adaptation strategies in wetland plants: links between ecology and physiology. proceedings of a...

Factors Associated with Reed Decline in a Eutrophic Fishpond, Rožmberk (South Bohemia,Czech Republic)Author(s): Hana Čížková, John A. Strand and Jaroslava LukavskáSource: Folia Geobotanica & Phytotaxonomica, Vol. 31, No. 1, Adaptation Strategies in WetlandPlants: Links between Ecology and Physiology. Proceedings of a Workshop (1996), pp. 73-84Published by: SpringerStable URL: http://www.jstor.org/stable/4181418 .

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Folia Geobot. Phytotax. 31: 73-84, 1996

FACTORS ASSOCIATED WITH REED DECLINE IN A EUTROPHIC FISHPOND, ROZMBERK (SOUTH BOHEMIA, CZECH REPUBLIC)

Hana Cizkoval), John A. Strand2) & Jaroslava Lukavskai)

1) Institute of Botany, Academy of Sciences of the Czech Republic, Dukelskd 145, CZ-379 82 Trebo4, Czech Republic; teL +42 333 721127; fax +42 333 721136; E-mail [email protected]

2) Limnology, Department of Ecology, University of Lund, Ecology Building, S-223 62 Lund, Sweden; fax +46 462224003

Keywords: Calcium, Nitrogen, Organic matter, Phragmites australis, Sediment

Abstract: Characteristics of the growth and performance of Phragmites australis as well as sediment characteristics were investigated along the western shore of Rozmberk fishpond. The reed performance decreased toward the southern end of the shore, proximate to outlets of wastewater effluent and untreated sewage. While the reed stand was closed and looked healthy at the northern end, gaps occurred within the flooded part of the reed belt further southwards; reed, was absent in water along the southernmost part of the shore, though dead shoot stubble indicated its presence in earlier times. In the latter site, the surface layer of sediment consisted of fine mud with a high organic matter content and a high oxygen demand. To a smaller extent, patches of partly decomposed reed litter inside the gaps showed the same properties. It is suggested that organic matter accumulating within the flooded part of the reed belt may have reduced plant performance which ultimately lead to the formation of gaps. At a later stage, the lakeside fringe of the reed belt collapsed, thus completing the retreat of reed from water. A protective effect of calcium against the adverse effects of organic matter is suggested.

INTRODUCTION

Eutrophication has frequently been associated with reed decline (OSTENDORP 1989, CIZKOVA- KONIALOVA et al. 1992). Reduced stem stiffness (KLOrzLI 1971, 1974, SUKOPP et al. 1975, SUKOPP & MARKSTEIN 1989), changed dynamics of nitrogen (KOHL & KOHL 1993, KOHL et al. 1996) and of reserve carbohydrates (KUBIN et al. 1994, Ci2KOvA et al. 1996, KUBIN & MELZER 1996), increased palatability for pests (KOHL 1989, FUCHS 1993) were described as related phenomena. However, most authors have remained sceptical as to whether all these effects are parts of a generally valid mechanism. This scepticism may partly be due to effects of interacting factors, which differ among lakes. These are: wind and wave action in large European lakes (PIRoTH & PLATE 1993), mechanical damage by waves from motor-boats, recreational activities, and waterfowl and muskrat grazing in the river Havel (SUKOPP & MARKSTEIN 1989, KRAUSS 1993), mechanical damage by reed harvesters and high levels of toxic sulphide in Lake Fert6/Neusiedler See (M. DINKA, pers. comm., ARMSTRONG et al. 1996).

Reed decline was observed also in several large fishponds of the Trebon' Basin (South Bohemia, Czech Republic). In common with most other sites where reed decline was recorded,



74 H. ti?kova et al.

the trophic status has increased considerably in most fishponds of the Treboi? Basin over the last 40 years (POKORNY et al. 1994, PECHAR 1995). In contrast to most European lakes, however, seasonal fluctuations of water level are minimal (usually not more than 20 cm) as a result of a very tight control.of water level by the Treboni Fisheries (PKIBAji et al. 1992). In addition, the fishponds are smaller than large European lakes (the largest fishpond covers an area of 5 km 2) and long segments of their shores are sheltered by woods, which moderate the effects of wind and waves. Unlike the brackish sites, the total sulphur levels are generally low in the fishponds (L. Pechar, pers. comm.), which eliminates the risk of sulphide toxicity.

The extremely strong eutrophication with only moderate to negligible action of main interacting factors (wind/waves, water level fluctuations, sulphide toxicity) makes the fishponds interesting model sites for investigating the relation between eutrophication and reed performance. We selected the western shore of Rozmberk fishpond as particularly suitable because the reed performance coincided with the distance from sources of pollution (discharge of untreated piggery sewage and partly treated domestic wastewater). While reed retreated completely from water along the southernmost part of the shore (proximate to the pollution), it formed z. closed and healthy-looking stand towards the apparently cleaner northern end.

In 1992, we measured selected characteristics of reed growth, stem morphology and shoot nutrient levels in order to evaluate reed performance. We also measured sediment characteristics along the reed belt. We aimed at answering the following questions for this habitat:

(1) What are typical signs of reduced reed performance? (2) Which plant characteristics change with decreasing reed vitality? (3) Is the reed performance related to (a combination of) sediment characteristic(s)?

SITE DESCRIPTION

Rozmberk fishpond is situated about 4 km north of the town of rreboni at 423 m a.s.l. Its 2 2 total water area is 4.89 km and its cadastral area is 6.75 km2. The average depth is 1 m and

the maximum depth (at the outlet) is 4 m. The shore is gently sloping. The littoral vegetation is formed by helophyte communities dominated by Phragmites australis (CAV.) TRIN. ex STEUD. and Glyceria maxima (HARTM.) HOLMB. which adjoin sedge meadows further landwards. Large parts of the littoral are surrounded by woodland. Further details on the littoral vegetation are given by HROUDOVA et al. (1988).

The main sources of nutrient and organic pollution include piggery sewage, crushed limestone and fish feed, applied by the Treboni Fisheries in order to promote fish production. Furthermore, insufficiently treated effluent is discharged into the southernmost bay from a nearby wastewater treatment plant processing wastewater from a pig farm (about 30 000 pigs) and municipal wastewater from the town of Treboni (about 8000 inhabitants). In addition, untreated sewage is discharged into the southernmost bay. By comparison, the northern part of the fishpond is relatively clean thanks to the waters of the through-flowing Luznice river (Fig. la).

For the purpose of the investigations, the investigated part of the shore (total length of 800 m) was divided into five segments (with number 1 assigned to the northern end) according to the shore topography and appearance of the aquatic part of the reed belt (Fig. lb). Segments 1 and 2 were covered by closed, healthy looking reed stands. The width of the reed belt was



Reed decline in Rozmberk fishpond 75

L D N

A'

/ I w r -IsfQQ

wvgrass /

Q oaks / / QQ' ! ,A& willows B -i

/ [ dib F F F

Fig. 1. Map of Rozmberk fishpond (a) and the investigated part of the shore (b). D - dam, LR - the Lu'znice River, F - landing site for the Fisheries' boats, S - discharge of untreated sewage, W - outlet of the wastewater treatment plant. Numbers indicate investigated segments of the shore

about 5 m and 10 to 15 m in segments 1 and 2, respectively. Openings (gaps) were found within the flooded part of the reed belt in segments 3 and 4. While these gaps were infrequent and separated from each other in segment 3, they formed a more or less continuous chain within segment 4. Reed of segment 4 was shorter compared to preceding segments. The width of the reed belt was 15 to 25 m and 5 to 10 m for segments 3 and 4, respectively. Segment 5 adjoins a small bay proximate to the site where raw piggery sewage was discharged. It is also closest to the outlet of the wastewater treatment plant. It was devoid of reed, with the exception of a single patch of sparse reed about 5 m in diameter. The stubble of dead shoots, which became exposed during the autumnal drawdown, indicated that reed had formerly been present also in bare parts of segment 5. Most of the shore of segment 5 was colonised

by Glyceria maxima, which formed floating mats in depths up to 30 cm. Typha angustijolia L. and Typha latifolia L. formed several scattered patches.

MATERIAL AND METHODS

Sediment characteristics

The surface layer of sediment (0 to 5 cm depth) was sampled both at the outer edge of the closed reed belt and, in segments 3 and 4, also inside the aforementioned gaps. Patches of partly decomposed litter overlay the sandy sediment inside these gaps. In segment 5, the sandy sediment was covered with a continuous, several centimetres thick, layer of fine black mud. These different types of material were sampled separately.

Sediment samples were collected in 100-ml plastic boxes, closed airtight under water and transferred to the laboratory. Redox potential was measured within 2 hours of sampling, as the potential of a Platinum redox electrode using a millivoltmeter with an internal resistance of 1010 Ohms. The redox potential was measured against a saturated argent chloride reference electrode and related to the standard hydrogen electrode. The values were not standardised in relation to pH.



76 H. 6i?kovd et al.

Table 1. Sediment characteristics along a reed belt in Rozmberk fishpond (mean ? s.e., n = 4). Values showing the same letter are not significantly different at the 0.05 probability level (LSD test).

Seg. Sediment Redox potential Water content Loss on ignition Oxygen demand No. character (mV) (% fresh weight) (%) [mg g-I(fw)h- ]

reed belt edge I sand -35 ? 57 a 19.8 ? 0.8 ac 0.60 ? 0.10 a 0.025 ? 0.002 ac 2 sand -81 ? 9 a 24.6 ? 0.8 acde 0.60 ? 0.10 a 0031 0.001 ac 3 sand -60 ? 33 a 19.1 ? 0.6 a 0.55 ? 0.05 a 0.019 ? 0.003 ac 4 sand 0 97 a 20.1 ? 0.9 ac 0.50 ? 0.10 a 0.015 0.003 a

gaps in reed belt 3 sand -82 ? 12 a 23.9 ? 1.5 ac 1.05 ? 0.19 a 0.020 ? 0.006 ac 3 mud with reed litter -72 ? 11 a 69.2 ? 7.0 b 16.63 ? 5.21 bc 0.056 ? 0.001 b 4 sand -100 ? 5 a 27.4 ? 1.6 cd 1.50 ? 0.2 a 0.026 ? 0.005 ac

reed retreated from water 5 sand -71 ?23 a 29.3 ? 2.8 de 7.35 ?5.75 ac 0.030 ? 0.009 c 5 black mud -137 23 a 93.0 ? 1.5 f 68.20 4.6 d 0.218 ? 0.007 d

F 0.878 94.8 35.6 170.7 d.f. 8/27 8/27 8/19 8/27 P 0.547 0.000 0.000 0.000

Respiratory oxygen demand of sediment suspension was used to characterize the actual oxygen debt in the root medium (i.e. the rate at which oxygen is consumed from the medium when present). Sediment samples were placed in 100 ml glass bottles, topped with air-saturated distilled water, and incubated at 20 ?C for 30 min. The suspension was mixed every 5 min. The initial and final oxygen concentrations were measured using a Clark-type oxygen sensor manufactured by Chemoprojekt Satalice (Prague, Czech Republic). The amount of sediment in the suspension was adjusted to reach final oxygen concentrations between 30 and 60 % of air saturation. This amount was either 5 or 20 g fresh weight, depending on sediment type. The fresh weight was estimated after decanting the small amounts of water standing on the surface of sediment samples. The dry weight was measured after drying the samples at 105 ?C. The loss on ignition was measured after ignition in a muffle furnace at 550 ?C.

Four samples for each site were pooled and used for analyses of mineral nutrient levels. The analyses were performed by an agricultural laboratory at Jindrichu'v Hradec using standard methodology for agricultural soils (JAVORSKY 1982). The material was finely ground and mineralised with sulphuric acid and hydrogen peroxide. Total nitrogen was determined coulometrically, total phosphorus was determined colorimetrically, and potassium, calcium and magnesium were determined by atomic absorption spectrometry.

Plant performance

The depth limit of reed (depth penetration in sense of WEISNER 1991) was estimated as the length of flooded stem parts at the lakeside edge of the reed belt. Characteristics of reed production (stand height, shoot density, total above-ground biomass) were recorded on 27




Table 2. Depth limit of Phragmites australis along the western shore of Rozmberk fishpond (mean ? s.e.). ANOVA: F = 32.7; d.f. = 4/123; P = 0.000. Values followed by different letters are significantly different at 0.05 probability level (LSD test).

Segment No. Segment length Depth limit n (m) (cm)

1 70 63.4? 1.5 a 18 2 110 53.8?0.5b 30 3 150 53.0 ? 0.7 b 33 4 280 53.8 ?0.4b 46 5 170 57.0 1

July (at the time of panicle emergence) in single 0.5 x 0.5 m quadrats along belt transects perpendicular to the shore. One transect was surveyed in a typical part of segments 1 to 4. The fifth transect was laid through the remnant of reed belt in segment 5. The transects started at the lakeside edge of the reed belt and ended either near the bank under the tree canopy (in segments 1

and 2) or in the terrestrial part of the reed belt on the bank (segments 3 to 5). Stand height was estimated as the average height of five tallest shoots for each quadrat. Total above-ground biomass included stems, live parts of leaves and inflorescences. Their dry weight was determined after drying to constant weight at 80 ?C.

Bulk samples of shoots (stems and leaves) from the 3rd quadrat (1 to 1.5 m away from open water, where the stand reached the highest values of density, biomass and height) were analyzed for mineral nutrient concentrations. The analyses were performed by an agricultural laboratory at Jindrichiuv Hradec using standard methods for estimating nutrient contents in crops (JAVORSKY 1982). Mineral nutrients were assessed after mineralization of the plant dry matter with sulphuric acid and hydrogen peroxide catalyzed by selenium. Nitrogen was determined using the Kjeldahl method. Phosphorus was determined colorimetrically with ammonium molybdenate in the presence of stannous chloride. Sodium, potassium and calcium were determined by flame photometry. Magnesium was determined by atomic absorption spectroscopy.

RESULTS

Sediment characteristics

Redox potential, oxygen demand and loss on ignition Redox potential ranged from -140 to 0 mV. Differences between segments (1 to 5) or

between different sediment types (sand, litter, black mud), tested using one-way analysis of variance, were not significant (Tab. 1). By contrast, significant differences were found between sediment types in their oxygen demand. Oxygen demand was higher in the litter layer from the gaps than in the sandy sediment both within the gaps and outside the reed belt. By far the highest values were found in the black mud of segment 5. This pattern correlated with the pattern of sediment fresh weight and with the loss on ignition. Close positive correlations were found between the loss on ignition on the one hand and sediment oxygen demand and fresh weight on the other hand; the correlation coefficients were 0.960 (n=28) and 0.907, respectively.

Mineral nutrient levels The levels of total nitrogen were much the same all along the shore of segments 1 to 4,

both at the reed belt edge and inside the gaps. In segment 5, sand had a similar nitrogen level



78 H. 6tikova et al.

Table 3. Mineral nutrient levels (% dry weight) in shoot biomass of Phragmites australis along the western shore of Rozmberk fishpond.

Segment No. N P K Ca Mg Na Silica

1 1.28 0.149 1.38 0.32 0.100 0.032 2.32 2 1.36 0.117 0.95 0.32 0.086 0.042 1.61 3 1.33 0.127 1.27 0.30 0.100 0.039 2.14 4 1.35 0.117 0.95 0.23 0.062 0.024 1.68 5 1.42 0.095 1.06 0.21 0.070 0.053 1.29

as substrates in previous segments, but the black mud had a value one order of magnitude higher (Fig. 2a). Total phosphorus levels followed a similar pattern as total nitrogen (Fig. 2a). The levels of nitrogen and phosphorus were closely positively correlated with the loss on ignition, yielding correlation coefficients of 0.986 and 0.974 (n=9), respectively. By comparison, the levels of potassium. were more uniform along the shore (Fig. 2b). The levels of calcium and magnesium were high in segments 1, 2, and in the black mud of segment 5, but were low to negligible in segments 3 and 4 (Fig. 2b). The regressions between the loss on ignition on the one hand and potassium, calcium and magnesium on the other, were insignificant.

Plant performance

Depth limit The depth limit was very similar for all segments, ranging from 53 to 63 cm (Tab. 2). It

was even less variable within segments (Tab. 2).

Stand height, shoot density and above-ground biomass The maximum stand height, shoot density and above-ground biomass was reached about

1 m from the open water edge (Fig. 3, 4, 5). All these characteristics declined toward the bank in segments 1 and 2. In segments 3 and 4, the lakeside dense part of the reed belt was followed by an opening (gap) with very sparse or no reed over a distance of one to several metres. The infrequent shoots found in the gaps were shorter than shoots of the lakeside part of the reed belt. The reed became denser again in the limosal ecophase (in sense of HEJNY 1960), i.e. in a depth of less than 10 cm. The limosal vegetation included also Phalaris arundinacea L. and Glyceria maxima, which made up 30 to 70 % of the total above-ground biomass.

Mineral nutrient levels While the nitrogen concentrations increased in reed shoots towards segment 5, the

concentrations of phosphorus, main cations (potassium, calcium, and magnesium) and silica decreased (Tab. 3). The nitrogen levels were closely negatively correlated with phosphorus and silica levels, with correlation coefficients of -0.986 and -0.954 (n=5), respectively. The correlations of nitrogen with potassium, calcium and magnesium were looser, yielding correlations coefficients of -0.667, -0.742 and -0.676, respectively.




12 3.00

10 a N P mK 2.50

cm 8, 2.00

6 1.50 _

4- ~~~~~~~~~~~1.00 (

2 0.50

0. 0.00 le 2e 3e 3gs 3gl 4e 4gs 5. 5o

5 0.30

b Ca mg 0.25 4-

cm 0.20 cm -)e 3, -9 cm

0.15 c

u 2 ]0.10

1 0O.05

1e 2e 3e 3gs 3g9 4e 4gs 5s 5o

Site

Fig. 2. Concentrations of main mineral nutrients in the surface layer of sediment along the western shore of Rozmberk fishpond. Numbers on the horizontal axis indicate shore segments. e - reed belt edge, g - gap inside the reed belt, 1 - layer of reed litter, s - sand, o - organic mud.

DISCUSSION

Signs of reduced reed performance

Gaps within the reed stand were considered signs of reduced reed performance in this study. Similar gaps, called lagoons, were observed earlier by Dykyjova' and coworkers (ULEHLOVA & PRIBIL 1978) in a similar fishpond habitat, and occur also in Lake Neusiedler See (L. Hammer, pers. comm.). Such gaps are considered by these authors a natural stage of stand development. Our observations may not contradict theirs, if eutrophication is viewed as a factor enhancing the speed of stand maturation and degeneration. If it is so, separate gaps (such as observed in segment 3) may indeed represent the first stage of reed decline in sheltered sites where other interacting factors are weak. In the next stage the gaps string together, which results in the situation observed along segment 4. In the last stage, the lakeside fringe of the reed belt collapses with only the terrestrial part of the reed belt remaining, as in segment 5. This assumption was supported by our observations two years later (1994), when the lakeside fringe of segment 4 collapsed (leaving the stubble field), and only the terrestrial part of the reed belt remained.



80 H. 6fizkovA et al.

E 3000t 1 t L 2 E~~~~~~~

2100

-10o _____________

0 2 4 6 8 10 0 2 4 6 8 10

300

3 ~~~~~~~4 E

LA I

*j 100

! _

468101214

0 2 4 6 8

10

3W- Distance from open water (i) 100 ~~~~~5

10 C

0 2 4 6 a 1 0

Distance from open water (in)

Fig. 3. Stand height above water level (triangles) and profile of the fishpond bottom (squares) in transects laid through the reed belt. The vertical distance between the two symbols indicates the shoot height. Numbers in upper right corner indicate the shore segments.

Plant characteristics changing with reed performance: mineral nutrient balance

Mineral nutrient analyses of plant material (Tab. 3) indicate a differential nutrient uptake by reed stands along the western shore, with the highest nitrogen and lowest phosphorus, potassium, magnesium and calcium uptake in segment 5. The differences in nitrogen concentrations are not very large and fall within the range reported from fishpond habitats (DYKYJOVA 1978). However, the negative correlations of nitrogen with phosphorus, silica, potassium, calcium and magnesium were not observed previously in such habitats. A negative effect of increased nitrogen uptake on the uptake of potassium was noted by DYKYJoVA (1978) and BOAR et al. (1989) in experimental cultures.

Relations between plant performance and sediment characteristics

The ANOVAs revealed a similar pattern in the organic matter content of the sediment (as determined by the loss on ignition) and the reed performance (classified as Present or Retreated; F = 9.62; d.f. = 1/14; P = 0.008). Segment 5, where Phragmites had retreated, was distinguished from all the other segments (where Phragmites was still present) by a much higher content of organic matter in sediment. Therefore, the fine organic mud is likely to be




160

152 120'

80A

w

40 'J 0

0 2 4 6 8 10 0 2 4 6 a 10 160.

E ~~~~3 4 >. 120,

80,

0 -C Cl) 0 4 6

0 246810 1214 0 2 4 6 8 10 160 Distance from open water (i)

5 120-

80.

40

0. 0 2 4 6 8 10

Distance from open water (in)

Fig. 4. Shoot density of Phragmites australis in transects laid through the reed belt. Numbers in upper right comer indicate the shore segments.

involved in the reed retreat from water in segment 5. Adverse effects of various sources of organic matter on reed growth have been demonstrated experimentally (VAN DER PUTrEN 1993, H.H. SCHIERUP, pers. comm.).

The surface layer of fine black mud may have inhibited the formation, or reduced the viability of terminal buds, which form on new rhizomes and protrude through the surface sediment layer at the end of summer. Similarly, ARMSTRONG et al. (in press) observed bud abortion on both English and Hungarian die-back sites and attributed the bud death to the effect of phytotoxins originating from anaerobically decomposing organic matter.

Patches of reed litter, such as found inside gaps in segments 3 and 4, may inhibit reed growth inside the reed belt in a similar manner as the surface layer of black mud. The accumulation of litter is likely to have the worst effects on reed vigour at intermediate water depths, i.e. inside the flooded part of the reed belt. This is supported by our data showing that the production is highest 1 to 2 metres from the water's edge (Fig. 5), i.e. the gaps start where the most litter is being produced. In the terrestrial part of the reed belt, the soil surface is in contact with air (at least for part of the season), and the litter is therefore decomposed, at least partly, aerobically. On the lakeside fringe of the reed belt, the effects of reed litter and other sources of organic matter will be ameliorated by direct contact with the lake water, which will wash away the anaerobic toxins. Similarly, MENDELSSOHN et al. (1982) have related



82 H. 6i?kova et al.

15001 1 2

E 5 - I 10001 .2' 0 2 4 6 8 10 0 2 4 6 8 10

Iw >< 1200G

*0 4W

0,1500 2 4 6 8 10

__________________ Distance from open water (i)

.0 I

100

100

0 2 4 6 8 10 Distance from open water (in)

Fig. 5. Above-ground biomass of Phragmites australis (triangles) and other species (asterisks) in transects through the reed belt. Numbers in upper right corner indicate the shore segments.

the vigour of a salt marsh grass, Spartina alterniflora LOISEL., to its proximity to open water: the more vigorous tall form occurs adjacent to tidal creeks and has a higher productivity than the more inland short form. The growth limitation of the short form was ascribed to a more severe anaerobiosis in inland habitats compared to the banks of tidal creeks.

Accumulated reed litter was observed also inside the reed belt of segments 1 and 2, where no gaps occurred. A question therefore remains whether an additional factor may ameliorate the adverse effect of accumulated organic matter. According to sediment analyses, calcium concentrations were extremely high in the sediments of segments 1, 2 and 5, while they were undetectable in segments 3 and 4 (Fig. 2). Calcium ions may chelate the dissolved organic acids, originating from the decomposing organic matter. In addition, calcium carbonate may neutralize acidic compounds such as organic acids (e.g. GOLTERMAN 1969). Rozmberk sediments are naturally poor in calcium; most of the calcium detected comes from crushed dolomitic limestone applied by the Treboni Fisheries, which accounts for its uneven distribution. The natural protective effect of calcium, via the mechanisms above, may explain why no substantial reed decline occurred in hardwater fishponds of South Moravia, which are managed as intensively as the South Bohemian fishponds. The high amounts of calcium




found in the black mud of segment 5 were probably insufficient to counteract the effects of the rich source of fine organic matter.

Only the surface layer of sediment was analysed in this study. Depth profiles of both redox potential and chemical composition of the sediment are needed in order to evaluate the possible links between sediment properties and the performance of Phragmites rhizomes and roots in more detail.

CONCLUSION

Our results bring correlative evidence that high content of organic matter in the sediment is linked with signs of reduced reed performance in eutrophic conditions. The sources of organic matter include fine organic mud of allochthonous origin and reed litter. The adverse effect of organic matter may be ameliorated by high levels of calcium.

Acknowledgement: This research was part of project No. 60564 of the Academy of Sciences of the Czech Republic. J.A.S. was supported by a IAESTE scholarship. We thank Dr Naomi Rea and Dr Jan Pokorny for useful comments on the manuscript.

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