Macrophytes of the Hungarian lower Danube valley (1498-1468 river km) Georg A. Janauer1 and Dóra Steták2

With 4 figures and 2 tables Abstract: Aquatic vegetation was surveyed in the main channel of the River Danube near Baja (Southern Hungary) in the “Gemenc” National Park and in an irrigation and drainage canal system on the left bank of the river The main river was species poor, compared to the oxbows and the canals, in which more species and much greater plant mass developed. Characteristic features were identified with regard to Relative Plant Mass, a measure of dominance, and the Mean Mass Index, which indicates the distribution pattern of species in a system. This is the first quantitative survey, which may form the baseline for future comparative investigations of the development of the aquatic vegetation in this section of the River Danube.

Introduction Apart from sparse floristic data in publications and manuscripts mainly concerned with the backwaters of the Danube, there was no information available about the aquatic vegetation of the Hungarian lower Danube valley prior to the 1960s. KÁRPÁTI (1963) and RÁTH (1978) presented detailed phyto-sociological and ecological description of the aquatic vegetation of many different water bodies (side arms, oxbows and canals) which occur in the Danube 1

Institute of Ecology and Conservation Biology, University of Vienna, Althanstrasse 14, A – 1090 Vienna, Austria 2 Hungarian Danube Research Station of the Hungarian Academy of Sciences, H-2131 Göd, Jávorka S. u. 14., Hungary, [email protected]

floodplain. However, detailed macrophyte spatial distribution data for the main arm, as well as in backwaters and canals, have not been reported to date .

Study site In Hungary, the Danube has a wide alluvial valley to the south of Budapest. Although the dykes are mainly adjacent to the river, the width of the inundation area rarely exceeds 500 m. There are only two exceptions which are the ”Gemenc” and the ”Béda-Karapancsa” floodplains. The ”Gemenc” floodplain is one of the last large, still functioning inundation areas of Europe with a surface area of 178 km2. It is situated between river-km 1498 and 1468, on the right side of the Danube (46° 08´ - 46° 22´ North, 18° 50´ - 18° 56´ East). A set of different water body types representing different steps of morphological succession is present. These represent several connectivity types (AMOROS et al. 1987), which are the main arm of the Danube, side arms, oxbows, ponds and flat areas with temporary water cover. This complex ensemble makes the Gemenc flood plain one of the most important natural resources of the area. Until the end of the 19th century the Danube meandered considerably in this area, leaving naturally cut off oxbows and creating new riverbeds after major flood events. For the development and separation of a meander some 150 years were required (SOMOGYI 1974). These natural dynamics were terminated when the river was regulated in the late 19th century. As a consequence the length of the riverbed was reduced by 30 km in this section and the incision of the riverbed turned out to be more than 1 m in the last 100 years.

The difference between the lowest and the highest point of the area is only some 6 m. The type of sediment present varies from fine gravel or coarse sand to fine silt or clay. The mean discharge at Baja is 2290 m3.s-1 (period: 1981 - 1990). Mean water temperature of the

Danube at Mohács was 18.3, 20.8, and 21.1 °C (period: 1985 – 1994) in June, July and August, respectively. This part of the floodplain has been under nature protection since 1977, and from 1996 it has been part of the Danube-Drava National Park. On the left side, the inundation area is narrow, but several oxbows and a network of drainage ditches and irrigation canals are beyond the dyke in the former floodplain of the river. The course of the canals partially follows the former river channels.

In this study, the main arm of the Danube, as well as side arms, a lake, oxbows and an a static pond in the inundation area, and canals beyond the dykes were surveyed. The names of the water bodies and numbering of survey stretches are listed in Table 1.

Vegetation The ”Gemenc” is known as a large, floodplain woodland. However, only 5-10 % of the forested area is in a near-natural state, the remainder is plantation. The natural woody vegetation of this area consists of willow bushes (Salix purpurea L., S. triandra L.), willow (S. alba L.), poplar (Populus alba L., P. nigra L.) and ash-elm-oak (Fraxinus angustifolia ssp. danubialis POUZAR, Ulmus laevis PALL., Quercus robur L.) forests, and probably oakhornbeam (Q. robur, Carpinus betulus L.) forests also occurred in the past. The treeless vegetation is formed by hay meadows, which are dominated by Alopecurus pratensis L. (originally anthropogenic, but in a good, species rich state), reed beds (Phragmites australis (CAV.) TRIN., Glyceria maxima (HARTM.) HOLMBG., Phalaris arundinacea L., large sedge beds (Carex acuta L. dominated), and some aquatic macrophytes. On periodically wet patches, representatives of different annual communities like pioneer communities on river gravel and sand banks (Nanocyperion), ruderal and semi-ruderal riverine and marsh communities (Bidentetea and Agropyro-Rumicion) are present. Based on literature, herbarium

data and field observations, 37 vascular aquatic macrophyte species were recently found in or near to the ”Gemenc” floodplain (STETÁK 2000a, STETÁK 2000b). Beyond the dykes, arable fields are almost the only land use.

Method: Field survey and data processing A standard method for field survey (KOHLER 1978), data processing and display of graphic results was used for the assessment of the aquatic vegetation. Aspects of dominance and characteristic types of hydrophyte distribution are discussed by use of numerical derivatives: distribution diagram, Relative Plant Mass (RPM), Mean Mass Index (MMO, MMT) and distribution ratio (d, KOHLER & JANAUER 1995, PALL & JANAUER 1995, JANAUER et al. 1993, for details of methods: see JANAUER in this volume).

Results Species list In this study a total number of 28 species was found (Table 2; not dealt with here: 3 helophytes and one filamentous alga). Species richness was relatively high in the main channel compared to the Danube near Vac (to the north of Budapest), where only 6 species were found (RATH 1994), but it is about the same as in the Szigetköz area (Western Hungary, RATH 1987). In the flood plain water bodies and in the canals the species richness was much higher (20 and 27, respectively).

Distribution Diagram In this diagram (Fig. 1) the survey units from the Danube, the oxbows, and the canals are displayed in order from the left to right, to enhance clarity. This does not relate to their

position with respect to each other. All survey units [SUs] in the main channel are 1 river-km long, except for the last one. In the diagram SUs are alternating between the right and left bank of the river, starting with the right bank. The last unit, only a few hundred metres long, is on the left bank.

The aquatic vegetation of the main channel shows a very peculiar distribution. Apart from small amounts of Lemna minor and Spirodela polyrhiza in the first survey unit on the right bank and a single stand of Ceratophyllum demersum in the second survey unit, all the other plant stands were found on the left bank. Most of the plant material was floating and no plants were more tightly attached to the substrate. The right bank was practically devoid of any aquatic vegetation. In the oxbows, species richness was much higher than in the main channel. Some species developed as very large stands and the plant mass estimate reached the highest level. The considerable variety of habitats and flood plain water types is represented by the high number of SUs of rather short length. In the canals an even larger number of species occurred. The neophyte Cabomba caroliniana and the submersed pleustophyte Ceratophyllum demersum were abundant in almost all SUs. Highly uniform aquatic vegetation conditions are obvious in the canal SUs.

Danube River Relative Plant Mass (Fig. 2a): In the main river channel the pleustophytes are the dominant aquatic species. It should be recalled that the plant material was either floating or just loosely attached to the sediment near the banks. Mean Mass Index and Distribution Ratio (Fig. 2b): Only Lemna minor, Spirodela polyrhiza, Ceratophyllum demersum, to a lesser extent Cabomba caroliniana, show wider distributions. All other species were present only in very few habitats and even here they developed very

little plant mass (values of MMO and MMT are very low, but the distribution ratio “d” is also much less than 0.5).

Oxbows and other flood plain water bodies Relative Plant Mass (Fig. 3a): In the oxbows and the other flood plain water bodies with low hydrological dynamics Ceratophyllum demersum is the dominant species. A large number of other aquatic plants, including floating leaf species like Nuphar lutea and Trapa natans, are also typical of this type of aquatic environment.

Mean Mass Index and Distribution Ratio (Fig. 3b): All species with a “d”-value above 0.5 are reasonably ubiquitous in the canals, e.g. Ceratophyllum demersum, Salvinia natans, Nuphar lutea and Trapa natans. Other species like Azolla filiculoides or Potamogeton nodosus are extremely limited with regard to their distribution in the canal system (“d” is very low).

Canals Relative Plant Mass (Fig. 4a): In the canals Ceratophyllum demersum and Cabomba caroliniana are of approximately equal dominance and yet the RPM value is lower than in the Danube and in the flood plain water bodies. In addition a much larger number of species was found to have RPM values above 1 %. This indicates that species richness is high in many parts of the canal system.

Mean Mass Index and Distribution Ratio (Fig. 4b): The diagram shows three different types of plant mass distribution. Three species developed a mean plant mass close to or above “3”, which is equivalent to “frequent” (note the exponential scale on the abscissa). Also, both bars (MMO, MMT) are of about equal height

(“d” > 0.5), which is an indicator of ubiquitous occurrence. However, many other species with only low MMO and MMT values have a “d” close to 0.5: they are found in a wide range of habitats, but in small amounts. Only one species is considered as being rare in this canal system: Trapa natans has very low MMO and MMT, but also a very low “d”.

Discussion In this study, 27 vascular aquatic macrophyte species, three helophytes and one filamentous alga, but no mosses were found (Table 2). Free floating species (acropleustophytes) were typical of small, shallow, more or less shaded ponds, while in large oxbows, side arms and canals benthopleustophytes (namely Ceratophyllum demersum L.), submerged rooting and floating leaf species were dominant.

In the main channel of the River Danube 12 species were found. Along the left side, considerable amounts of plants and plant fragments drifted in the river during the survey period. This material originated from the cutting of macrophytes in the canal ”Vajas-fok”, which is a management procedure repeated annually to ensure the free discharge of the water. With increasing distance from the mouth of the canal the abundance of the drifting plants decreased. However, there were still viable plant fragments even 20 km downriver. 21 species occurred in the oxbows and other flood plain water bodies. This high number is not surprising as a wide range of habitats are represented (see Fig. 1) and the many different levels of connectivity that modulate the flood impact of the river. Azolla filiculoides, Polygonum amphibium, and Potamogeton panormitanus (syn. P. berchtoldii FIEDER) were confined to the flood plain waters. The canals were the aquatic environments with the highest species richness (27 species). Moderate to high nutrient levels (the canals take in the run-off from adjacent fields) and the constant flow conditions in all parts of the canal system favour

macrophyte growth, both in terms of the occurrence of numerous species as well as the development of considerable plant mass.

The middle and lower reaches of the River Danube , where water flow is less rapid than in reaches with alpine characteristics (Austria, Germany), feature pleustophytes as dominant elements of the aquatic vegetation even in the main river channel. They frequently occur in spur dyke bays, where the flow is moderate to non-existent during periods of mean and low discharge. In the Danube, near the town of Baja, the situation was different. The littoral near the right bank was almost devoid of any aquatic vegetation (Fig. 1), but near the left bank a rather constant flow of several pleustophytic species, mainly Lemnids, but also Ceratophyllum demersum and the neophyte Cabomba caroliniana was found. Most of that plant material stems from the canal system, which is part of the agricultural area to the East of the Danube and extends into the large plain between Danube and Tisza River. Close to the mouth of the canal system the amount of Lemnids is high, but with increasing distance from that point the plant mass decreases (Fig. 1). In very few cases submerged anchored species (Myriophyllum spicatum, M. verticillatum and Potamogeton lucens) were found. Compared to the flood plain water bodies and canals the River Danube is species poor and excluding the floating material the river is practically devoid of any aquatic plant growth.

The aquatic vegetation of the flood plain waters of the Gemenc National Park, which is located on the right bank of the river, was highly diverse. Not only were many species present, but the plant mass was very different in the manifold types of oxbows, side channels, lake and pond (Fig. 1, Tab. 1). A most peculiar characteristic of these water bodies was: (i) the absence of Cabomba caroliniana, which is ubiquitous in both the main river and the canal system; and (ii) the development of a large plant mass of true stagnant species like Elodea canadensis, Nuphar lutea, Nymphoides peltata, Potamogeton lucens and Trapa natans. It is

interesting that not a single survey unit, long or short, is without plant growth. On the other hand not all the species were found everywhere, as the characteristics of the water bodies are much different. In contrast the canals support a few more species, including Cabomba caroliniana, but most of these species were present almost everywhere. Upon comparison the distribution diagram (Fig. 1) is characteristic, but different, for each of the three environments.

With regard to dominance Lemna minor is the most frequent plant along the left bank of the main river, with Ceratophyllum demersum subdominant. In the oxbows and in the canal system this latter species is dominant. The RPM diagrams (Fig. 2a, 3a, 4a) are characteristic for the three environments: only five species (out of 12) represented the Danube, 13 species (out of 21) reached more than 1% in the oxbows, and in the canals 19 species (out of 27) are represented in the diagram. The more regular the flow dynamics, the more species can exist and may develop a plant mass high enough to be represented in the diagram (species with values below 1% are combined in the “residual”).

The type of distribution and the development of the Mean Plant Mass are also characteristic for the three types of environment (Figs. 2b, 3b, 4b). In the Danube, Lemna minor, and to a lesser extent Ceratophyllum demersum, Spirodela polyrhiza and Cabomba caroliniana (Note: floating) are ubiquitous (”d” ca. 0.5), but on average in none of these cases was much plant mass developed. In the oxbows and side channels three species developed extensive plant stands and a large plant mass in their specific survey units (MMO, white bar): Ceratophyllum demersum, Nuphar lutea and Potamogeton lucens. However, their distribution in the whole oxbow systems was not the same. N. lutea, and to a greater extent P. lucens were confined to only a few habitats (MMT and d are small, “clumped distribution”). This is unlike several other species, e.g. Trapa natans, Salvinia natans, Hydrocharis morsus-ranae, which did not

develop much plant mass (MMO and MMT ca. 2), but were present almost everywhere (d > 0.5). A few other species developed at very low plant mass, e.g. Najas minor, P. crispus, P. pectinatus, and P. nodosus, but only for the latter was a “d” close to 0,1 calculated, which puts this species into the category of rare plants in this system. Apart from Cabomba caroliniana, C. demersum, and Lemna minor, which developed high plant mass (MMO and MMT > = 3), many other species, e.g. Spirodela polyrhiza, occurred in minor amounts, but they were distributed over most of the canal system (“d” > 0.5, ubiquitous). The only rare species in the canals is Trapa natans (MMO, MMT < 1, “d” < 0,1)

Conclusion This is a first overview on the aquatic vegetation of the Danube River near the town of Baja in southern Hungary, the flood plain water bodies in the “Gemenc” National Park, and a part of the irrigation and drainage canal system in Central Hungary. Most of the vascular aquatic macrophyte species found in this study are typical of standing or slowly flowing, mainly shallow, eutrophic waters with silty sediments. The occurrence of species sensitive to organic pollution indicates that this area is free of, or low in, organic pollution.

The main arm of the Danube is, as expected, poor in aquatic macrophytes. The only sites available for macrophyte development are the spur bays where there is shallow standing water during low water periods.

Although the report of drifting plants is only a snapshot, it provides information about the possible means of dispersion and establishment of macrophytes carried by the Danube. It is particularly important in case of potentially invasive species such as Cabomba caroliniana A. GRAY.

Running water species were not found, although there are some side arms where water is flowing when the water level is above the mean. This may be due to the fact that the water is not permanently flowing and under these conditions it is turbid and the flow velocity is high (high disturbance, AMOROS & BORNETTE, 1999). In periods of low water levels there are still water ponds in the bed of these side arms where some species accustomed to lentic conditions, e.g. Trapa natans L., can be found.

Acknowledgements The authors thank Karin Pall for creating the standard graphics (Figures 1 to 4) and the ÖNIAD for supporting part of the work via the “Pilot Study Danube”.

References AMOROS, C. & G. BORNETTE (1999): Antagonistic and cumulative effects of connectivity: a predictive model based on aquatic vegetation in riverine wetlands – Large Rivers 11(3): 311-327. Arch. Hydrobiol. Suppl. 115/3. AMOROS, C., A. L. ROUX and J. L. REYGROBELLET (1987): A method for applied ecological studies of fluvial hydrosystems – Regulated Rivers 1: 17-36. FEHÉR, G. & A. SCHMIDT (1998): An adventive water fern in an oxbow at ”Gemenc”: Azolla filiculoides LAM. – Botanikai Közlemények 85(1-2): 57-61. JANAUER, G. A., ZOUFAL, R., CHRISTOF-DIRRY, P., ENGLMAIER, P. (1993). Neue Aspekte der Charakterisierung und vergleichenden Beurteilung der Gewässervegetation. Ber. Inst. Landschafts-Pflanzenökologie Univ Hohenheim, 2, 59–70. KÁRPÁTI, V. (1963): Die zönologischen und ökologischen Verhältnisse der Wasservegetation der Donau-Überschwemmungsraumes in Ungarn – Acta Botanica Acad. Sci. Hung. 9(3-4): 323-385. KOHLER, A. & G. JANAUER (1995): Zur Methodik der Untersuchung von aquatischen Makrophyten in Fließgewässern – in: Ch. Steinberg, H. Bernhardt, H. Klapper (Hrsg.): Handbuch Angewandte Limnologie KOHLER, A. (1978): Methoden der Kartierung von Flora und Vegetation von Süßwasserbiotopen – Landschaft + Stadt 10(2): 73-85. PALL, K., JANAUER, G. A. (1995). Die Makrophytenvegetation von Flußstauen am Beispiel der Donau zwischen Fluß-km 2552.0 und 2511.8 in der Bundesrepublik Deutschland. Arch. Hydrobiol. Suppl . Large Rivers 9: 91-109. RÁTH, B. (1978): Untersuchung der Laichkrautvegetation eines toten Armes in der Umgebung von Baja – Annales Univ. Sci. Budapestinensis 20-21: 137-153. RATH, B. (1987): The macrophyte vegetation of a small branch-system of the Danube at

Dunaremete (Szigetköz, River km 1826). Acta Bot. Hungarica 33: 187 – 197. RATH, B. (1994): Botanische Aufnahme der Wassermakrophytenbestände mit der Kohler – Methode im ungarischen Donauabschnitt bei Vac (Strom-km 1670 – 1697). Ber. Arbeitstagung der IAD 30: 245 – 249. SOMOGYI, S. (1974): Reconstruction of the development of the riverbed and the floodplain of the Danube at the ”Sárköz” section using maps made between 1782 and 1950 – Földrajzi Értesítő 23(1): 27-36. STETÁK, D. 2000a: Floristical data concerning the Gemenc Region of the Danube-Drava National Park, Hungary (in Hungarian). Kitaibelia 5(1): 145-176. STETÁK, D. 2000b: Aquatic macrophytes in the ”Gemenc” Floodplain of the Danube-Drava National Park (South Hungary) – Limnological Reports Vol. 33, Proceedings 33rd Conference of IAD, Osijek, pp 137-142.

Figure captions: Figure 1: Distribution Diagram – Danube River, Flood Plain Water Bodies, Canals. The width of the columns is equivalent to the real length of the survey units. Horizontal bars indicate the plant mass estimate (PME) for each species in each survey unit. Small bar: estimate levels 1 and 2. Medium bar: estimate level 3. Large bar: estimate levels 4 and 5.

Figure 2: Danube River a) Relative Plant Mass: The proportion of each species is weighted with the length of the survey units of occurrence. b) Mean Mass Index and Distribution Ratio: White bar - Mean Mass Index, calculated on the basis of survey units of occurrence of a species (MMO). Black bar - Mean Mass Index, calculated on the basis of all survey units (MMT). Distribution ratio (d): Ratio between MMO and MMT. It is of special importance when comparing very low numerical values (= very small bars) of MMO and MMT. Detailed explanation in the “Methods” chapter of this volume.

Figure 3: Oxbows and other Flood Plain Water Bodies a) Relative Plant Mass b) Mean Mass Index and Distribution Ratio

Figure 4: Canals a) Relative Plant Mass b) Mean Mass Index and Distribution Ratio

Tables

Table 1: Name and type of the studied water bodies and numbers of survey stretches Table 2: Species list

Table 1: Name and type of the studied water bodies and numbers of survey stretches Name water Danube, right bank, river km

Type of water body main arm

1497-1474

Number of stretch 1-3, 17, 19, 21, 25, 27, 29, 31, 33, 35, 37, 39, 41, 43, 52, 54, 56-61

Danube, left bank, river km

main arm

1495-1480

6, 18, 20, 22, 26, 28, 30, 32, 34, 36, 38, 40, 42, 44, 53, 55

Cserta-Duna

side arm on the inundation area

47-48, 51

Szeremlei-Duna

side arm on the inundation area

63-70

Forgó-tó

lake on the inundation area

4-5

Decsi-Kis-Holt-Duna

oxbow on the inundation area

23-24

Káposztás-Duna

oxbow on the inundation area

45-46

Nyéki-Holt-Duna

oxbow on the inundation area

49-50

Nagybaracskai-Holt-Duna

oxbow beyond the dyke

72

Füzesi-Holt-Duna

oxbow beyond the dyke

74

Vajas-fok

canal beyond the dyke

7-16

Ferenc-csatorna

canal beyond the dyke

71, 73

pond at Kádár-zátony

astatic pond on a sandbank

62

Table 2: Species list Abbrev. = standardised species name abbreviation. D = Danube, O = oxbows and other flood plain water bodies, C = canals and ditches. O = presence. Species name

Abbrev.

D

O

C [O]

[Filamentous Alga]

[fil alg]

[O]

Azolla filiculoides Lam.

Azo fil

O

Cabomba caroliniana Gray

Cab car

O

Ceratophyllum demersum L.

Cer dem

O

Ceratophyllum submersum L.

Cer sub

Elodea canadensis L.C.Rich

Elo can

O

Equisetum palustre L.

Equ pal

O

Hydrocharis morsus-ranae L.

Hyd mor

O

O

O

Lemna minor L.

Lem min

O

O

O

Lemna trisulca L.

Lem tri

O

O

O

Myriophyllum spicatum L.

Myr spi

O

O

O

Myriophyllum verticillatum L.

Myr ver

O

Najas marina L.

Naj mar

O

O

Najas minor Allioni

Naj min

O

O

Nuphar lutea (L.) Sibth. & Sm.

Nup lut

O

O

Nymphaea alba L.

Nym alb

O

O

Nymphoides peltata (S.G.Gmel.) O.Kuntze

Nym pel

O

O

Polygonum amphibium L.

Pol amp

O

Potamogeton crispus L.

Pot cri

O

Potamogeton gramineus

Pot gra

O

Potamogeton lucens L.

Pot luc

Potamogeton nodosus Poir.

Pot nod

Potamogeton panormitanus *

Pot pan

Potamogeton pectinatus L.

Pot pec

Sagittaria sagittifolia L.

Sag sag

Salvinia natans (L.) All.

Sal nat

Sparganium erectum L.

Spa ere

Spirodela polyrhiza (L.) Schleiden

Spi pol

Stratiotes aloides L.

Str alo

Trapa natans L.

Tra nat

Utricularia vulgaris L.

Utr vul

Number of species [excluding filamentous algae]

O O

O O

O

O

O O

O

O

O

O

O

O O

O

O O

O

O

O O

O

O

O O

O

O O

12

21

27

Fig. 1

Hungaria, Danube River Azo fil Cab car Cer dem Cer sub Elo can Equ pal Hyd mor Lem min Lem tri Myr spi Myr ver Naj mar Naj min Nup lut Nym alb Nym pel Pol amp Pot cri Pot gra Pot luc Pot nod Pot pan Pot pec Sag sag Sal nat Spa ere Spi pol Str alo Tra nat Utr vul fil alg

Oxbows

Canals

Fig. 2a

Fig. 2b:

Fig. 3a:

Fig. 3b:

Fig. 4a:

Fig. 4b: