**4.2 Qualitative structure of benthofauna in oxbow-lakes with various type of connection to the main river**

#### **4.2.1 Lentic oxbow-lakes**

The oxbow-lakes with limited hydrological connection to the main river were inhabited by 27 taxa. The most abundant were Chironomidae larvae (*Chironomus* sp. and *Chaoborus* sp.) as well as leeches of *Erpobdella* genus (Table 7). Those taxa are characterized by considerable

Fig. 4. Benthofauna constancy of occurrence (C, %) in the oxbow-lakes of the Słupia River Collating the data from the studied river basins one can conclude that the most frequent were *Chironomus* sp. larvae, although they did not place the first position in any of the river basins (Table 6). *Erpodbella* sp. (Cav.=32%), *Chaoborus* sp. (Cav.=31%) and Oligochaeta (Cav.=26%) were also important elements of benthofauna structure. None of the identified

*Chironomus* sp. 15.7 (3) 50.0 (2) 40.0 (5) *Erpobdella* sp. 10.2 (6) 22.9 (3) 61.8 (2) *Chaoborus* sp. 25.3 (1) 58.3 (1) 10.0 (14) Oligochaeta 20.0 (2) 10.4 (6) 48.4 (3) *Procladius* sp. 12.6 (4) 14.6 (4) 48.3 (4) *A.aquaticus* 7.8 (8) 2.1 (9) 65.2 (1) *Glossiphonia* sp*.* 5.0 (9) 2.1 (9) 3.0 (7) *Cloëon* sp. 0.1 (40) 4.2 (7) 23.3 (10) *Bezzia* sp. 3.7 (11) 4.2 (7) 3.3 (19) Table 6. Benthofauna constancy of occurrence (C, %) and their ranks in the studied river

**4.2 Qualitative structure of benthofauna in oxbow-lakes with various type of** 

The oxbow-lakes with limited hydrological connection to the main river were inhabited by 27 taxa. The most abundant were Chironomidae larvae (*Chironomus* sp. and *Chaoborus* sp.) as well as leeches of *Erpobdella* genus (Table 7). Those taxa are characterized by considerable

River basin **Łyna Drwęca Słupia** 

invertebrate representatives reached the status higher than accesoric taxon.

Taxa

basins

**connection to the main river 4.2.1 Lentic oxbow-lakes** 

adaptability to environmental conditions and therefore are common in various aquatic ecosystems, including oxbows (Galardo et al., 2008, Obolewski, 2011a). However, they were only accesoric taxa and their constancy of occurrence did not exceed half of the samples.


Table 7. Constancy of occurrence with classification of the most frequent benthofauna taxa (Cav. > 25.0 %) in lentic oxbow-lakes

Both leeches and Diptera larvae are often observed in reservoirs polluted with organic substances, where the concentration of oxygen is low and limits the existence of other aquatic invertebrates. The remaining representatives of benthic fauna were classified as accidental taxa. The frequency of 5 taxa, including *Procladius* sp. larvae and Oligochaeta representatives, exceeded 10%,

The comparison of constancy of occurrence between the studied river basins gives interesting observations. The oxbow-lakes of the Drwęca River are closed and the refreshment of waters takes place during spring and autumn spates. Macrozoobenthos was represented there by 12 taxa and predominated by Diptera larvae (*Chironomus* sp., *Chaoborus* sp.), classified as constant taxa (OLD1) or euconstants (OLD2-OLD3). Particularly low macrozoobenthos diversity was observed in OLD1, where dense pleustonic fauna limited the mixing of waters and decreased the oxygenation (Glińska-Lewczuk, 2009). In the Łyna River Basin there was only one cut-off oxbow-lake, in which 14 taxa were observed and 85% of them occurred once. The most frequent was *V. pulchella* but it was still classified as accidental taxon (C<25%). The two closed oxbow-lakes in the Słupia river basin were rich with 20 macrozoobenthos taxa. 35% of them were constant taxa. Comparing to the oxbowlakes of Drwęca and Łyna, *Erpobdella* and *A. aquaticus* occurred the most frequently in the Słupia river basin. No *Chaoborus* sp. larvae were observed which indicated good water oxygenation (Kajak, 2001).

#### **4.2.2 Semi-lotic oxbow-lakes**

Semi-open oxbow-lakes are hydrologically classified between cut-off and open reservoirs. They form environmental conditions favourable to fauna typical of both lotic and lentic ecosystems. The total number of taxa identified in the studied semi-lotic oxbow-lakes was equal to 40 which was reflected by the qualitative structure of benthofauna (Table 8)

The most frequent taxa in the oxbow-lake of the Łyna River were predatory larvae *Procladius* sp., bivalves *Pisidium* sp. and *Chaoborus* sp. larvae. However, the last two taxa were classified as accesoric, similarly to Oligochaeta and *Chironomus* sp. The remaining taxa were accidental. Different situation was observed in the oxbow-lake located in the Słupia River Basin, where the highest constancy of occurrence reached *Erpobdella* sp., *Procladius* sp., *A.* 

Biodiversity of Macroinvertebrates

were classified as eudominant taxa (Table 10).

typical of degraded aquatic ecosystems (Kajak, 2001).

**4.3.1 Łyna River basin** 

indiv.m-2), (Fig. 5).

around 50-60% (Table 10).

in Oxbow-Lakes of Early Glacial River Basins in Northern Poland 155

The oxbow-lakes of the Łyna River were predominated by *Chaoborus* sp., Oligochaeta and *Chironomus* sp. which jointly constituted 66% of the total benthofauna abundance. They

As for the other macrozoobenthos representatives, *V. viviparus* reached the status of dominant taxa and six taxa belonging to Crustacea, Hirudinea and Insecta were subdominants. Relatively large group of taxa was observed with low density which is

The average density in the studied reservoirs was low and did not exceed 200 indiv. m-2. However, each oxbow-lake was predominated by one taxa- OLŁ1-OLŁ3 by *Chaoborus* sp. (maximum density 1400 indiv.m-2 ) and OLŁ4-OLŁ5 by Oligochaeta (maximum density 2600

Fig. 5. Benthofauna density (indiv. m-2) in the oxbow-lakes of the Łyna River

Such high abundance was reflected by the values of domination index (D, %). The share of *Chaoborus* sp. varied between 70% in OLŁ1 to 20% in OLŁ2, where they were eudominants. In OLŁ1 their domination was so high that no other taxa exceeded 10% of the total density. That situation indicates OLŁ1 as the most degraded reservoir which needs revitalization (Obolewski et al., 2009). The level of Oligochaeta domination in two oxbow-lakes was

**4.3 Quantitative structure of benthofauna in oxbow-lakes of the studied river basins** 


*aquaticus*, *H. stagnalis* and *Glossiphonia* sp. (constant taxa). Oligochaeta, *Chironomus* sp., *S.fuliginosa* and *Sphaerium* sp. were accesoric taxa and the remaining were accidental.

Table 8. Benthofauna constancy of occurrence with classification of the most frequent taxa (Cav. >25.0 %) in semi-open oxbow lakes

#### **4.2.3 Lotic oxbow-lakes**

The most frequent taxon in lotic oxbow-lakes was *Chironomus* sp. which was classified as euconstants (C>50%). Quite often occurred *Erpobdella* sp., *A. aquaticus,* oligochaetes and *Chaoborus* sp. (Table 9).


Table 9. Benthofauna constancy of occurrence and classification of the most frequent taxa (Cav. > 25.0 %) in lotic oxbow-lakes

The frequency of occurrence of other benthofauna representatives differed between the river basins. The open oxbow-lakes of the Łyna River were predominated by *Chironomus* sp. (Cav.=60%, euconstant taxon). The larvae of *Chaoborus* sp. also occurred quite often (Cav.=44%) but they were not present in OLŁ5. It was the youngest of the studied reservoirs while *Chaoborus* sp. prefers stagnant waters and therefore was absent in the discussed oxbow-lake (Kajak, 2001). The qualitative structure of benthofauna in the oxbows of the Łyna River Basin indicated decreased importance of *A. aquaticus* which was not observed in OLŁ3. This species is typical of oxbow-lakes in northern Poland (Obolewski, 2011a; Obolewski & Glińska-Lewczuk, 2011) rich with organic matter. They are absent in reservoirs which are uncovered or with high flow velocity washing out organic substances (Stańczykowska, 1986). Lotic oxbow-lakes of the Słupia River were predominated by *A. aquaticus* (Cav.=76%) and *Erpobdella* sp. (Cav.=54%) which were classified as euconstants.

*aquaticus*, *H. stagnalis* and *Glossiphonia* sp. (constant taxa). Oligochaeta, *Chironomus* sp.,

*Procladius* sp. **44.4** 58.3 51.4 *Erpobdella* sp*.* 22.2 **66.7** 44.4 *A. aquaticus* 22.2 58.3 40.3 *Glossiphonia* sp*.* 22.2 58.3 40.3 Oligochaeat 27.8 50.0 38.9 *Chironomus* sp. 27.8 33.3 30.6 *H. stagnalis* 0.0 58.3 29.2 *S. fuliginosa* 22.2 33.3 27.8 Table 8. Benthofauna constancy of occurrence with classification of the most frequent taxa

The most frequent taxon in lotic oxbow-lakes was *Chironomus* sp. which was classified as euconstants (C>50%). Quite often occurred *Erpobdella* sp., *A. aquaticus,* oligochaetes and

*Chironomus* sp. **75.0 75.0 44.4** 25.0 41.7 52.2 *Erpobdella sp.* 41.7 16.7 33.3 42.0 66.7 40.1 *A.aquaticus* 16.7 0.0 22.2 **67.7 83.3** 38.0 Oligochaeta 41.7 16.7 44.4 16.7 67.0 37.3 *Chaoborus* sp. 58.3 **75.0** 0.0 25.0 8.3 33.3 Table 9. Benthofauna constancy of occurrence and classification of the most frequent taxa

The frequency of occurrence of other benthofauna representatives differed between the river basins. The open oxbow-lakes of the Łyna River were predominated by *Chironomus* sp. (Cav.=60%, euconstant taxon). The larvae of *Chaoborus* sp. also occurred quite often (Cav.=44%) but they were not present in OLŁ5. It was the youngest of the studied reservoirs while *Chaoborus* sp. prefers stagnant waters and therefore was absent in the discussed oxbow-lake (Kajak, 2001). The qualitative structure of benthofauna in the oxbows of the Łyna River Basin indicated decreased importance of *A. aquaticus* which was not observed in OLŁ3. This species is typical of oxbow-lakes in northern Poland (Obolewski, 2011a; Obolewski & Glińska-Lewczuk, 2011) rich with organic matter. They are absent in reservoirs which are uncovered or with high flow velocity washing out organic substances (Stańczykowska, 1986). Lotic oxbow-lakes of the Słupia River were predominated by *A. aquaticus* (Cav.=76%) and *Erpobdella* sp. (Cav.=54%) which were classified as euconstants.

Łyna River Basin Słupia River Basin

**OLŁ 2 OLŁ 3 OLŁ 5 OLS 4 OLS 5** 

Oxbow-lakes Mean

Łyna River Basin Słupia River Basin

**OLŁ1 OLS 3** 

Oxbow-lakes Mean

*S.fuliginosa* and *Sphaerium* sp. were accesoric taxa and the remaining were accidental.

Taxa

(Cav. >25.0 %) in semi-open oxbow lakes

**4.2.3 Lotic oxbow-lakes** 

*Chaoborus* sp. (Table 9).

Taxa

(Cav. > 25.0 %) in lotic oxbow-lakes

#### **4.3 Quantitative structure of benthofauna in oxbow-lakes of the studied river basins 4.3.1 Łyna River basin**

The oxbow-lakes of the Łyna River were predominated by *Chaoborus* sp., Oligochaeta and *Chironomus* sp. which jointly constituted 66% of the total benthofauna abundance. They were classified as eudominant taxa (Table 10).

As for the other macrozoobenthos representatives, *V. viviparus* reached the status of dominant taxa and six taxa belonging to Crustacea, Hirudinea and Insecta were subdominants. Relatively large group of taxa was observed with low density which is typical of degraded aquatic ecosystems (Kajak, 2001).

The average density in the studied reservoirs was low and did not exceed 200 indiv. m-2. However, each oxbow-lake was predominated by one taxa- OLŁ1-OLŁ3 by *Chaoborus* sp. (maximum density 1400 indiv.m-2 ) and OLŁ4-OLŁ5 by Oligochaeta (maximum density 2600 indiv.m-2), (Fig. 5).

Fig. 5. Benthofauna density (indiv. m-2) in the oxbow-lakes of the Łyna River

Such high abundance was reflected by the values of domination index (D, %). The share of *Chaoborus* sp. varied between 70% in OLŁ1 to 20% in OLŁ2, where they were eudominants. In OLŁ1 their domination was so high that no other taxa exceeded 10% of the total density. That situation indicates OLŁ1 as the most degraded reservoir which needs revitalization (Obolewski et al., 2009). The level of Oligochaeta domination in two oxbow-lakes was around 50-60% (Table 10).

Biodiversity of Macroinvertebrates

**4.3.2 Drwęca River basin** 

SR – subrecedents)

in Oxbow-Lakes of Early Glacial River Basins in Northern Poland 157

Macrozoobenthos in the oxbow-lakes of the Drwęca River was predominated by Diptera larvae: *Chaoborus* sp., *Chironomus* sp. and *Procladius* sp. which jointly constituted 76% of the total density. *Chaoborus* sp. was the most abundant in OLD1 and OLD4 while *Chironomus* sp. in OLD2 and OLD3 (Table 11). The increase in *Chironomus* sp. density was accompanied by higher abundance of predatory *Procladius* sp. but they were not observed in the reservoirs predominated by *Chaoborus* sp. In terms of domination, larvae of *Chironomus* sp.

*Chaoborus* sp. **37.3** 26.5 22.6 **68.3** 38.7

*Procladius* sp. 4.8 17.1 18.7 0.0 10.2

*Cloëon* sp. 4.8 0.0 2.6 0.0 1.9

*H. stagnalis* 0.0 0.0 0.0 2.5 0.6

Fig. 6. Benthofauna density (indiv. m-2) in the oxbow-lakes of the Drwęca River

*Chironomus* sp. 14.5 **46.3 30.2** 19.4 27.6 **ED**

*Erpobdella* sp. 24.1 1.4 2.0 9.8 9.3 **D** *A. aquaticus* 14.5 0.0 0.0 0.0 3.6 **SD** *Glossiphonia* sp. 0.0 0.0 8.7 0.0 2.2

**<sup>R</sup>** Oligochaeta 0.0 2.7 3.9 0.0 1.7 *Sergentia* sp. 0.0 0.0 6.1 0.0 1.5 *Chironomus* sp. pupa 0.0 1.8 3.5 0.0 1.3 *Bezzia* sp. 0.0 1.5 1.7 0.0 0.8

*Cyrnus* sp. 0.0 2.7 0.0 0.0 0.7 **SR**

Table 11. Benthofauna domination index (D, %) in the oxbow-lakes of the Drwęca River with classification (ED – eudominants, D – dominants, SD – subdominants, R – recedents,

Taxa Oxbow-lakes Mean Classification **OLD 1 OLD 2 OLD 3 OLD 4**


Table 10. Domination index (D, %) of benthofauna in the oxbow-lakes of the Łyna River with classification (ED – eudominants, D – dominants, SD – subdominants, R – recedents, SR – subrecedents) +<0.1%

#### **4.3.2 Drwęca River basin**

156 Ecosystems Biodiversity

Taxa Oxbow-lakes Mean Classification **OLŁ 1 OLŁ 2 OLŁ<sup>3</sup> OLŁ 4 OLŁ<sup>5</sup>**

**SD** 

**SR** 

Oligochaeta 6.5 9.0 2.2 **58.3 52.9** 25.8 **ED** 

*V. viviparus* 3.7 6.7 3.4 1.3 10.5 5.1 **D** 

*Pisidium* sp. 3.1 0.0 0.0 0.3 5.1 1.7 **R** 

Table 10. Domination index (D, %) of benthofauna in the oxbow-lakes of the Łyna River with classification (ED – eudominants, D – dominants, SD – subdominants, R – recedents,

SR – subrecedents) +<0.1%

*Chaoborus* sp. **69.1 20.8 58.6** 0.3 0.0 29.8

*Chironomus* sp. 2.3 13.9 10.4 14.7 9.9 10.2

*A. aquaticus* 2.2 2.3 0.0 0.0 14.4 3.8

*Limnephilus* sp*.* 0.0 1.5 1.7 10.8 0.0 2.8 *Erpobdella sp.* 1.0 8.1 2.3 0.8 1.9 2.8 *Bezzia* sp. 0.0 5.4 7.5 0.0 0.2 2.6 *S. fuliginosa* 0.6 9.3 1.7 0.3 0.0 2.4 *Procladius* sp*.* 0.1 5.7 4.7 0.7 0.0 2.2 *Cyrnus* sp. 0.0 7.0 1.7 1.0 0.0 1.9

*M. culiciformis* 5.9 0.0 0.0 0.0 0.0 1.2 *Dytiscus* sp. 0.0 0.0 0.0 4.0 0.0 0.8

*Glossiphonia* sp*.* 1.0 0.0 1.8 0.0 0.9 0.7 *Notonecta* sp. 0.0 0.0 0.0 3.7 0.0 0.7 *H. stagnalis* 0.0 1.6 1.7 0.0 0.0 0.7 *A. anatina* 0.3 2.1 0.0 0.0 0.4 0.6 *Sergentia* sp. 0.0 0.0 0.0 2.5 0.2 0.5 *Chironomus* sp. pupa 0.0 0.0 2.3 0.0 0.0 0.5 *Lestes* sp. 0.0 2.1 0.0 0.0 0.0 0.4 *L. stagnalis* 0.5 0.0 0.0 0.0 1.5 0.4 *E. bimaculata* 0.0 1.5 0.0 0.0 0.0 0.3 Phryganaidae 0.0 0.0 0.0 0.0 1.5 0.3 *D. polymorpha* 0.0 1.5 0.0 0.0 0.0 0.3 *Caenis sp.* 0.0 1.5 0.0 0.0 0.0 0.3 *V. pulchella* 0.0 0.0 0.0 0.9 0.0 0.2 *Polypedilum* sp. 0.9 0.0 0.0 0.0 0.0 0.2 *V.piscinalis* 0.4 0.0 0.0 0.0 0.0 0.1 *P. antipodarum* + 0.0 0.0 0.4 0.0 0.1 *V. cirstata* 0.3 0.0 0.0 0.0 0.0 0.1 *A. cygnea* 0.3 0.0 0.0 0.0 0.0 0.1 *E. tenellus* 0.4 0.0 0.0 0.0 0.0 0.1 *Goera* sp. 0.5 0.0 0.0 0.0 0.0 0.1 Psychomidae 0.4 0.0 0.0 0.0 0.0 0.1 *G. truncatula* 0.2 0.0 0.0 0.0 0.0 + *Sphaerium* sp. 0.4 0.0 0.0 0.0 0.0 + *Lepidostoma* sp. 0.2 0.0 0.0 0.0 0.0 + *Hydropsyche* sp. + 0.0 0.0 0.0 0.0 + Limoniidae + 0.0 0.0 0.0 0.0 + *Cloëon* sp. 0.0 0.0 0.0 0.0 0.2 + Macrozoobenthos in the oxbow-lakes of the Drwęca River was predominated by Diptera larvae: *Chaoborus* sp., *Chironomus* sp. and *Procladius* sp. which jointly constituted 76% of the total density. *Chaoborus* sp. was the most abundant in OLD1 and OLD4 while *Chironomus* sp. in OLD2 and OLD3 (Table 11). The increase in *Chironomus* sp. density was accompanied by higher abundance of predatory *Procladius* sp. but they were not observed in the reservoirs predominated by *Chaoborus* sp. In terms of domination, larvae of *Chironomus* sp.


Table 11. Benthofauna domination index (D, %) in the oxbow-lakes of the Drwęca River with classification (ED – eudominants, D – dominants, SD – subdominants, R – recedents, SR – subrecedents)

Fig. 6. Benthofauna density (indiv. m-2) in the oxbow-lakes of the Drwęca River

Biodiversity of Macroinvertebrates

*Chironomus* sp.

pupa

in Oxbow-Lakes of Early Glacial River Basins in Northern Poland 159

The comparison of averaged domination indices and ranks of the consecutive taxa in the studied river basins shows the frequent occurrence of *Chaoborus* sp. larvae which predominated in the oxbow-lakes of Drwęca and Łyna (Table 13). Its importance was lower in the Słupia River Basin. Opposite situation was observed in case of *A. aquaticus* and *Cloëon*  sp*.* which were not abundant in the Drwęca and Łyna river basins but constituted the most importand quantitative component of macrozoobenthos in the Słupia River Basin. High

Taxa Oxbow-lakes Mean Classification **OLS 1 OLS 2 OLS 3 OLS 4 OLS 5**

**SD**

**SR** 

0.0 0.2

ranks were also reached by *Chironomus* sp*.* and Oligochaeta.

*A. aquaticus* 27.1 **29.0 36.3 29.3** 11.4 26.6

Oligochaeta 2.4 19.9 26.1 7.9 13.5 14.0 *Cloëon* sp. 3.5 7.8 0.4 0.8 **44.1** 11.3 *Procladius* sp*.* 4.6 7.6 2.1 4.4 1.4 4.0

*Gammarus* sp. 0.0 0.0 0.8 18.1 0.4 3.9 *Erpobdella* sp*.* 1.3 1.8 2.4 1.5 8.6 3.1 *S. fuliginosa* 2.3 4.6 3.8 2.4 0.4 2.7 *Sphaerium* sp. 2.3 3.8 1.5 3.8 0.8 2.4 *Limnephilus* sp*.* 1.2 2.0 4.8 1.3 1.1 2.1 *Corixa* sp. 0.7 1.0 0.5 0.8 7.3 2.1 *Sergentia* sp. 0.0 1.7 5.7 2.4 0.0 2.0 *Glossiphonia* sp*.* 0.7 1.0 3.2 0.0 2.3 1.4

**ED** *Chironomus* sp. **49.2** 11.6 4.3 23.3 0.7 17.8

*V. viviparus* 0.7 4.2 0.0 0.0 0.4 1.1 **R** 

0.8

*Bezzia* sp. 0.0 1.0 0.0 0.0 3.9 1.0 *H. stagnalis* 0.7 1.0 2.7 0.0 0.0 0.9

*Chaoborus* sp. 0.5 0.0 0.4 1.6 0.6 0.6 *Dytiscus* sp. 0.5 1.0 0.4 0.0 0.9 0.6 *Hydrovatus* sp. 0.0 0.0 1.9 0.0 0.0 0.4 *Lestes* sp. 0.8 0.0 0.0 0.0 0.8 0.3 *Tabanus* sp*.* 1.0 0.0 0.0 0.0 0.0 0.2 *Cyrnus* sp. 0.0 0.0 0.0 0.8 0.4 0.2

0.0 0.0 0.4

*Platambus* sp. 0.0 1.0 0.0 0.0 0.0 0.2 *E. bimaculata* 0.0 0.0 0.0 0.8 0.0 0.2 *Ephemera* sp. 0.0 0.0 0.8 0.0 0.0 0.2 *Notonecta* sp. 0.0 0.0 0.0 0.0 0.5 0.1 *Libellula* sp. 0.0 0.0 0.0 0.4 0.0 0.1 *P. geometra* 0.0 0.0 0.0 0.0 0.5 0.1 *A. grandis* 0.5 0.0 0.0 0.0 0.0 0.1 *N. cinera* 0.0 0.0 0.4 0.0 0.0 0.1

Table 12. Benthofauna domination index (D, %) in the oxbow-lakes of the Słupia River with classification (ED – eudominants, SD – subdominants, R – recedents, SR – subrecedents)

predominated and reached even 70% of benthofauna (OLD4) while *Chironomus* sp.- 30-50% (OLD2-OLD3). The only exception was OLD1 where leeches of *Erpobdella* sp. as well as *Chironomus* sp. appeared (Table 11).

The density of benthofauna did not exceed the average value of 50 indiv.m-2 and more abundant macrozoobenthos was observed only in OLD3 (Fig. 6). The larvae of *Chironomus* sp. appeared massively (density above 500 indiv.m-2) in that reservoir as well as in OLD2. OLD4 seems to be the most degraded oxbow-lake in the Drwęca River Basin due to low benthofauna density and the predomination of *Chaoborus* sp. (density above 400 indiv.m-2). The only reservoir with no predominant taxa was OLD1 and represented different zoocenotic type.

#### **4.3.3 Słupia River Basin**

The oxbow-lakes in the Słupia River Basin were abundantly inhabited by taxa belonging to Oligochaeta, Crustacea and Insecta (Table 12). They constituted 70% of benthofauna and the share of *A. aquaticus* reached 40%. That species predominated in three oxbows (D=30-36%). *A. aquaticus*, particularly large individuals are typical of β-mezosaprobic waters (Turoboyski, 1979). Almost half of benthofauna density in OLS1 constituted the larvae of *Chironomus* sp. but their share was lower in the remaining reservoirs. In turn, the density of Oligochaeta was similar in all the oxbows (Table 12).

Quite atypical domination structure was observed in OLS5, where benthofauna was predominated by *Cloëon* sp. (25% of total density). That taxon is regarded as bioindicator and its presence is a sign of good environmental conditions, particularly high water oxygenation (Turoboyski, 1979).

Average macrozoobenthos density in the oxbow-lakes of the Słupia River was at the level of 100 indiv.m-2 with the domination of *A. aquaticus* (density between 400 and 700 indiv.m-2)*, Chironomus* sp*.* (maximum density over 1500 indiv.m-2) or *Cloëon* sp. (1600 indiv.m-2), (Fig. 7).

Fig. 7. Benthofauna density (indiv. m-2) in the oxbow-lakes of the Słupia River

predominated and reached even 70% of benthofauna (OLD4) while *Chironomus* sp.- 30-50% (OLD2-OLD3). The only exception was OLD1 where leeches of *Erpobdella* sp. as well as

The density of benthofauna did not exceed the average value of 50 indiv.m-2 and more abundant macrozoobenthos was observed only in OLD3 (Fig. 6). The larvae of *Chironomus* sp. appeared massively (density above 500 indiv.m-2) in that reservoir as well as in OLD2. OLD4 seems to be the most degraded oxbow-lake in the Drwęca River Basin due to low benthofauna density and the predomination of *Chaoborus* sp. (density above 400 indiv.m-2). The only reservoir with no predominant taxa was OLD1 and represented different

The oxbow-lakes in the Słupia River Basin were abundantly inhabited by taxa belonging to Oligochaeta, Crustacea and Insecta (Table 12). They constituted 70% of benthofauna and the share of *A. aquaticus* reached 40%. That species predominated in three oxbows (D=30-36%). *A. aquaticus*, particularly large individuals are typical of β-mezosaprobic waters (Turoboyski, 1979). Almost half of benthofauna density in OLS1 constituted the larvae of *Chironomus* sp. but their share was lower in the remaining reservoirs. In turn, the density of

Quite atypical domination structure was observed in OLS5, where benthofauna was predominated by *Cloëon* sp. (25% of total density). That taxon is regarded as bioindicator and its presence is a sign of good environmental conditions, particularly high water

Average macrozoobenthos density in the oxbow-lakes of the Słupia River was at the level of 100 indiv.m-2 with the domination of *A. aquaticus* (density between 400 and 700 indiv.m-2)*, Chironomus* sp*.* (maximum density over 1500 indiv.m-2) or *Cloëon* sp. (1600 indiv.m-2), (Fig. 7).

Fig. 7. Benthofauna density (indiv. m-2) in the oxbow-lakes of the Słupia River

*Chironomus* sp. appeared (Table 11).

zoocenotic type.

**4.3.3 Słupia River Basin** 

oxygenation (Turoboyski, 1979).

Oligochaeta was similar in all the oxbows (Table 12).

The comparison of averaged domination indices and ranks of the consecutive taxa in the studied river basins shows the frequent occurrence of *Chaoborus* sp. larvae which predominated in the oxbow-lakes of Drwęca and Łyna (Table 13). Its importance was lower in the Słupia River Basin. Opposite situation was observed in case of *A. aquaticus* and *Cloëon*  sp*.* which were not abundant in the Drwęca and Łyna river basins but constituted the most importand quantitative component of macrozoobenthos in the Słupia River Basin. High ranks were also reached by *Chironomus* sp*.* and Oligochaeta.


Table 12. Benthofauna domination index (D, %) in the oxbow-lakes of the Słupia River with classification (ED – eudominants, SD – subdominants, R – recedents, SR – subrecedents)

Biodiversity of Macroinvertebrates

**4.4.2 Semi-lotic oxbow-lakes** 

mezosaprobic waters (Turoboyski, 1979).

**4.4.3 Lotic oxbow-lakes** 

most abundant in OLŁ5 (Table 16).

bioindicator of oligotrophic waters.

Hirudinea.

in Oxbow-Lakes of Early Glacial River Basins in Northern Poland 161

hour oscillation of water level caused by the work of hydroelectric power stations (so called effect of Intensive Flood Pulse IFP) (Obolewski, 2011b). As a result, benthofauna abundance

Semi-open oxbow lakes were predominated by the larvae of *Chaoborus* sp. which were accompanied by *A. aquaticus* and Oligochaeta. They formed the group of eudominants and constituted 70% of total benthofauna density (Table 15). Other important macrozoobenthos components were 6 taxa belonging to Insecta (Diptera, Trichoptera, Megaloptera) and

The structure of benthofauna domination in Łyna and Słupia river basins was specific. The diversity of lotic and lentic habitats causes that those biocenosis are unique biological systems (Tockner et al., 1999; Obolewski, 2011a). *Chaoborus* sp. and *M. culiciformis* predominated in the oxbow-lake of the Łyna River, opposite to the Słupia River Basin. That indicates possible differences in the level of water refreshment. Moreover, water currents may wash away *Chaoborus* sp. (Mikulski, 1974) but favour the presence of *A. aquaticus* thanks to the inflow of organic matter carrying by water. As for other taxa, *Limnephilus* sp. and leeches *Glossiphonia* sp. were classified as subdominants. Their occurrence indicates β-

Taxa Oxbow-lakes Mean Classification **OLŁ1 OLS 3** 

**SD** 

is higher with the domination of Oligochaeta and Crustacea.

*Chaoborus* sp. **69.1** 0.4 34.7

Oligochaeta 6.5 26.1 16.3 *Chironomus* sp. 2.3 4.3 3.3

*Sergentia* sp. 0.0 5.7 2.9 *M. culiciformis* 5.5 0.0 2.8 *Limnephilus* sp. 0.0 4.8 2.4 *S.fuliginosa* 0.6 3.8 2.2 *Glossiphonia* sp*.* 1.0 3.2 2.1

*A. aquaticus* 2.2 **36.3** 19.2 **ED** 

Table 15. Domination index of the most import ant benthofauna representatives (mean D>2 %) in semi-lotic oxbow-lakes with classification (ED – eudominants, SD – subdominants)

Lotic oxbow-lakes are reservoirs with constant inflow of fresh water which intensity depends on water flow in the river (Ward et al., 2002). The studied open oxbows were predominated by Oligochaeta, Diptera larvae and Crustacea. Their share in benthofauna qualitative structure reached 56% (eudominants). The higher was hydrological connectivity the lower was the percentage share of eudominants but higher number of important benthofauna components. Regardless the level of contact between oxbows and rivers, the same taxa prevailed. However, the importance of Oligochaeta increased and they were the

The domination structure also depended on hydrological connectivity. The oxbow connected with the main river by melioration ditches was predominated by *Cloëon* sp.-


Table 13. Domination index (D, %) and ranks of benthofauna representatives in the oxbowlakes of consecutive river basins
