternary by theformation of the Skradin fluvial travertine barrage loc-ated downstream (Fig. 2). The epilimnetic water fromthe lake flows out through numerous channels overa travertine barrier creating the 47 m high SkradinFalls (commonly known as Krka Falls). Our artificialsubstrata were exposed in microhabitats in one of themany channels on the travertine barrier.

Sites were under the influence of constant flows ofplankton-rich water. Because of the hydrological char-acteristics of the channels, current velocities do notvary significantly. At the beginning of the experimentsin April 1995 the water temperature was 13.6 ◦C. Themaximal temperature was in August (23.4 ◦C), and atthe end of investigations in December it was 10.4 ◦C.The water is slightly alkaline (pH 8 ± 0.2), calcareous(alkalinity = 200 ± 10 mg CaCO3 l−1) and well oxy-genated. The mean value of concentrations of nitrateswas 0.28 mg N l−1, and of total phosphorus 0.26 mgP l−1.

Figure 2. Study site on the Krka River: position of exposed arti-ficial substrata on the travertine barrier between Visovac Lake andSkradin Falls.

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Figure 3. Seasonal changes in tufa deposition rate (TDR) after atwo-month exposure at current velocities of 5 cm s−1 and 50 cms−1.

Results

Seasonal changes of tufa deposition and periphytonovergrowth rate on the two microhabitats with respectto the current velocity

The aim of this examination was to establish to whatdegree the travertinisation process influences the com-munity composition of periphytic ciliates and its di-versity. For this reason, we analysed firstly the sea-sonal changes in the tufa deposition and periphytonovergrowth rate in the travertine barrier habitats as-sociated at current velocities of 5 cm s−1 and 50 cms−1. After 2-months, from April to December, therewas a considerable difference in tufa deposition andperiphyton overgrowth between the two velocities(Figs 3 and 4). Under a slow current, there was a sig-nificantly higher tufa deposition rate on glass slidesA1, A2, A3, A4 and A5 than on glass slides A6 andA7, exposed from September to December. Using theplanned comparison in ANOVA (n = 21, F = 6.7, F0= 4.6, d.f. = 1,14, P(F > F0) < 0.05) we found thatour hypothesis (H0): (1/5A1 + 1/5 A2 + 1/5A3 + 1/5A4 + 1/5A5) > (1/2A6 + 1/2A7) was confirmed at a95% confidence level. In addition, in the microhabitatsexposed to 50 cm s−1 a significantly greater tufa de-position rate was observed on the glass slides exposedfrom August to November (B5 and B6) than on glassslides B1, B2, B3, B4 and B7. Our planned compar-ison by ANOVA (n = 21, F = 20.02, F0 = 17.7, d.f. =1,14, P(F>F0) < 0.001) confirmed the (H0): (1/5 B1+1/5B2 + 1/5B3 + 1/5B4 + 1/5B7) < (1/2 B5 +1/2 B6).

Comparing the temporal curves of the tufa depos-ition and periphyton overgrowth rate on glass slidesexposed to the two current velocities, we found thatthe increase in tufa deposition rate (TDR) can be as-

Figure 4. Seasonal changes in the periphyton overgrowth rate(POGR).

Figure 5. Relationship between tufa deposition rate (TDR) andperiphyton overgrowth rate (POGR). The linear regression, accord-ing to model y = ax, is based upon the untransformed data (R2 =0.94; d.f. = 1,12; P<0.05).

sociated with an increase in periphyton overgrowthrate (POGR). Regression analysis showed that a linearmodel (y = ax) fitted the relation TDR = f (POGR)satisfactorily (R2 = 0.94 at P < 0.05; N = 14, d.f. =1,12; F > F0) (Fig. 5).

Relationship between the seasonal changes in thecommunity composition of ciliates and tufadeposition rate associated with periphytonovergrowth rate and current velocity

Based on features of the community, the number ofciliate species and their population density were re-lated seasonally to tufa deposition rate and periphytonovergrowth rate (Fig. 6). A considerable decrease ofciliate species was found after a two-month expos-ure of glass slides at slow current velocity (5 cms−1) in the autumn period when the tufa depositionand periphyton overgrowth rate were decreased. Onthe other hand a greater population density of ciliateswas a response to lower values of tufa deposition andperiphyton overgrowth. On the glass slides exposed

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Figure 6. Changes in species number and number of individuals ofciliates after two-month exposure periods of glass slides.

in habitats with a current velocity of 50 cm s−1, wefound a similar relation. The greater number of ciliatespecies and the decrease of their population densitywas related also to the increase of tufa deposition andperiphyton overgrowth rate. The glass slides exposedin a lotic habitat carried a higher ciliate populationdensity than the lentic habitat. At 5 cm s−1 the over-all mean ciliate density was 122 ind. cm−2, and at50 cm s−1 497 ind. cm−2. Generally, the greatestshare in total number of ciliate individuals belongedto attached species (Table 1). At 5 cm s−1, 65% ofthe total ciliate abundance belonged to sessile speciesand at 50 cm s−1 approximately 87% were sessile.In the lentic area the numerically dominant specieswere the peritrichs Vorticella picta (from A1 to A5)and Carchesium polypinum (A6–A7). In the lotic area,the dominant species were also peritrichs (V. picta andZoothamnium procerius).

The second question was how species diversity, anattribute of ciliate association, is related to the tufadeposition and periphyton overgrowth rate. To answerthis we used Brillouin’s index, the most popular meas-ure of species diversity. As seen in Figure 7, species

Figure 7. Relationship between diversity index (H′) of ciliates andtufa deposition rate (TDR). The nonlinear regressions, accordingto model y = axb , are based upon the untransformed data. (a)Periphyton developed at 5 cm s−1(R2 = 0.872; d.f. = 1,5; P<0.05);(b) Periphyton developed at 50 cm s−1(R2 = 0.849; d.f. = 1,5;P<0.05).

diversity was related nonlinearly to the tufa depositionrate.

The main question was how ciliate communitycomposition is related to tufa deposition and currentvelocity. From the 2-month colonization periods fromApril to December, 73 ciliate species were identi-fied on the glass slides exposed at current velocitiesof 5 cm s−1 and 50 cm s−1. There was no differ-ence between the number of species found in differentcurrent velocity conditions (50 species at 5 cm s−1,and 49 species at 50 cm s−1). Considering their fre-quency and population density at a current velocityof 5 cm s−1, the most common ciliate taxa wereVorticella picta, Zoothamnium procerius, Euplotes af-finis, Cinetochilum margaritaceum, Ascobius lentus,Acineta fluviatilis, Heliophrya rotunda, Loxophyllummeleagris and Lembadion lucens, and at 50 cm s−1

V. picta, Z. procerius, Acineta flava, A. fluviatilis,Aspidisca cicada and Lacrymaria olor in the lotic area(Table 1).

According to the various periods of tufa depos-ition, the colonized ciliates can be separated into

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Table 1. The most frequent ciliate species present after two-month exposures of glass slides at two current velocities (ind. cm−2)

Current velocity 5 cm s−1 50 cm s−1

Denotation of glass slides A1 A2 A3 A4 A5 A6 A7 B1 B2 B3 B4 B5 B6 B7

Acineta flava 34.0 46.9 5.0 17.4

Acineta fluviatilis 1.0 2.1 1.0 4.0 11.9 12.2 12.6 9.9 29.4

Ascobius lentus 1.0 5.0 4.2 3.0 11.9 4.0 6.3

Aspidisca cicada 1.0 2.8 5.0 6.3 11.1 1.0

Carchesium polypinum 113.0 147.0 179.2 6.0

Chilodonella uncinata 35.0 47.8 5.0

Cinetochilum margaritaceum 15.8 6.3 2.0

Coleps hirtus 1.0 4.0 20.5 12.6 2.0

Cristigera sp. 5.1 6.3 9.9

Dileptus margaritifer 2.0 5.1

Euplotes affinis 3.0 3.0 3.3 3.0 5.0 6.0

Heliophrya rotunda 1.0 2.0 2.0 4.0

Holosticha pullaster 3.0 13.0 5.0

Hypotricha div. sp. 6.0 8.1 4.2 5.5 5.0 18.0 12.6

Lacrymaria olor 2.0 2.0 2.0 4.3 5.0 6.1 6.3

Lembadion lucens 1.0 4.0 5.1

Loxophyllum meleagris 2.0 1.0 2.0 5.1

Metacineta sp. 4.3 6.1 9.4

Pleuronema crassum 6.1 12.6 1.0

Prorodon sp. 6.6 4.0 3.0

Stentor igneus 20.0 187.7

Stentor multiformis 5.0 18.9 22.2

Stentor roeselii 3.5 6.9 6.3 3.3

Trochilia minuta 1.0 3.0 3.3 9.4

Vorticella campanula 7.3 12.6 33.3

Vorticella picta 32.0 30.0 20.0 33.0 30.0 26.4 26.0 561.2 403.0 258.0 195.5 70.0 74.4 186.0

Zoothamnium procerius 1.0 6.2 6.0 7.0 10.0 1.0 1.0 263.8 140.8 99.0 96.0 76.3 66.7 17.4

period of increased tufa period of

deposit rate increased tufa

deposit rate

three groups (Table 1): ciliates associated with theperiod of increased tufa deposition rate, ciliates asso-ciated with a period of decreased tufa deposition rateand ciliates found during all periods of investigation,independently of the tufa deposition rate.

As evidence that the composition of colonizingciliates in periphyton communities can be associatedwith the tufa deposition rate, we used Principal Com-ponents Analysis (PCA). In a three-dimensional scat-terplot spatial ordination of seven ciliate associations,performed by PCA, we summarized the differences incommunity composition (Figs 8 and 9). In the hab-itat exposed to the current velocity of 5 cm s−1, theciliate communities (which colonized the glass slides

A1, A2, A3, A4 and A5) associated with higher tufadeposition rate were spatially separated from A6 & A7which were colonized in the autumn period charac-terized by the lower deposition rate. The PCA spatialordination of seven colonized ciliate communities at acurrent velocity of 50 cm s−1 showed their spatial sep-aration considering the varied tufa deposition rate too.The ciliate communities that colonized glass slides B5& B6 (summer period with the greatest tufa depositionrate) were spatially separated from the ciliate com-munities that colonized glass slides B1, B2, B3, B4& B7 during the periods of lower deposition rate. Theunrotated factor structure showed that the first threeprincipal components (P1, P2 and P3) explained a total

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Figure 8. Three-dimensional principal ordination of seven ciliateassociations developed under greater (A1, A2, A3, A4 and A5) andlower (A6 and A7) tufa deposition conditions at a current velocityof 5 cm s−1.

of 89.8% of the total variances in the spatial distri-bution of ciliate communities developed at a currentvelocity of 5 cm s−1, and a total of 96.0% of theciliate communities developed at a current velocity of50 cm s−1 (Table 2). In addition, the unrotated loadingshowed that at a current velocity of 5 cm s−1 the ciliatecommunities that colonized glass slides A1, A2, A3,A4 and A5 contributed to PC1 and glass slides A6 andA7 to PC2 with a loading higher than 0.70. Colonizingciliate communities at 50 cm s−1 showed a differentcontribution. With loadings higher than 0.70 all sevenciliate communities contributed to PC1 (Table 3). Thefactor score of ciliates which contributed to PC or-dination of ciliate communities showed that peritrichsCarchesium polypinum, Vorticella campanula, V. pictaand heterotrichs Stentor igneus, S. multiformis werethe main determinants of the derived PC ordination ofciliate communities.

Discussion

The observed relation between current velocity, ratesof periphyton overgrowth, and tufa deposition on arti-ficial substrates has shown that on travertine barrierhabitats the periphyton overgrowth rate after a 2-month period of colonization increased with currentvelocity. In the literature, there are different data aboutthe influence of current velocity on the production of

Table 2. Eigenvalues extracted by the first three principal com-ponents

Current PC Eigenval. % total Cumul. Cumul.

velocity Variance Eigenval. %

1 3.851 55.021 3.851 55.021

5 cm s−1 2 1.959 27.995 5.811 83.016

3 0.475 6.786 6.286 89.802

1 5.936 84.797 5.936 84.797

50 cm s−1 2 0.506 7.226 6.442 92.023

3 0.279 3.989 6.721 96.012

Figure 9. Three-dimensional principal ordination of seven ciliateassociations developed under greater (B5 and B6) and lower (B1,B2, B3, B4 and B7) tufa deposit conditions at a current velocity of50 cm s−1.

periphytic communities. Some reports showed greaterbiomass accumulation at higher current velocities dueto the stimulatory effect of water current (McIntire,1966), while Rolland et al. (1997) found the greatestdiversity of epilithic algae and highest chlorophyllconcentration where the current velocity was moder-ate. Ghosh & Gaur (1994) found the maximum of algalbiomass at low flow (9–12 cm s−1). Finally, Jowett& Biggs (1997) obtained opposite results in differentbiotopes with the same methods.

Analyzing the relationships between periphytongrowth rate and tufa deposition rate we found a pos-itive correlation. This result suggested that the livingbiota plays an important role in calcite deposition and

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that the microflora are active inducers of precipita-tion. According to Golubic & Fisher (1975), Kempe& Emeis (1985) and Pedley (1992), the biota plays afocal role in removing CO2 from solution, acting asa substrate for precipitation and encrustation, and intrapping micritic calcite in its extracellular mucus.

It would be expected, therefore, that there wouldbe a relation between tufa deposition rate and currentvelocity. In our investigations, tufa deposition was onaverage three times greater at a current of 50 cm s−1

than at 5 cm s−1. During the late summer and earlyautumn, when the tufa deposition was the most intens-ive, the values were even six to ten times higher. Ourresults tally with Srdoc et al. (1985). On the travertinebarriers in Plitvice Lakes (Croatia) they observed thatthe tufa formation was greater in lotic areas than inlentic habitats.

The population density of ciliates on exposed ar-tificial substrates ranged from 70 to 879 ind. cm−2.For comparison, on glass slides exposed in verticalprofile (in the same period as these experiments) inLake Visovac, ciliate numbers were from 40 up to2400 ind. cm−2 (Primc-Habdija et al., 1997). Sleighet al. (1992) reported that on stones in a chalk streamwith typical flow rates between 10 and 55 cm s−1, thepopulation density of sedentary peritrich ciliates wasnot large (annual mean 10 ind. cm−2). In the investig-ations of Taylor (1983), the mean number of sessile,filter-feeding ciliates on nitex strips was 95 cm−2.

The aim of the investigations was to define howtufa deposition in the karstic region is reflected in cili-ate composition in the periphyton community. PCAanalysis, which summarizes differences in ciliate com-position, separates assemblages associated with ahigher tufa deposition rate from those associated witha lower tufa deposition rate. Species diversity of cili-ates (Brillouin’s index) was related to the tufa depos-ition rate positively, but nonlinearly (Fig. 7). For anexplanation of our results, it seems to us proper torefer to the first Thienemann principle of biocenot-ics: the more variable the conditions of a place, themore species there are in the community inhabitingit. We considered that a higher tufa deposition rateand periphyton overgrowth rate offer favorable con-ditions for ciliate assemblages. This can be explainedby the fact that the biocrystallization by prokaryoticand eukaryotic periphytic algae changed the microen-vironmental conditions, the physical structure of theperiphyton, and that porosity of tufa gives a new mi-crosubstrate for colonization by ciliates. At greatertufa deposition rate, the specific travertine substrate,

Table 3. Contribution of community composition of colonized cili-ates at a current velocity of 5 cm s−1 (A1–A7) and 50 cm s−1

(B1–B7) associated with seven different conditions of tufa depositaccording to unrotated loading (∗ >0.70)

Current Ciliate PC1 PC2 PC3

velocity community

5 cm s−1 A1 0.815∗ 0.061 −0.057

A2 0.939∗ 0.06 −0.243

A3 0.789∗ 0.114 0.441

A4 0.964∗ 0.058 −0.024

A5 0.833∗ 0.087 0.047

A6 0.188 −0.978∗ 0.010

A7 0.146 −0.985∗ −0.139

50 cm s−1 B1 0.989∗ −0.030 −0.057

B2 0.911∗ −0.192 −0.243

B3 0.857∗ −0.255 0.441

B4 0.983∗ −0.026 −0.024

B5 0.884∗ 0.423 0.047

B6 0.896∗ 0.376 0.010

B7 0.917∗ −0.286 −0.139

with rough surfaces and numerous microtopographicalfeatures, offers available settlement sites for coloniz-ation and refuge for ciliates. Besides increasing thespace available for colonization, the irregularities al-low for sedimentation of detritus – potential food formany micro-organisms, including ciliates.

Several authors have pointed out the importanceof the morphological (topographical) structure of asubstrate for colonization. As was shown by Baker(1984), bacteria colonize the surfaces of roughenedglass or polystyrene about 14 times as fast as theequivalent smooth surfaces, and the author suggestedthat surface roughness might be important in enhan-cing colonization by providing more anchoring points.In their comparison of the colonization by peritrichsof smooth and grooved slides, Harmsworth & Sleigh(1993) concluded that surface irregularities appear toenhance attachment. It is suggested that in general thedensities of peritrich colonization are determined byavailable settlement sites, available food and preda-tion. Microtopographical features offer refuges fromcurrents and create localized turbulence (Davis & Bar-muta, 1989) resulting in increased impingement andsettlement of algal spores (Stevenson, 1983, 1984).Local differences in habitat characteristics provide‘safe sites’ (Harper, 1977). Depressions also providesites in which the magnitude of physical disturbance

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is reduced (Woodin, 1978; Miller et al., 1987). Be-cause refuges provide a habitat in which mortalityand the probability of extinction are reduced, they areimportant to dynamics of populations (McNair, 1986).

In the same relation, we explain the results whichreveal that the substrata exposed in lotic habitats carryon average four times higher ciliate populations thanthose in the lentic habitats (the overall mean numberof ciliates at 5 cm s−1 was 122 cm−2, and at 50 cms−1 497 cm−2). In the literature, very little informationis available concerning the effect of current velocityon the structure of periphytic ciliates. According toBick et al. (1975), Schmerenbeck (1974) and Schön-born (1982), ciliate density decreases with increasingcurrent velocity. Similar results are given by Baldocket al. (1983), who found that protozoan populationsmainly occur in sheltered microhabitats and also thatpopulations are largest at locations of lowest currentvelocity. Amoebae, flagellates and small ciliates werefound, primarily in the form of flat species. These find-ings all suggest the importance the water current as anagent for dislodging cells.

There can be several factors causing populationdensities of ciliates to be so obviously higher in loticthan in lentic areas. Among them we consider greatertufa deposition to be very important – with all the ad-vantages of surface irregularities. The second reasonfor greater ciliate density at higher flow speed mightbe better conditions for food capture. Microfilter feed-ers, mostly stalked bacterivorous peritrichs, made upthe majority of the ciliate community. They probablyconsume the food suspension from plankton-rich lakewater where the quantity of food particles dependsupon the discharge. The swift current rapidly removesthe relatively impoverished water near the community,thus bringing water of full food concentration closerto the ciliates.

Furthermore, type of periphyton can be also re-sponsible for different rates of ciliate accumulation inthe two microhabitats. On the substrata exposed to thefaster current velocity, crusty assemblages were de-veloped. Here, together with stalked peritrichs, adnatediatoms and cyanobacterial colonies (Gomphonema,Cymbella, Achnanthes and Phormidium) dominated.In the lentic area motile-planktonic algae (Diatoma,Navicula, Synedra, Cyclotella and Fragilaria) weremore important in percentage terms. Consequently,the periphyton developing in slow flow conditions wasmore loosely attached than that developing in a fasterflow. A loose attachment to the substratum could makethis assemblage unstable because it would probably be

scoured more easily by flooding. Cattaneo et al. (1997)pointed out that a looser attachment to the substratumcould make these assemblages more readily availableto grazers.

In karstic regions, a strongly attached periphytonis closely correlated not only with current velocity butalso with tufa deposits. The core species are thosethat are known to have a role in calcification, e.g.Gomphonema, Cymbella and Phormidium. We canconclude that the effect of current velocity in a karsticarea has different implications than in a ‘non-karstic’area. Namely, with increased current velocity (up toabout 100 cm s−1) tufa deposit increases (Srdoc et al.,1985). Tufa has a rough surface from calcite crystalsand high porosity. We consider that increased tufa-deposition offers greater possibilities for refuge formicroorganisms. Consequently, in karstic biotopes,active drift is not the primary factor influencing thebiomass and structure of the periphyton.

Acknowledgements

The research upon which this study is based was sup-ported by grant from the Ministry of Science andTechnology of the Republic of Croatia.

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