Hydromorphological conditions, potential fish habitats and the fish community in a mountain river subjected to variable human impacts, the Czarny Dunajec, Polish Carpathians

RIVER RESEARCH AND APPLICATIONS

River. Res. Applic. 25: 517–536 (2009)

Published online 4 February 2009 in Wiley InterScience

HYDROMORPHOLOGICAL CONDITIONS, POTENTIAL FISH HABITATS ANDTHE FISH COMMUNITY IN A MOUNTAIN RIVER SUBJECTED TO VARIABLE

HUMAN IMPACTS, THE CZARNY DUNAJEC, POLISH CARPATHIANS

B. WYZGA,a* A. AMIROWICZ,a A. RADECKI-PAWLIKb and J. ZAWIEJSKAc

a Polish Academy of Sciences, Institute of Nature Conservation, al. Mickiewicza 33, 31-120, Krakow, Polandb Department of Water Engineering, Agricultural University, al. Mickiewicza 24/28, 30-059 Krakow, Poland

c Institute of Geography, Pedagogical University, ul. Podchora zych 2, 30-084 Krakow, Poland

(www.interscience.wiley.com) DOI: 10.1002/rra.1237

ABSTRACT

The Czarny Dunajec River, Polish Carpathians, has been considerably modified by channelization and gravel mining-inducedchannel incision and it varies in morphology from a single-thread, incised or regulated channel to an unmanaged, multi-threadchannel. For twelve cross-sections with between 1 and 4 flow threads, the abundance and diversity of fish fauna were determinedby electrofishing and compared with an assessment of hydromorphological river quality and the variation in flow depth, velocityand bed material size. Hydromorphological quality of the river varied between high-status conditions (Class 1) in unmanaged,multi-thread cross-sections with varying proportions of islands and Class 4 in channelized cross-sections. The increased numberof low-flow channels in a cross-section was associated with a larger aggregated width of low-flow channels and greater variationin flow depth, velocity and bed material size. Single-thread cross-sections hosted only 2 fish species and 13 individuals onaverage, whereas 3–4 species and 82 individuals on average were recorded in cross-sections with four low-flow channels.Regression analysis indicated that both the number of fish species and individuals increased linearly with increasing variation indepth within a cross-section and exponentially with improving hydromorphological river quality, while they were unrelated toflow width, suggesting that it is the increase in variability of habitat conditions rather than simple habitat enlargement, thatsupported the increased abundance and diversity of fish fauna in the multi-thread cross-sections. This study shows that thesimplification of flow pattern and the resultant degradation of hydromorphological river quality, caused by human impacts, isreflected in remarkable impoverishment of fish communities and that recovery of these communities will require an increase inmorphological complexity of the river. Copyright # 2009 John Wiley & Sons, Ltd.

key words: mountain river; human impact; hydromorphological river quality; fish habitats; fish fauna

Received 19 December 2008; Accepted 23 December 2008

INTRODUCTION

During the 20th century most reaches of Polish Carpathian rivers were greatly modified by human activities. Intense

channelization works resulted in considerable channel narrowing, accompanied by significant channel shortening

in the lower reaches of rivers and the replacement of former, multi-thread channels by single-thread, artificial ones

in the middle and upper reaches (Wyzga, 2001a, 2008; Bojarski et al., 2005). In the 1940s–1960s huge amounts of

gravel were mined from some Carpathian rivers (Rinaldi et al., 2005) and in the subsequent decades illegal

exploitation of bed material from mountain stream and river channels was a common practice (Wyzga, 2001a;

Radecki-Pawlik, 2002). The increase in transport capacity of Carpathian rivers caused by channelization and also

removal of bed material resulted in rapid bed degradation (Wyzga, 2008), which was strengthened in the second

half of the century by a reduction in sediment delivery resulting from an increase in forest cover in the montane

parts of catchments (Lach and Wyzga, 2002). As a result, Carpathian rivers experienced channel incision of

between 0.5 and 3.8m during the 20th century, with rates of incision being particularly high in many river sections

during the second half of the century. Incision has increased the flow capacity of channels and so has reduced the

frequency and lateral extent of inundation of valley floors, inducing a marked increase in flood hazard to

*Correspondence to: B. Wyzga, Polish Academy of Sciences, Institute of Nature Conservation, al. Mickiewicza 33, 31-120 Krakow, Poland.E-mail: [email protected]

Copyright # 2009 John Wiley & Sons, Ltd.

518 B. WYZGA ET AL.

downstream reaches (Wyzga, 1997, 2008) and a reduction in the potential of Carpathian floodplains to store

sediment (Wyzga, 2001b, 2008). With these and many other detrimental effects of channel incision becoming

apparent at a local scale, concrete weirs were constructed in many Carpathian stream and river channels to arrest

bed degradation and reduce bedload transport (Bednarczyk et al., 2003). This construction has disrupted the

continuity of the watercourses for fish (Bojarski et al., 2005; Wisniewolski, 2005).

In the 20th century, many European mountain and piedmont rivers were subjected to similar human disturbances

and the resultant loss of vertical channel stability (e.g. Bravard et al., 1997; Surian and Rinaldi, 2003) and decrease

in the biodiversity of aquatic and riparian ecosystems (e.g. Roux et al., 1989; Muhar et al., 2008). With increasing

understanding of the adverse effects of human activities in river channels, restoration measures are being

undertaken widely to improve their geomorphological and ecological conditions (Habersack and Piegay, 2008),

strengthened by the Water Framework Directive of the European Union (Directive 2000/60/EC), which requires

that rivers will attain good ecological status by 2015.

To restore good ecological status of watercourses, it is essential to determine the relations which exist between

the structure of river biocoenoses and the hydromorphological characteristics of the habitats. Such relationships

support assessment of the degree to which the good quality of river ecosystems may be re-established through the

improvement of physical characteristics as well as the flow and sediment regime of the watercourse (i.e. its

hydromorphological quality), or whether ecosystem degradation has resulted from other factors such as a decrease

in water quality. Moreover, determining gradients in the biodiversity of riverine communities in relation to the scale

of habitat modification with respect to reference conditions may support identification of the range of restoration

measures which need to be undertaken.

This paper aims to determine relationships between the abundance and diversity of fish fauna and

hydromorphological conditions in a number of cross-sections of the Czarny Dunajec draining the Polish

Carpathians. Among the groups of biota used as indicators of the ecological status of watercourses (fish,

macroinvertebrate and diatom communities), fish have the longest life cycle and so are likely to be more sensitive to

long-term changes in physical characteristics of their habitats than to short-term changes in water quality. Thus, the

status and condition of fish fauna are a useful monitoring tool to identify degradation in the ecological integrity of

rivers caused by human-induced modifications of hydromorphological conditions (Schiemer, 2000).

STUDY AREA

The gravel-bed Czarny Dunajec (Figure 1) constitutes the upper part of the Dunajec, the second largest river of the

Polish Carpathians. The Czarny Dunajec rises at about 1500m a.s.l. in the high-mountain Tatra massif. In the Tatra

Mountains foreland, it flows for a length of about 38 km to its confluencewith the Biały Dunajec River at an altitude

of 578m (Figure 1B).

During the second half of the 20th century, the river channel was affected by considerable, spatially variable

human modification (Krzemien, 2003; Zawiejska and Krzemien, 2004; Zawiejska and Wyzga, submitted to press).

In the 1950s–1960s intense gravel mining from the river bed occurred at several locations (Dudziak, 1965),

followed in subsequent decades by widespread, illegal extraction of larger cobbles (Krzemien, 2003). These

activities induced up to 3.5m of channel incision and transformation from an alluvial to bedrock bed in many

sections of the river within the Gubałowka Hills (Zawiejska and Wyzga, submitted to press). In the 1960s–1990s, a

7 km long stretch in the middle river course was progressively channelized using 0.7–2.1m high concrete drop

structures, which disrupted fish passage, and the former multi-thread channel was replaced by a nearly straight,

single-thread one. Immediately downstream of the channelized stretch, the river remains unmanaged over a length

of about 4 km, with its channel pattern varying from braided through island-braided to heavily island-braided.

Further downstream, throughout the lower course of the river, the channel was considerably straightened and

narrowed by channelization works, which were completed by the 1980s, but the channel gradient was not reduced

by construction of drop structures.

The study reported in this paper was performed in a 17 km long reach in the middle river course, along which the

catchment area increases relatively little and the river receives no major tributaries (Figure 1B). This reach

comprises a deeply incised channel in its upper part, a regulated channel with drop structures in its middle part, an

Copyright # 2009 John Wiley & Sons, Ltd. River. Res. Applic. 25: 517–536 (2009)

DOI: 10.1002/rra

Figure 1. (A) Location of the Czarny Dunajec River in relation to physiogeographic regions of southern Poland. (B) Drainage network andphysiography of the Czarny Dunajec catchment and detailed setting of the investigated sections of the river: (1) high mountains; (2) mountains ofintermediate and low height; (3) foothills; (4) intramontane and submontane depressions; (5) the Czarny Dunajec catchment to the beginning ofthe study reach; (6) catchment area increment along the study reach; (7) boundary of the Czarny Dunajec catchment; (8) boundaries ofphysiogeographic units; (9) river cross-sections investigated; (10) flow-gauging stations; PKB, Pieniny Klippen Belt; STT, Sub-Tatran Trough

HYDROMORPHOLOGY AND FISH COMMUNITY IN MOUNTAIN RIVER 519

unmanaged channel in the lower part and again a regulated channel at its downstream end. The reach is typified by

considerable variation in river morphology (Wyzga and Zawiejska, 2005) including single- and multi-thread

sections, sections with alluvial and bedrock boundaries and with incised and vertically stable beds. Together with

differences in channel management, these contrasting channel conditions are associated with high variability in

physical habitat conditions and are likely to be reflected in differences between local river biocoenoses. Indeed,

considerable variation in the ecological status of the Czarny Dunajec in its middle course was revealed by combined

evaluation of both biotic and abiotic characteristics of the river performed in the 1990s (Kulesza, 1998). Since


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520 B. WYZGA ET AL.

channel incision in the upper part of the study reach has continued, the present downstream variations in

hydromorphological conditions within the reach may be even larger than were observed in the 1990s.

STUDY METHODS

Investigations were conducted at twelve river cross-sections, representing the range of hydromorphological

conditions present in the study reach of the Czarny Dunajec. Hydromorphological river quality in the cross-sections

was assessed through scoring of ten groups of features of the channel, river banks, riparian zone and floodplain

(Table I), mainly according to their specification in the European Standard EN-14614 (CEN, 2004), although two

additional features were added (Table I). First, an appraisal of in-channel, aquatic vegetation was replaced by

separate appraisal of aquatic vegetation and island vegetation within the river’s active zone. This reflected a

considerable role played by island vegetation in the Czarny Dunajec and the observation that the island vegetation

may significantly differ in naturalness from the vegetation in the riparian zone. Second, in the assessment of

modifications to river flow, we completed an appraisal of discharge regime and flow hydraulics by evaluating the

degree of connectivity between river channel and hyporheic zone. While the significance of water exchange

between river channel and hyporheic zone for river biota is well recognized (e.g. Baxter and Hauer, 2000), its

occurrence may be drastically reduced or even eliminated by channel incision which induces a change from an

alluvial to bedrock river bed.

A reference state for the river was not derived from historical information because its previous braided state,

particularly in the 19th century, was partly a response to intense agricultural and pastoral activities. Instead, we

considered reference hydromorphological conditions as those that would exist under present environmental

Table I. Assessment categories and features of the channel, banks, riparian zone and floodplain of the Czarny Dunajec assessedto evaluate the hydromorphological quality of the river according to the European Standard EN-14614 (CEN, 2004)

No. Assessment category Assessed feature

Channel1 Channel geometry Channel planform

Channel cross-section and longitudinal profile2 Substrate Artificial/natural bed substrate

Degree of modification of substrate material3 In-river vegetation and organic debris Aquatic vegetation and island vegetation within

the river’s active zone�

Organic material (leaves, woody debris)4 Erosion/deposition features Presence of erosional and depositional channel forms5 Flow Modification of natural flow hydraulics by engineering

works and structuresModification of natural flow regimeDegree of connectivity between river channel andhyporheic zone�

6 Longitudinal river continuity Impact of engineering structures on longitudinal rivercontinuity—sediment transport and biota migration

Banks and Riparian Zone7 Bank structure Modification of river banks (material, profile, height)8 Riparian zone Vegetation and land use in the riparian zoneFloodplain9 Land use and associated features in the river corridor Land use in the river corridor/floodplain

Presence of remnant channels, oxbows and mires withinthe floodplain

10 Lateral connectivity and channel migration Degree of lateral connectivity of river and floodplain,and continuity of floodplain along the riverConstraint on lateral channel movement

�feature modified or added to the list indicated in the European Standard EN-14614.


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conditions in the catchment but without human influence on the channel, riparian zone and floodplain of the river.

The assessment was performed simultaneously by four specialists in fluvial geomorphology, river engineering and

hydrobiology following field inspection of the cross-sections, the analysis of river change in their vicinity over the

past few decades as well as the presentation of diagrams of the cross-sections and river appearance in particular

cross-sections on orthophotos and ground photos (Figure 2). Each assessed category was scored on the scale from

1 (for near-natural conditions) to 5 (for extremely modified conditions). The aggregated score, averaged from the

scores of the four specialists, allowed each of the cross-sections to be associated with a particular class of

hydromorphological quality.

Figure 2. Examples of the sources of information used to evaluate hydromorphological quality of the Czarny Dunajec River in the surveyedcross-sections, illustrated for cross-section D with single-thread, regulated channel and cross-section K with multi-thread, partly reinforced

channel. This figure is available in colour online at www.interscience.wiley.com/journal/rra


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522 B. WYZGA ET AL.

The investigated cross-sections were located to run consistently across pools, so as to examine differences

between particular types of river morphology and channel management rather than those related to pool-riffle

sequences in the river. Detailed levelling of the cross-sections was performed during baseflow conditions and water

depth, mean flow velocity and mean grain size of surface bed material were determined at 1m intervals within the

low-flow channel(s). For gravelly sediments, transect sampling (which yields results equivalent to bulk sieve

analysis; Diplas and Sutherland, 1988; ISO 9195, 1992) was used, with 15 particles collected from the bed at each

site; this number of particles was chosen to avoid extension of the sampling beyond the area characterized by

hydraulic measurements. Subsequently, the distribution of the ‘b’ axis diameters of collected particles was

determined and their mean size was calculated as an average of the diameter of the 3rd, 8th and 13th grain in the

sequence (reflecting the 20th, 50th and 80th percentiles of the grain size distribution), this being the closest

available approximation of the formula of Folk and Ward (1957). The grain size distribution of sandy and silty

sediments was established in a laboratory using sieving or hydrometer analyses and their mean grain size was

calculated from the same percentiles of the distribution. For each cross-section, means and coefficients of variation

of water depth, mean flow velocity and mean grain size of surface bed material were then calculated.

Species composition of fish communities in the investigated cross-sections was estimated on the basis of single

electrofishings conducted on 4 September 2006. Fishes were caught in a 10m wide strip along each cross-section

using the backpack electroshocker (DEKA-Lord). Number and approximate total lengths of fish specimens caught

in particular low-flow channels were recorded. Two age categories were distinguished: juveniles (YOY) as well as

subadult and adult fishes (1þ age class and older). Specimens were assigned to these categories on the basis of their

total length, taking the length of 10 cm (brown trout) or 5 cm (other species) as the size limit.

RESULTS

Hydromorphological river quality

Evaluation of the hydromorphological quality of the river indicated considerable variation in this parameter

among the surveyed cross-sections, with the values ranging between 1.08 and 3.96 (Table II). For all cross-sections

with four low-flow channels, the river was classified as representing high status (reference) conditions. Cross-

sections I and J, showing heavily island-braided morphology, were considered to be only slightly modified by

human activity (scores 1.08 and 1.26), while a somewhat greater degree of modification, largely as a result of

artificial reinforcement of one of the channel banks, typifiedmulti-thread cross-sections H and Kwith moderate and

scarce occurrence of islands, respectively (scores 1.57 and 1.69). Single-thread cross-sections with regulated

Table II. Averages of the scores given by four evaluators for particular assessment categories and average scores of thehydromorphological river quality in the investigated cross-sections of the Czarny Dunajec. The average scores for particularcross-sections are shown against hydromorphological quality classes

Evaluated cross-section A B C D E F G H I J K LNumber of flow threads I III I I I I II IV IV IV IV I

Channel geometry 2.63 2.25 2.75 4.38 4.50 3.75 2.88 1.25 1.13 1.00 1.50 4.50Substrate 2.25 2.25 2.25 2.75 3.13 2.75 1.63 1.50 1.13 1.00 1.25 2.75In-river vegetation and organic debris 2.81 2.63 3.13 4.00 4.00 4.00 3.25 2.63 1.88 1.63 2.25 4.00Erosion/deposition character 2.75 2.00 2.00 4.25 4.75 3.75 2.25 1.50 1.00 1.00 1.00 4.38Flow 1.92 2.00 1.92 2.42 2.67 2.46 1.59 1.17 1.08 1.00 1.25 2.79Longitudinal river continuity 1.00 1.00 1.00 4.50 4.50 5.00 1.25 1.00 1.00 1.00 1.00 1.00Bank structure 2.00 2.00 2.00 3.50 4.00 3.88 2.63 1.75 1.00 1.00 2.38 3.88Vegetation/land use in riparian zone 1.63 2.38 2.13 2.13 2.63 2.38 1.88 1.88 1.75 1.00 1.38 2.50Land use in the river corridor 2.06 3.00 2.75 3.38 4.75 3.88 3.25 2.00 1.63 1.13 1.63 4.75Lateral connectivity/channel migration 2.88 2.38 2.25 4.13 4.63 4.56 3.38 2.25 1.06 1.00 2.06 4.88Average score 2.19 2.19 2.22 3.54 3.96 3.64 2.40 1.69 1.26 1.08 1.57 3.54

Key for hydromorphological quality classification: 1.00–1.79, High (reference conditions); 1.80–2.59, Good; 2.60–3.39, Moderate; 3.40–4.19,Poor; 4.20–5.00, Bad.


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channels (D–F and L) were scored between 3.54 and 3.96, hence falling into Class 4 of hydromorphological quality.

This reflected radical modification of channel geometry in the cross-sections (Figure 2—cross-section D), absence

of erosional and depositional forms, disturbance of fish migration by weirs, loss of hydraulic connectivity of the

river with its floodplain and lateral channel stabilization (Table II). Finally, unmanaged cross-sections A–C (two

single-thread cross-sections and one cross-section with three low-flow channels) with deeply incised channel as

well as cross-section G with two low-flow channels and partly reinforced banks were considered to represent

hydromorphological quality Class 2, as a result of modest deviation from reference conditions for a number of

features that were evaluated (Table II).

As the evaluation of hydromorphological quality of the river was based on expert judgement, it is interesting to

see the degree of agreement between scores assigned to particular cross-sections by the four specialists and the high

similarity in the opinions of experts for most of the evaluated cross-sections (Figure 3). The opinions differed by

about 1 quality class for only three cross-sections: cross-section C, in which the river had incised by about 3m over

the second half of the 20th century; cross-section F with narrow, regulated channel and moderately wide cross-

section G with partly reinforced banks. For each of these cross-sections, better opinions on a number of evaluated

features were expressed by a hydrobiologist than by both fluvial geomorphologists who were more aware of

human-induced changes to the channel morphology and their impact on hydraulic and sedimentary characteristics

of the river (increase in flow velocity, simplification of the flow pattern, coarsening of bed material) in these

locations (cf. Wyzga et al., 2009).

Physical parameters of riverine habitats

Measurements of hydraulic parameters were performed at discharges varying between 3.15 and 3.74m3 s�1

among the surveyed cross-sections. This variation reflected run-off variability during the measurement campaign

and the specific hydrographic setting of the study reach (Figure 1). No significant systematic change in the

measured discharge was found either with the position of a particular cross-section in the downstream sequence

(linear regression, p¼ 0.48) or with the distance of a cross-section from the beginning of the study reach (p¼ 0.36).

Therefore, it was possible to consider that the data from all cross-sections represented similar flow conditions and

Figure 3. Scores given by four evaluators for the hydromorphological river quality in the investigated cross-sections of the Czarny Dunajec andthe average of the four estimates. The number of flow threads in each cross-section is also indicated


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524 B. WYZGA ET AL.

thus to analyse whether the physical parameters of fish habitats in the river were dependent on the complexity of

flow network. Multi-thread cross-sections were typified by significantly greater (Mann–Whitney test, p¼ 0.03)

aggregated width of low-flow channels than single-thread cross-sections (Figure 4), with widths of 19.8m on

average in single-thread cross-sections and 34.1m in cross-sections with four low-flow channels (Figure 5). An

Figure 4. Examples of cross-sectional morphology of the Czarny Dunajec in channelized river sections (upper) and an unmanaged section(lower). For low-flow channels, mean grain size of the sediment on the bed surface and mean flow velocity are indicated at 1m intervals. The

scale for velocity commences at the water surface for each low-flow channel


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Figure 5. Scatter plots of physical characteristics of the Czarny Dunajec versus the number of flow threads in the investigated cross-sections.The significance of the difference of the parameter means between the cross-sections with one and four low-flow channels were determined by

Mann–Whitney test. The dashed lines link the parameter means between the two groups of cross-sections


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526 B. WYZGA ET AL.

increasing number of low-flow channels in a cross-section was also associated with a reduction in mean flow

velocity, whereas mean flow depth was not related to the number of flow threads (Figure 5). Moreover, cross-

sections with four low-flow channels were typified by significantly finer material on the bed surface than those with

a single channel (mean grain size 49.5 and 88.3mm, respectively; Mann–Whitney test, p¼ 0.01) (Figure 5).

However, the most striking difference between single- and multi-thread cross-sections was in the variation of

physical characteristics of fish habitats (Figures 4 and 5). Single-thread cross-sections were typified by significantly

lower variation in flow depth (Mann–Whitney test, p¼ 0.01), velocity (p¼ 0.02) and mean size of bed material

(p¼ 0.02) than the cross-sections with four low-flow channels (Figure 5). In the former, the pattern of flow depth

and velocity was relatively regular and a gravel bed occurred across the whole channel width (Figure 4; cross-

sections E and L). In the latter, braids with fast-flowing water were accompanied by those with a slow current and,

consequently, the dominating gravelly parts of the bed were accompanied by areas covered with sand or mud

(Figure 4; cross-section I).

Examination of scatter diagrams for the pairs of physical parameters measured at 1m intervals emphasizes

differences which exist between fish habitats occurring in single- and multi-thread cross-sections of the Czarny

Dunajec. First, locations with slow-flowing, shallow water were considerably more abundant in multi-thread cross-

sections than in single-thread ones (Figure 6). For instance, sites with water depth not exceeding 0.2m and flow

velocity less than 0.2m s�1 represented 11% of all sites in six investigated single-thread cross-sections, whereas in

six multi-thread cross-sections, they comprised 36% of all sites and were 5.7 times more abundant in absolute

numbers.

Second, single-channel sections lacked fine bed sediments in sites with low-flow depth and slow water current

(Figure 6). Even if deposited on the bed at low to medium flows, such sediments are apparently easily flushed out

from these sites with every flow increase, indicating that shallow-depth, slow-flow conditions are transient in these

sections. At the same time, grain size of the cobble bed was unrelated to flow velocities measured at baseflow

(Figure 6) as the bed forms at substantially higher velocities during floods. In contrast, multi-thread channel

sections exhibited two populations of material covering the bed: pebble to cobble sediments in the main braids

conveying most flow, indicating depositional conditions during flood flows, and muddy-sandy sediments in the

braids with slow water current, their grain size being adjusted to the hydraulic conditions at low to medium flows

(Figure 6). The persistence and sometimes quite considerable thickness of such fine sediment overlying gravelly

material in the latter braids reflects relatively long-lasting disconnection of their upstream end from the main water

current, when the braids are at least partly fed by groundwater.

Finally, in single-thread cross-sections, flow velocity generally increased with increasing water depth (Figure 6).

This reflected the occurrence of slow water currents in shallow areas close to the river banks and faster flow towards

the channel centre, where the depth was greatest (Figure 4; cross-sections E and L). In multi-thread cross-sections,

no relation between water depth and flow velocity was observed (Figure 6). Here, in some braids low velocities

occurred in shallow channel areas where flow retardation due to bed roughness was greatest, while in others low

velocities were found in deep pools due to backwater effects (Figure 4; cross-section I). Consequently, fine

sediments in these multi-thread cross-sections were deposited in both shallow and deep channel areas (Figure 6).

Fish communities

The fish fauna of the investigated reach of the Czarny Dunajec consisted of four species. Both the species

composition and abundance of fish varied among the surveyed cross-sections (Figures 7 and 8). Brown trout, Salmo

trutta L. and Alpine bullhead, Cottus poecilopus Heckel were recorded in all cross-sections, while the remaining

two species, Eurasian minnow, Phoxinus phoxinus (L.) and stone loach, Barbatula barbatula (L.), occurred in only

five and two cross-sections, respectively. Among 1463 fishes (1010 juveniles and 453 older individuals) recorded,

minnow was the most abundant, with 953 juveniles and 316 older individuals representing a majority of the total

sample. Considerably less abundant were Alpine bullhead (38 and 92, respectively) and brown trout (16 and 40),

whereas only a few stone loach (3 and 5) were recorded in the investigated cross-sections.

In general, multi-thread cross-sections hosted greater numbers of both species and older individuals than

single-thread cross-sections (Figure 8). In single-thread cross-sections, fish assemblages consisted of only 2 species

and 4–22 individuals (mean: 13), whereas in the cross-sections with four low-flow channels, they comprised


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Figure 6. Scatter plots for the pairs of physical characteristics of the Czarny Dunajec measured at all sampling sites in the investigated single-thread cross-sections (left diagrams) and multi-thread cross-sections (right diagrams) of the river


3–4 species and 36–119 individuals (82 on average), with the differences in the number of both species and

specimens being statistically significant (Mann–Whitney test, p¼ 0.01 in both cases). Multi-thread cross-sections

also hosted numerous juveniles (mean: 166) in comparison with very low numbers of such fishes in single-thread

cross-sections (only two specimens on average) (Figure 7). Moreover, these two types of cross-sections differed in

the proportion of juvenile and older specimens (x2 test, p< 0.0001). In the multi-thread cross-sections, juveniles

were 2.7 times more abundant than subadult and adult individuals (998 and 375), whereas in the single-channel

cross-sections they were 6.5 times less numerous (12 and 78).

However, these relations did not follow a simple spatial pattern within the study reach (Figures 7 and 8).

The greatest numbers of both juvenile and older specimens occurred in cross-sections I and J which had a heavily

island-braided morphology and were located in the central part of a 4 km long, multi-thread channel section.

Despite the same number of low-flow channels, cross-sections H and K, which supported a moderate to scarce

occurrence of islands and were located close to the ends of the multi-thread channel section, hosted less abundant


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Figure 7. Species composition of fish assemblages recorded during single electrofishing surveys within low-flow channels of the investigatedcross-sections of the Czarny Dunajec. This figure is available in colour online at www.interscience.wiley.com/journal/rra

528 B. WYZGA ET AL.

juvenile and older fishes. Two-species fish assemblages with relatively low numbers of individuals were found in

single-thread cross-sections located both upstream and downstream of the unmanaged, multi-thread channel

section. A similar assemblage with only two species and quite low number of individuals was also recorded in

multi-thread cross-section B, which had a bar-braided morphology and was located in the deeply incised, upper part

of the study reach.

Relationships between the abundance and diversity of fish and explanatory variables

With the detailed recognition of a set of physical parameters for the surveyed cross-sections, it was possible to

verify whether they can explain the observed variability in the abundance and species diversity of fish among the


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Figure 8. Comparison of the number of low-flow channels, the assessment of hydromorphological quality and the results of electrofishingcarried out in 12 cross-sections of the Czarny Dunajec River: St, Salmo trutta L.; Cp, Cottus poecilopus Heckel; Pp, Phoxinus phoxinus (L.);Bb, Barbatula barbatula (L.); n, total number of subadults and adults (�1 year old) caught; dashed line indicates the occurrence of

juveniles (YOY) only. This figure is available in colour online at www.interscience.wiley.com/journal/rra


cross-sections. Results from regression analysis indicated that the number of fish species as well as subadult and

adult individuals increased linearly with increasing variation in water depth in a cross-section (Figure 9, Table III).

The number of individuals also increased with decreasing grain size of material on the channel bed and with

increasing variation of grain size of the material in a cross-section. Other physical parameters under consideration

had no significant effect on fish communities (Table III).

The species diversity of fish communities and fish abundance in the surveyed cross-sections were greater in the

cross-sections of better hydromorphological quality (Figure 8). Regression analysis showed that fish diversity and

abundance increase exponentially with improving hydromorphological river quality (i.e. decreasing quality scores)

(Figure 9, Table III). For fish abundance, this non-linear relationship was especially pronounced and the

greatest increase in the number of individuals occurred with a relatively small improvement in hydromorphological

quality of the river that accompanied the change from braided morphology with occasional islands to heavily


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Figure 9. Scatter plots and estimated regression relationships between the number of fish species (left diagram) and the number of subadult andadult fish (right diagram) caught in the investigated cross-sections of the Czarny Dunajec, and the coefficient of variation of flow depth and

hydromorphological river quality in the cross-sections

530 B. WYZGA ET AL.

island-braided morphology. Importantly, the relationship with hydromorphological river quality explains a

considerably greater portion (88%) of the total variation in fish abundance than the regression models estimated for

single physical parameters of fish habitats (between 34% for the coefficient of grain size variation and 50% for the

coefficient of flow depth variation) (Table III).

Table III. Explanation and significance of regression models estimated between the number of fish species and the number ofsubadult and adult fishes caught in the investigated cross-sections of the Czarny Dunajec River, and potential explanatoryvariables. Relationships with p-values < 0.05 are indicated in bold

Independent variable Dependent variable Type ofregression

Number of species Number of specimens

Low-flow channel width R¼ 0.35 p¼ 0.26 R¼ 0.42 p¼ 0.17 linearMean flow depth R¼�0.34 p¼ 0.28 R¼ 0.01 p¼ 0.96 linearFlow depth—coefficient of variation R¼ 0.75 p¼ 0.005 R¼ 0.71 p¼ 0.009 linearMean flow velocity R¼�0.32 p¼ 0.30 R¼�0.43 p¼ 0.16 linearFlow velocity—coefficient of variation R¼ 0.39 p¼ 0.21 R¼ 0.51 p¼ 0.09 linearMean grain size R¼�0.56 p¼ 0.056 R¼S0.59 p¼ 0.04 linearGrain size—coefficient of variation R¼ 0.47 p¼ 0.12 R¼ 0.58 p¼ 0.047 linearHydromorphological river quality R¼S0.76 p¼ 0.000001 R¼S0.94 p¼ 0.000001 non-linear


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DISCUSSION

The study performed in the mountain Czarny Dunajec River provided quantitative data on the differences in

hydromorphological conditions between channel sections heavily modified by channelization and gravel mining-

induced channel incision, and those which remained in a relatively undisturbed state. The latter distinguish from the

modified channel sections by their greater low-flow channel width, lower flow velocity and finer surface bed

material. It should be emphasized that the disparity in mean size of channel gravels between multi-thread channel

section located in the lower part of the study reach and single-thread sections in its upper and middle parts is not

simply a manifestation of a downstream-fining trend of bed material in the river but is enhanced by differences in

the active channel width (Wyzga et al., 2009) that induce distinct differences in unit stream power during flood

flows between these sections (Wyzga and Zawiejska, 2005; Wyzga, 2007). During low to medium flows, deposition

of fine sediments on the bed of less active braids additionally accentuates the difference in grain size of surface bed

material between the single-thread and multi-thread channel sections.

The greater complexity of the flow network in the undisturbed channel section is also reflected in the higher

cross-sectional variability in flow depth, velocity and mean size of bed material. An important feature of the multi-

thread cross-sections is the occurrence of different combinations of habitat conditions such as the association of

slow water current with both shallow and deep channel areas. In contrast, in single-channel sections, variation in

one physical parameter typically parallels that in another, resulting in a simple gradient from shallow, low-energy

habitats to deep, high-energy ones. The multi-thread channel sections not only exhibit significantly greater spatial

variability in habitat conditions than single-channel sections but they also seem to be typified by greater temporal

stability of shallow-water, slow-velocity conditions useful for juvenile fishes. Moreover, among the multi-thread

cross-sections, a trend of increasing persistence of such conditions/habitats is apparent with an increase in the share

of vegetated islands within the active zone of the river as evidenced by the lack of fine sediment deposition in cross-

section B which had a bar-braided morphology and the considerable extent of such deposition in the cross-sections

with a heavily island-braided morphology. This observation agrees with greater rates of floodplain reworking and

habitat restructuring observed in a bar-braided reach of the Tagliamento River, Italy, in comparison with an island-

braided reach (van der Nat et al., 2003), and probably reflects the stabilizing effect of hydraulic shelter and root

strength provided by island vegetation.

Despite some subjectivity inherent in expert judgements on hydromorphological quality of the river (cf. CEN,

2004), the independent evaluations of quality by four specialists in different disciplines generally yielded quite

similar results, but also enabled averaging of extreme opinions in a few cases. The evaluation indicated

considerable differences in hydromorphological river quality within the study reach. The unmanaged, multi-thread

channel section with varying island extent within the active zone of the river was assessed to represent high quality

hydromorphological conditions. In contrast, channelized river sections, with their radically modified channel

geometry and lost longitudinal (due to the construction of weirs) and lateral connectivity (as a result of the

elimination of lateral channels and the river isolation from its floodplain), were considered to be in a poor

hydromorphological state. Finally, deeply incised but presently unmanaged channel sections were classified as

representing good hydromorphological conditions.

Five fish species were reported from this reach of the Czarny Dunajec in the second half of the 19th century

(Nowicki, 1883) and in the years 1975–1980 (Starmach, 1984). This small number of species did not reflect poor

environmental quality of the river but a relatively high position of the reach within the hydrographic network

(Starmach, 1984) as well as the characteristics of Central European biogeographic region. Rivers of this region

exhibit a relatively small diversity of fish species as a result of elimination of some species during the last glaciation

and difficulties in re-colonization from refuges in theMediterranean area across the Alpine-Carpathian water divide

(Hewitt, 1999; Reyjol et al., 2007).

The present study documented the occurrence of four fish species in the investigated reach of the Czarny

Dunajec. Grayling, Thymallus thymallus (L.), which until three decades ago was recorded over most of the river’s

length (Starmach, 1984) was absent, and the extent of Eurasian minnow and stone loach, which were previously

found throughout the study reach (Starmach, 1984), was limited to the multi-thread channel section in its lower

part. These changes can be explained by alterations to the hydromorphological conditions in the river caused by

channel regulation in the middle part of the study reach and channel incision in its upper part. The construction of


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532 B. WYZGA ET AL.

weirs in the channelized river section prevents fish migration from the downstream sections that have maintained

high quality conditions. An almost complete elimination of multi-thread channel pattern and shallow areas in the

regulated channel has resulted in the homogenization of habitat conditions, and the concentration of flow has led to

an increase in flow velocity and mean size of bed material. Habitat heterogeneity and the availability of shallow

habitats have been shown to be important for grayling, especially juveniles, in the Drau River (Muhar et al., 2008).

The increase in flow velocity in impacted sections of the study reach may also have been an important factor

reducing the extent and abundance of stone loach (cf. Santoul et al., 2005). An additional negative impact on this

species, especially at the limit of its occurrence, may be co-existence with brown trout, as a result of competition

with its juveniles or predation by larger individuals (cf. Nilsson and Persson, 2005).

The parts of the study reach modified in recent decades by channelization and channel incision are currently

typified by the occurrence of two species of fish: brown trout and Alpine bullhead. This simple fish assemblage is

characteristic of Polish Carpathian streams and rivers with relatively steep channel gradients, coarse bed material

and high flow velocities (Starmach, 1984; Starmach et al., 1988, 1991). However, larger numbers of juveniles of

both species were recorded only in the incised cross-section B that supported three low-flow channels. Single-

thread, incised and channelized cross-sections hosted a very small number of juveniles. The considerable

coarsening of bed material that accompanied channel incision in the upper part of the study reach (Wyzga et al.,

2009), has most likely made spawning more difficult there. The single-thread channel sections are typified by few

undercut banks and small amounts of largewoody debris (Wyzga and Zawiejska, 2005; Wyzga, 2007), with most of

the scarce wood accumulations deposited along channel margins beyond the low-flow channel (Wyzga, 2007).

Moreover, the channelized sections also exhibit an almost complete lack of in-channel boulders. Because of the

lack of hydraulic shelter provided by these features (Piegay et al., 2000) coupled with the generally strong water

current and the transient nature of shallow-depth, slow-velocity habitats in these heavily modified sections,

juveniles can be easily swept away to downstream sections (cf. Harvey, 1987). As a result, the populations of brown

trout and Alpine bullhead in the single-channel sections are most likely sustained by the influx of older individuals

from less disturbed, upstream reaches (migration from downstream reaches is prevented by weirs) that are more

able to cope with the conditions existing in the single-thread sections.

In the multi-thread channel section, the brown trout—Alpine bullhead assemblage is completed with Eurasian

minnow and stone loach. Minnow represents another environmental guild (Welcomme et al., 2006) associated with

pools (Starmach, 1963; Langford and Hawkins, 1997). As a result of their shoaling behaviour, the relatively

abundant occurrence of minnow in suitable habitats increases the number of fish species and individuals in multi-

thread relative to single-thread river cross-sections. In turn, the occurrence of occasional stone loach in the multi-

thread cross-sections increased the distinction in the number of fish species between the two types of cross-sections.

The observed relations between the abundance and species richness of fish in the Czarny Dunajec and physical

characteristics of the channel probably reflect genuine links between channel morphology, habitat conditions and

riverine communities (Lamouroux et al., 2002; Smiley and Dibble, 2005; Dauwalter et al., 2008). This study

documented the dependence of the number of fish species and specimens on the variability in water depth (and also

in grain size of bed material in the case of fish abundance), while their dependence on an aggregated width of low-

flow channels was not confirmed. This indicates that greater abundance and diversity of fish in the multi-thread

cross-sections does not reflect enlargement of habitat area but is an effect of greater habitat heterogeneity and

increased importance of those habitats which are crucial for juveniles, mainly as nursery areas and refuges (cf.

Baras and Nindaba, 1999; Langler and Smith, 2001; Dolinsek et al., 2007). Fish, especially juveniles, benefit from

the greater share and persistence of shallow-water, slow-velocity habitats and the existence of lateral channels

partly disconnected from the main water current. The occurrence of accumulations of large wood may also be

important in providing shelter from predators and strong water currents (Everett and Ruiz, 1993; Piegay et al.,

2000; Zalewski et al., 2003). An inventory of largewood performed after a moderate flood during 2001 documented

the occurrence of considerable amounts of debris in the multi-thread section of the Czarny Dunajec, exceeding the

quantity retained in single-thread channel sections by two orders of magnitude (Wyzga and Zawiejska, 2005;

Wyzga, 2007). A considerable proportion of thewood accumulations deposited in the multi-thread section occurred

in low-flow channels (Wyzga, 2007). During the present survey in 2006, wood accumulations in the river were

much less abundant but they occurred only in the multi-thread channel section. Moreover, the good condition of the

population of adult fishes in this section might have been a legacy of the former high quantities of stored wood.


DOI: 10.1002/rra


It is not surprising that the relationship with hydromorphological quality of the river explained a considerably

greater portion of the total variation in fish abundance than did regression models developed for any single physical

parameter. Increased complexity of the flow network is associated with greater variability in many habitat

parameters, such as the co-existence of patches of coarse and fine bed substrate, slow and fast water current, zones

of the inflow of hyporheic water and the infiltration of riverine water into the channel bed, or zones of different

water shading by the tree canopy, which together are reflected in a better assessment of hydromorphological river

quality. As this variability of habitat conditions is beneficial for various fish species and their different life stages, it

explains the strong relation that exists between the number of fish specimens and the hydromorphological quality of

the river.

CONCLUDING REMARKS

This study has shown that high variability of hydromorphological quality of the Czarny Dunajec, caused by

spatially varied human impacts, is clearly reflected in the degree of heterogeneity of fish habitats and in the diversity

and abundance of fish fauna. More diverse and relatively abundant fish communities were recorded only in a

channel section in which natural river dynamics and a multi-thread channel pattern were preserved due to the lack

of significant human intervention. This section is typified by high variability of habitat conditions, reflected in the

highly valued hydromorphological river quality in the examined multi-thread channel cross-sections. In contrast,

single-thread cross-sections in the river sections modified by channelization and channel incision exhibited

relatively low variability in habitat conditions and hosted fish communities consisting of two species with a small

number of subadult and adult specimens and almost a complete absence of juvenile fishes.

By identifying relationships between the number of fish species and specimens and hydromorphological river

quality, this study has confirmed that the standardized assessment of habitat conditions may serve as a simple,

indirect procedure to identify problems with the ecological integrity of watercourses caused by man-induced

alterations of the physical environment (cf. Muhar and Jungwirth, 1998). While high hydromorphological river

quality is not sufficient for river biota to thrive (if, for instance, water quality is poor), poor hydromorphological

conditions will inevitably lead to a degradation of the condition of riverine communities. The assessment of

hydromorphological quality of a watercourse does not require a professional hydrobiological survey but can be

done relatively quickly by river managers, hence allowing locations in which an improvement in habitat conditions

is necessary to enhance the ecological integrity of the watercourse to be quickly identified.

Three aspects of the relationship between the status of fish fauna in the Czarny Dunajec and the

hydromorphological quality of the river should be emphasized. First, although the multi-thread cross-sections

located in a 4 km long, unmanaged river section were consistently evaluated as representing high quality

hydromorphological conditions, this was not reflected in species richness of the respective fish communities, with

only three or four of the five previously recorded species confirmed in the present survey. This discrepancy may

reflect isolation of the section within a longer river reach with poor hydromorphological quality and indicates that

habitat integrity at a greater longitudinal scale is necessary to ensure resilience of the riverine ecosystem to human

and natural disturbances. Second, hydromorphological river quality in the single-thread, deeply incised cross-

sections of the Czarny Dunajec was evaluated to be notably higher (Class 2) than in the channelized cross-sections

(Class 4), whereas the condition of fish communities in both types of cross-sections was similarly poor. This

indicates that the guidance standard for assessing the hydromorphological conditions in rivers (CEN, 2004)

underestimates the negative impacts of channel incision (increase in flow velocity, coarsening of bed material, loss

of lateral connectivity) on the habitat quality in rivers. Third, the greatest increase in the number of fish occurred

with a relatively small improvement in the hydromorphological quality of the river that was associated with the

change from a braided morphology with occasional islands to heavily island-braided morphology. This indicates

that fishes benefit from a greater proportion of island vegetation within the active zone of the river, probably due to

the high spatial variability in habitat conditions coupled with a positive effect of the vegetation on temporal stability

of habitats.

Similar to the situation in other mountain regions of Europe (cf. Muhar et al., 2000; Habersack and Piegay, 2008),

in the Polish Carpathians most river reaches have been considerably modified by human activities. The unmanaged,


DOI: 10.1002/rra

534 B. WYZGA ET AL.

multi-thread channel section in the lower part of the study reach of the Czarny Dunajec constitutes ‘an island of

biodiversity’ (cf. Ward et al., 1999) within the generally highly modified middle course of this river. Preservation of

the undisturbed channel dynamics and the high morphological complexity of the river in this section will be

essential to preserve species diversity of fish communities in the studied reach of the Czarny Dunajec and to restore

in the future this diversity in the modified river sections.

The dependence of the abundance and diversity of fish fauna on hydromorphological river quality and the degree

of variability in channel morphology, demonstrated in this study, indicates that the future recovery of fish

communities in this and other modified mountain rivers will require a renewed increase in morphological

complexity of their channels and improvement of hydromorphological conditions in the rivers. In channelized river

reaches, an important goal in the improvement of fish habitats is restoration of the lost longitudinal and lateral

connectivity of river ecosystems (Kondolf et al., 2006), while in the deeply incised reaches, restoration of lateral

connectivity as well as the reduction in flow velocity and bed material size are necessary.

ACKNOWLEDGEMENTS

This research was financed by Research Grant 2 P04G 092 29 of the Polish Ministry of Science and Higher

Education. The authors thank two anonymous reviewers for their helpful comments and Professor Angela Gurnell

for improving the language and style of the paper.

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