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PERMANENT MACROZOOBENTHOS IN THE DANUBE AREA BEFORE AND DURING THE OPERATION OF THE GABCIKOVO BARRAGE

####
Vladimir KOSEL

Department of Zoology, Faculty of Natural Sciences, Comenius University, Mlynska dolina, 842 15 Bratislava, SLOVAKIA

CONCLUSIONS After the drop of the discharge in the original Danube in October 1992, in the next year (1993) no significant changes in the species composition of fauna were found. In 1994 the changes in the Danube fauna were more pronounced and tended to enrichment of the spectrum of species observed. At Gabcikovo appeared Gastropods Lithoglyphus naticoides and Physella acuta (syn. Physa) expanded to the entire shore from Dobrohost to Gabcikovo. Water flowing from the reservoir to the old riverbed is partially devoid of suspended solids and enables a substantially better growth of algae on the stones. These significantly extend the nutritional offer and enable the substantial multiplication of grazers and scrapers (Gastropoda). The quantity of other species of zoobenthos (leeches and crustaceans (Amphipoda)) remained at the previous level. In Gabcikovo there is a remarkable increase of quantity of filtrators (Bivalvia), which is a probable consequence of the water enrichment by bioseston from the inundated river branches. In Klucovec, below the Gabcikovo structures, no increase in abundance was observed, its values did not exceed the variability observed in previous years. The immediate cause of the extinction of the fen biotope on the monitoring place Dobrohost-Kriviny was the decrease of the ground water level and the inundation supplying canal has no effect on this locality yet. It is probable that despite of changes of branch types from plesiopotamal to parapotamal ones, with regard to the variable water discharge, the general changes of fauna will not occur and that there will be a recovery of populations of previous original species will happen, especially of those which are indifferent to the current velocity. However, individual species could occur in different numbers as compared with pre(dam conditions. The conversion of branches to the eupotamal types could have an effect on their stream-lines together with the decrease of the water temperature. One has to keep in mind that the supplying of branches with water reshaped the branches and increase the area covered by water. This rearranged to some extent occurrence of eupotamal and, parapotamal, along the branches. Altogether, one could expect a slight reduction of the distribution and a decrease in abundance in stagnicolous species. The monitoring station Kralovska luka is of a special position. The station is located out of the inundation area and is charged with the ground water only. The development of fauna is still in the process of the recovery of populations of original species and the appearance of additional species seems to be very probable. The low abundance will, however, persist also in the future. I suppose that the limiting and selecting factor here will be the oxygen deficiency at the low water level under the ice surface in winter, when the extinct vegetation decays.

#### INTRODUCTION

During the monitoring of the impact of Hydroelectric power structure Gabcikovo (HPSG) on the aquatic fauna in the main stream of the Danube river and in other monitoring stationary stands we have observed permanent components of the macrozoobenthos, i.e. Porifera, Tricladida, Hirudinea, Polychaeta, Malacostraca, Mollusca, and Bryozoa. The aim of our effort was the qualitative and quantitative analysis of these groups before and after starting the operation of the HPSG, whereby we regarded as crucial data obtained in 1992 and 1993. The results of the monitoring of the whole aquatic fauna during 1989 - 1993 were elaborated in the review article which was presented at the Conference about HPSG in April 1994 [5]. In this paper we are presenting more detailed results on the macrozoobenthos of the inundation area from 1991 to 1994. The results obtained from the main stream of the Danube river will be mentioned only briefly as they have been published in detail [6].

#### OBSERVATION PLACES

The samples of zoobenthos were taken at three places in the main stream of the Danube river:

- Dobrohost, river kilometre (rkm) 1840.5
- Gabcikovo, rkm 1819.7
- Klucovec, rkm 1804

The riverbed of the Danube between Dobrohost and Gabcikovo after damming the Danube (and re-directing the Danube stream) will be called the old Danube and the river before damming - the original Danube.

From the river arm system we report here only the results from three monitoring places where we found the most evident effects of the operation of the water engineering structures. They are:

- Dobrohost - Kriviny, at rkm 1840.5, monitoring place No. 6b. The remnants of the original Danube branch were four shallow depressions with diameters about 20-40 m which were already isolated from the Danube. There is a continuous covering boggy vegetation. Depressions were charged with the ground water, in the second half of year they were mostly without water.
- Bodiky - Bodicka brana, at rkm 1830, monitoring place No. 9b. A large branch (40 - 60 m wide), of parapotamal type, without an aquatic vegetation.
- Trstena, Kralovska luka. Monitoring place No. 10. Remnants of the potamal branch 300 m long and 20 m wide, of paleopotamal type (without connection with Danube). Water is present throughout the year, however, only to the depth of 1 m. The abundant aquatic vegetation exists (Myriophyllum, Ceratophyllum, Alisma).

#### THE RIVER DANUBE

#### Material and Methods

The samples of the zoobenthos in the Danube were taken on the shore covered with block stones of the river bank fortification lining, on the places where the river water evidently streamed. The sampling procedure was following: a stone (about 30 cm diameter) resting about down to 40 cm below the water level and protruding to the surface was put into the hydrobiological net, and rinsed. The fauna stuck on the stone surface was collected with tweezers and, if necessary, scraped with a wire brush. Dimensions of the stone were measured taking into account its actual shape and the corresponding projection surface was calculated (not the real stone surface). At every monitoring stationary stand no less than 5 stones were sampled. The results are expressed in the number of individuals per m2 of stones lining the (hypothetical) horizontal stone layer. Beside of quantitative samples (described above), the zoobenthos was collected semi-quantitatively every 5-10 minutes be a hydrobiological net and manually with tweezers every 25-30 minutes. These sampling procedures do not include the fauna of a small ize (smaller than 2-4 mm), so called mesobenthos, (e.g. Corophium spp. and Jaeria sarsi). Sessile and colony-forming species of zoobenthos which created plaques continuously covering stone surfaces (Porifera and Bryozoa) as well as a bulk appearance of Dreissena we defined and expressed as a surface coverage, i.e. a per cent of a stone projection surface covered by the concerned species.

#### Qualitative composition of the zoobenthos in the Danube

The qualitative composition of the permanent zoobenthos at individual monitoring stations at species level has been published by Kosel [6], see Tables 1, 2, 3).

During 1991-1992 the character of the riparian macrozoobenthos in the monitored stretch of the river was qualitatively uniform. Species exhibiting higher abundance (eudominant and dominant species) were found in all monitoring places. Several species exhibiting lower abundance were sometimes missing. Certain differences in the fauna were found in the station Klucovec which is located below the Danube change of gradient. Only here we found the Gastropod Lithoglyphus naticoides. On the stone riparian substrate of the main stream of the Danube it was missing in the monitoring places located upstream Klucovec. The most distant habitat of L. naticoides was found at Palkovicovo (Sap), in September 1991.

After the drop of the discharge in the original Danube in October 1992, in the next year (1993) no significant changes in species composition of the fauna were found (in Gabcikovo we observed the presence of Asellus aquaticus and Lymnomysis benedeni). In 1994 the changes in the fauna were more pronounced and tended to an enrichment of the spectrum of species observed. At Gabcikovo appeared and expanded to the entire shore from Dobrohost to Gabcikovo Lithoglyphus naticoides and Physella acuta (syn. Physa).

#### Quantitative changes

Quantitative changes in zoobenthos were pronounced more significant. The time-course of these changes at the indicated cross-sections is shown in the Fig. 1. In 1991 and 1992, hence, in "native" hydrological conditions, the abundance of the zoobenthos was below the average with some oscillations.

Since 1993 the abundance of the zoobenthos has began to increase, but only in the Old Danube, i.e., in the stretch between Dobrohost and Gabcikovo. The largest increase in abundance was found in all species of Gastropoda that were dominant before: Ancylus fluviatilis, Lymnaea ovata and Bithynia tentaculata. Quantities of other species remained at previous levels. The abundance (of dominant species) increased also in 1994. Besides the species mentioned above, the increase in 1994 was caused by the appearance of new species Physella acuta and a species occurred rarely in the past - Potamopyrgus antipodarum (syn. P. jenkinsi). The largest increase in abundance of Gastropoda was in Dobrohost. Here we found large differences in abundance between June and September. In June 25.937 individuals/m2 were found (from this 22.379 individuals of Lymnaea ovata). In September this snail was also eudominant, while the abundance of Ancylus fluviatilis increased almost twice and that of Bithynia tentaculata 24 times as compared with June. Such differences in abundance were not observed at other monitoring places. Between monitoring places Dobrohost and Gabcikovo differences in abundance changes of Bivalvia were found: it remained unchanged in Dobrohost, but in Gabcikovo species Dreissena polymorpha and Sphaerium corneum augmented substantially. The abundance of Dikerogammarus haemobaphes decreased at both monitoring stations and was more pronounced in Dobrohost, less in Gabcikovo.

In Klucovec, below the estuary of the navigation canal, no increase in abundance was observed, its values did not exceed the variability observed in previous years. In June we could not obtain representative samples of zoobenthos from this station because of the high water level.

#### THE RIVER BRANCH SYSTEM

#### The monitoring stand Dobrohost - Kriviny, (Tab. 1.)

This monitoring stand is represented by a complex of four relatively small fens (marshes) located 200 m away from the Danube. They are older remnants of the former river branch. I denote them as fen A, B, C, D. The fen A (of a 50 m diameter) is the largest and deepest one and has a western location at the forest boundary. The other fens are smaller (diameters of about 30 m), shallower, and are situated in the forest. The fen B is located north-east of A, C lies to the south of B and the fen D is west of C. In 1989-1992 the water level was present here until the summer period. The aquatic submerged vegetation was missing here. Since 1993 the fens are devoid of water and without an aquatic fauna. In the Table 1 there is a list of species found in August 1991, when we found the largest number of species on the moist bottom.

The prevalent fauna here were Gastropoda, less Bivalvia, and from other groups exceptionally occurred Haemopis sanguisuga (Hirudinea) (fen A (1991)). From Gastropoda we found 15 species, from Bivalvia two species. The fens located in the forest (B, C, D) had the highest number (12 - 13) of species. The family Planorbidae was found the most abundant in all fens, with the dominant species Planorbarius corneus and Planorbis planorbis which are the most resistant to desiccation. On the surface of the moist soil we ascertained the absolute numbers of molluscs (932 to 1240 individuals/m2). Of the rare species we found here Hippeutis complanatus and Planorbis carinatus should be mentioned. Large-size species belonging to Prosobranchiata (Viviparus spp.) were missing.

#### Bodiky - The river branch of Bodiky, (Tab. 2.)

The samples of benthos were collected from the southern part of the branch closest to the Danube (The Gate of Bodiky) from the shore stones similarly as from the Danube shore (see above).

In 1991-1992 (a para-plesiopotamal period) we used to find up to 28 species here. The most abundant was the flatworm Dugesia tigrina and Dreissena polymorpha with the coverage of stones 50 - 95%. The medium abundance exhibited leeches Erpobdella octoculata and gastropods Viparus acerosus, Bithynia tentaculata and Lymnaea peregra (Table 2).

In October 1992 the Danube was dammed and the water level in the branch decreased within two weeks to the extent that the largest part of the branch surface became uncovered (naked). This situation continued also during the winter until March 1993, when the re-filling of the branch with the water from the by(pass canal started. In 1993 we noted the absence of Dugesia tigrina, three species of leeches, Viviparus acerosus, and the abundance of Dugesia lugubris, Erpobdella octoculata, Dreissena polymorpha and Bithynia tentaculata dropped substantially. A higher abundance than in 1992 was found out in Lymnaea stagnalis, Physella acuta and, mainly, in Gyraulus albus. Because of the increase of the water level we were unable to monitor the occurrence of fauna on the soft bottom, where we earlier noticed the presence of Unio pictorum (5-8 individuals/m2), Viviparus acerosus and Lithoglyphus naticoides.

In 1994 we noticed the absence of Erpobdella octoculata and the abundance of several, mostly stagnicolous Gastropoda: Lymnaea stagnalis, L. peregra, L. auricularia, Physella acuta and Gyraulus albus. The increased abundance was found out in Turbellaria, Bithynia tentaculata and Dreissena polymorpha. Several species (which are labelled with "+" sign in the Table) were found out in qualitative collections (sampling procedures).

#### Trstena, Kralovska luka, (Tab. 3.)

The dominating substrate (on the bottom) close to the shore is the gravel, further away from the shore is a deep, muddy, bottom with a submerged vegetation. The quantities of species in the Table 3 are expressed relatively (per a unit of time) and they are related to the day (period) when the maximal number of species was found in individual year(s).

The most variable fauna on the vegetation (mostly on Alisma plantago-aquatica) was found out on 28 August 1991. As to the number of species, molluscs are prevalent (Gastropoda - 9 spp., Bivalvia - 2 spp.), and leeches were represented by 4 species. All species were found in low numbers. In 1993 until September there was a very low water level in the branch (0.3 m on the water gauge) and the inundation of the adjacent branches was manifested by the increase of the water level (of Trstena, Kralovska luka) only after seven months - in October. In the period of low water level (on 9 June 1993 the water temperature reached 33o C) we found only 2 gastropod species and 1 species of leech, but an increased quantity of Asellus aquaticus and, mainly, of Bryozoa (coverage up to 50%). In 1994 the number of species found started to increase: 7 spp. Gastropoda, 2 spp. Hirudinea, three new species of Gastropoda and Limnomysis benedeni of Crustacea. L. benedeni and Gyraulus albus. exhibited the highest abundance

#### DISCUSSION

The project of HPSG on the Slovak - Hungarian stretch of the Danube required prognostic studies before the start of building which had to consider the possible influences on the biota of the Danube itself as well as the influence of newly built hydrological objects. These studies [4, 7, 8]) were based on the different project documentation and only generally considered the fate of zoobenthos. The studies concentrated on the dam with a re-servoir and on the by(pass canal. The improvement of the quality of water in the old Danube and the increased abundance of zoobenthos was predicted only under conditions if the old Danube downstream Dunakiliti would be charged with seepage water. The present variant, where the old Danube is charged with water from the reservoir, the deterioration of the water quality was predicted [4].

The extent of qualitative and, mainly, quantitative changes in zoobenthos of the old Danube are, undoubtedly, significant, but is linked to the changes subsequent to putting the HPSG into operation. Of these the hydrological changes should be mentioned: decreased and nearly constant discharge (about 200 - 400 m3/s), nearly constant water level, slow down of the current velocity, more extensive creation of places with slow flowing water. Osci-llations of the water levels in the past caused the dissipation of populations, and the decrease of the water level, on the contrary their concentration, or the extinction of mainly sessile forms. The lateral surf driven by passing vessels which disintegrated shore (ripal( sediments and increased the turbidity of water ceased. At present, water flowing to the old riverbed from the reservoir is partially without suspended solids and enables a substantially better growth of algae on the stones. This significantly extends the nutritional offer and enables the substantial multiplication of grazers and scrapers (Gastropoda). The quantity of other species of zoobenthos (leeches and crustaceans (Amphipoda)) remained at the previous level. In Gabcikovo there is a remarkable increase of quantity of filtrators (Bivalvia), which is a probable consequence of the water enrichment by bioseston from the inundated branches.

So far, it is not possible to explain satisfactorily the conspicuous decrease of crustacean Dikerogammarus haemobaphes, mainly in Dobrohost. Because in para-plesiopotamal branches this species was almost the only representative, it could be expected that in the old Danube its abundance will be increased and that of more rheophilic D. villosus will be decreased.

The sudden and numerous appearance of the Physella acuta in 1994 is very interesting. In the original Danube it was absent and its appearance in the new Danube enables to hold the new Danube for a kind of big river branch. Physella acuta occurs regularly in branches, but not as frequently as in the old Danube at present. Similar situation exists with Potamopyrgus antipodarum. This is not indigenous to the Danube, similarly is not Physella, but it occasionally occurred also before the Danube damming. For the first time it was registered in Bratislava in 1986 (the Karlova Ves branch) and approximately at the same time it was registered in Austria and Hungary [1,3]. Cejka [2] reported its occurrence in Dobrohost. These acquisitions of species, however, do not represent a significant augmentation of the value of the gene pool of the Danube's fauna.

It is highly probable that the development of the zoobenthos in the old Danube has not still been completed and additional changes of the spectrum of species are to be expected. The immediate cause of the extinction of the fen biotope on the monitoring station Dobrohost - Dunajske Kriviny was the decrease of the ground water level and the inundation supplying canal has no effect on this locality yet.

The changes in the benthic fauna at the Bodiky branch as compared with the original status are caused mainly by the rapid and extensive decrease of the water level that occurred after the Danube damming. The substantial part of the bottom of branches was uncovered and water remained in the deepest locations only. In the following winter the fauna became extinct as a consequence of desiccation and freezing. This state persisted from November 1992 till March 1993 when the inundation of the branch had began.

The reshaping of the Bodiky branch to an eupotamal type in 1993 and an increase of the area covered by water has a selective effect. The fauna in the first stage originated from refuges or it was drifted by the flow from upstream regions. This could be the way how to explain the increased abundance of stagnicolous Gastropoda in 1993. This intervention into the Bodiky branch could bring some effects only in 1994 and probably also later. It is probable that despite of changes of branch types from plesiopotamal to eupotamal ones, with regard to the variable water discharge, general changes of the fauna will not occur and that there will be a recovery of populations of original species, especially of those which are indifferent to the current velocity. However, individual species could occur in different numbers and at different places as compared with pre(dam conditions. One has to keep in mind that the supplying of branches with water reshaped the branches and increase the area covered by water. This rearranged to some extent the occurrence of eupotamal, parapotamal, plesiopotamal and paleopotamal along the branches. The conversion of the main branches to the eupotamal types could have an effect in their stream-lines together with the decrease of the water temperature. Altogether, one could expect a slight reduction of the area biotope and a decrease in abundance in stagnicolous species.

The monitoring station Kralovska luka is of a special position. The station is located out of the inundation area and is charged with the ground water only. The development of fauna is still in the process of the recovery of populations of original species and the appearance of additional species seems to be very probable. The low abundance will, however, persist also in the future. I suppose that the limiting and selecting factor here will be the oxygen deficiency at the low water level under the ice surface in winter, when the extinct vegetation decays.

#### REFERENCES

[1] Csanyi, B., 1994: The Macrozoobenthon community of the upper Hungarian Danube. 30. Arbeitstagung der IAD, ZUOZ - Schweiz. Limnologische Ber. Donau 1994, Bd. I: 74-78.

[2] Cejka, T., 1994: First record of the New Zealand molluscs Potamopyrgus antipodarum (Gray, 1843), (Gastropoda, Hydrobiidae) from the Slovak section of the Danube river. Biologia, 49: 657-658.

[3] Frank, C., Jungbluth, J., Richnowsky, A., 1990: Die Mollusken der Donau vom Schwartzwald bis zum Schwarzen Meer, Budapest, 142 pp.

[4] Holcik, J., Bastl, I., Ertl, M., Vranovsky, M., 1981: Hydrobiology and ichtiology of the Czechoslovak Danube in relation to predicted changes after the construction of the Gabcikovo(Nagymaros river barrage system. Prace Lab. rybar. hydrobiol., 3: 19-158

[5] Kosel, V., at al. 1994: Notions from the monitoring of hydrofauna in Danubian lowland in connection with the hydropower structure Gabcikovo (in Slovak(. In: Skusenosti z vyuzitia slovensko-madarskeho useku Dunaja. Bratislava, 10 pp. [6] Kosel, V., 1995: Ripal macrozoobenthos of the Danube before and after putting hydropower structures Gabcikovo into operation (in Slovak(, Vysledky a skusenosti z monitorovania bioty uzemia ovplyvneneho vodnym dielom Gabcikovo, Ustav zoológie a ekosozológie SAV Bratislava, pp 123-131.

[7] Rothschein, J., 1973: Über den Einfluss der geplanten Donaukraftwerke auf die Hydrofauna des tschechoslowakischen Donauabschnittes. Zbornik SNM, Bratislava, 18: 79-97.

[8] Rothschein, J., 1976: Prognoses of Danube regime qualitative changes after construction of hydropower stations (in Slovak(. Prace a studie 82. VUVH, 89 pp.

Tab. 1. Permanent zoobenthos at the Dobrohost-Kriviny station.

LOCALITY | A | B | B | C | C | D
--- | --- | --- | --- | --- | --- | ---
SPECIES | 1991 | 1991 | 1992 | 1991 | 1992 | 1992
Bithynia tentaculata | 6,0 | 15,3 | 4,8 | 1,1 | 29,3 | 13,6
Valvata cristata |  | 5,8 | 1,0 |  | 0,3 |
Valvata piscinalis | 3,4 | 7,4 | 1,3 |  | 3,3 |
Lymnaea peregra | 0,4 |  | 1,6 |  | 0,7 | 0,8
Lymnaea stagnalis | 10,8 | 0,3 | 2,3 |  | 0,3 |
Lymnaea turricula |  | 0,6 | 1,0 | 6,1 | 6,8 | 15,2
Aplexa hypnorum |  |  | 0,6 | 1,1 | 1,0 | 18,2
Anisus spirobris |  | 2,6 | 1,9 |  | 14,0 | 3,0
Anisus vortex |  | 4,5 | 6,5 |  | 3,3 | 1,5
Gyraulus albus |  | 0,3 |  |  |  |
Hippeutis complanatus |  |  | 0,6 |  |  |
Planorbarius corneus | 65,6 | 34,9 | 27,4 | 7,3 | 8,1 | 6,8
Planorbis carinatus |  | 3,0 |  |  | 3,3 | 1,5
Planorbis planorbis | 15,9 | 22,7 | 50,0 | 82,2 | 27,3 | 37,8
Sphaerium lacustre | 0,9 | 2,6 |  | 2,2 | 2,3 | 0,8
Pisidium casertanum |  |  | 1,0 |  |  | 0,8
TOTAL % | 100.0 | 100.0 | 100.0 | 100.0 | 100.0 | 100.0
QUANTITY | 932 /m2 | 1237 /m2 | 1240 /m2 | 179 | 307 | 132

Tab.2. Permanent zoobethos in the Bodiky branch.
Species with the asterisk were only found in qualitative samples.

DATUM | 03.10.91 | 16.09.92 | 12.10.93 | 06.09.94
--- | --- | --- | --- | ---
SPECIES | ex / m2 | ex / m2 | ex / m2 | ex / m2
Porifera |  |  |  |
Eunapius fragilis | * | 10% | * | 15%
Turbellaria |  |  |  |
Dugesia lugubris | 18 | 49 | 6 | 50
Dugesia tigrina | 2740 | 3011 |  | 15
Hirudinea |  |  |  |
Erpobdella apathyi | 61 | 23 | 22 | 12
Erpobdella octoculata | 122 | 165 | 12 |
Erpobdella nigricollis | 3 |  |  |
Helobdella stagnalis | 2 | 14 |  |
Glossiphonia complanata | 17 |  |  |
Gastropoda |  |  |  |
Bithynia tentaculata | 43 | 145 | 18 | 50
Lithoglyphus naticoides | 6 | 11 |  |
Valvata piscinalis | * | * | * | *
Viviparus acerosus | 49 | 83 |  |
Aplexa hypnorum |  | * |  |
Physa acuta | 5 | 54 | 102 | 5
Lymnaea auricularia | 9 | 56 | 68 | *
Lymnaea peregra | 83 | * | 117 | 40
Lymnaea stagnalis | 1 | 10 | 42 | *
Anisus spirobris |  | * |  |
Gyraulus albus | 5 | * | 105 | 8
Gyraulus laevis | 2 |  | 6 | *
Planorbis planorbis |  | * |  |
Acroloxus lacustris | 13 | * |  |
Dreissena polymorpha | 95% | 80% | 6 | 70
Pisidium supinum |  | * |  |
Sphaerium corneum | 3 |  |  |
Unio pictorum | * | * |  |
Bryozoa |  |  |  |
Cristatella mucedo |  |  | * |
Paludicella articulata | * | * | * |
Plumatella fungosa | * | * |  |
Crustacea |  |  |  |
Asellus aquaticus | * | 84 |  |
Corophium curvispinum | * | * | * |
Dikerogammarus haemobaphes | 32 | 14 | 6 | 4
Dikerogammarus villosus | 6 |  |  | 16
Chaetogammarus ischnus | 5 |  |  |
Limnomysis benedeni |  |  | * | *
TOTAL QUANTITY ex/m2 | 3225 | 3719 | 504 | 270
NUMBER OF SPECIES | 29 | 27 | 18 | 16

Tab. 3. Permanent zoobenthos at the Kralovska luka branch.
Species with the asterisk were only found in qualitative samples.

DATUM | 17.09.91 | 12.09.92 | 09.06.93 | 15.06.94
--- | --- | --- | --- | ---
SPECIES | ex / h | ex / h | ex / h | ex / h
Turbellaria |  |  |  |
Dugesia tigrina |  |  | * |
Hirudinea |  |  |  |
Glossiphonia heteroclita | 3 |  |  |
Helobdella stagnalis | 3 | 2 | 10 | 16
Hemiclepsis marginata | 4 |  |  |
Erpobdella octoculata | 12 | 16 |  | 4
Haemopis sanguisuga |  | 4 | 2 |
Gastropoda |  |  |  |
Bithynia tentaculata | 15 |  |  | 3
Valvata cristata | * |  |  |
Viviparus contectus | 2 |  |  |
Physa acuta |  |  |  | 3
Lymnaea auricularia | 9 | 1 | 2 | 48
Lymnaea peregra | 1 |  |  |
Lymnaea stagnalis | 16 | * | 1 | 28
Gyraulus albus | 21 | * |  | 72
Hippeutis complanatus |  |  |  | 8
Planorbarius corneus | 10 |  |  |
Planorbis planorbis | 3 | 3 |  | 8
Acroloxus lacustris | * | * |  |
Bivalvia |  |  |  |
Anodonta cygnaea | * |  |  |
Bryozoa |  |  |  |
Plumatella funfosa | * | * | 25% |
Plumatella repens |  | * | 25% |
Crustacea |  |  |  |
Asellus aquaticus | 26 | 132 | 442 | *
Lymnomysis benedeni |  |  |  | 200
TOTAL ex/h | 125 | 158 | 457 | 390
NUMBER OF SPECIES | 17 | 11 | 8 | 11