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

#### **4.4.1 Lentic oxbow-lakes**

Limited hydrological connectivity between cut-off oxbow-lakes and the river causes that those reservoirs are predominated by taxa belonging to eurybionts (Kajak, 2001). The main components of benthofauna in the studied lentic oxbows were Diptera larvae (*Chironomus* sp. and *Chaoborus* sp.), Oligochaeta and Isopoda (*A. aquaticus*) *–* 8 taxa in total. They formed the group of eudominants and constituted 71% of benthofauna abundance (Table 14). They were accompanied by predatory larvae of *Procladius* sp. as well as leeches *Erpobdella* sp. (dominants). As for the other benthofauna representatives, the presence of *Cloëon* sp. *Limnephilus* sp. should be noticed.

Benthofauna structure in lentic oxbow-lakes depended on the specific features of consecutive river basins. In the Drwęca River Basin the inflow of fresh water occurs during floods and therefore benthofauna was predominated by taxa resistant to low oxygen concentration in water- some of the Diptera larvae (Mikulski, 1974; Stańczykowska, 1986). The cut-off oxbows of Łyna and Słupia are additionally refreshed by regular, twenty-four


Table 14. Domination index of the most import ant benthofauna representatives (mean D>2 %) in lentic oxbow-lakes with classification (ED – eudominants, D – dominants, SD – subdominants)

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 is higher with the domination of Oligochaeta and Crustacea.

## **4.4.2 Semi-lotic oxbow-lakes**

160 Ecosystems Biodiversity

*Chaoborus* sp. 29.8 (1) 38.7 (1) (0.6) 17 *Chironomus* sp. 10.2 (3) 27.6 (2) 17.8 (2) Oligochaeta 25.8 (2) 1.7 (8) 14.0 (3) *A.aquaticus* 3.8 (5) 3.6 (5) 26.6 (1) *Procladius* sp. 2.2 (10) 10.2 (3) 4.0 (5) *Erpobdella* sp. 2.8 (6) 9.3 (4) 3.1 (7) *Cloëon* sp. 0.1 (36) 1.9 (7) 11.3 (4) *Glossiphonia* sp*.* 0.7 (15) 2.2 (6) 1.4 (13) *Bezzia* sp. 2.6 (8) 0.8 (11) 1.0 (15) *Sergentia* sp. 0.5 (19) 1.5 (9) 2.0 (12)

**Łyna Drwęca Słupia** 

Taxa River basin

Table 13. Domination index (D, %) and ranks of benthofauna representatives in the oxbow-

Limited hydrological connectivity between cut-off oxbow-lakes and the river causes that those reservoirs are predominated by taxa belonging to eurybionts (Kajak, 2001). The main components of benthofauna in the studied lentic oxbows were Diptera larvae (*Chironomus* sp. and *Chaoborus* sp.), Oligochaeta and Isopoda (*A. aquaticus*) *–* 8 taxa in total. They formed the group of eudominants and constituted 71% of benthofauna abundance (Table 14). They were accompanied by predatory larvae of *Procladius* sp. as well as leeches *Erpobdella* sp. (dominants). As for the other benthofauna representatives, the presence of *Cloëon* sp.

Benthofauna structure in lentic oxbow-lakes depended on the specific features of consecutive river basins. In the Drwęca River Basin the inflow of fresh water occurs during floods and therefore benthofauna was predominated by taxa resistant to low oxygen concentration in water- some of the Diptera larvae (Mikulski, 1974; Stańczykowska, 1986). The cut-off oxbows of Łyna and Słupia are additionally refreshed by regular, twenty-four

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

*Chironomus* sp. 14.5 **46.3 30.2** 19.4 14.7 **49.2** 11.6 26.6

**ED** *Chaoborus* sp. **37.3** 26.5 22.7 **68.3** 0.3 0.5 0.0 22.2 Oligochaeta 0.0 2.7 3.9 0.0 **58.3** 2.4 19.9 12.5 *A.aquaticus* 14.5 0.0 0.0 0.0 0.0 27.1 **29.0** 10.1

*Procladius* sp. 4.8 17.1 18.7 0.0 0.7 4.6 7.6 7.6 **D** *Erpobdella* sp. 24.1 1.4 2.0 9.8 0.8 1.3 1.8 5.9

*Cloëon* sp. 4.8 0.0 2.6 0.0 0.0 3.5 7.8 2.7 **SD** *Limnephilus* sp. 0.0 0.0 0.0 0.0 10.8 1.2 2.0 2.0

Table 14. Domination index of the most import ant benthofauna representatives (mean D>2 %) in lentic oxbow-lakes with classification (ED – eudominants, D – dominants, SD –

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

lakes of consecutive river basins

**connection to the main river 4.4.1 Lentic oxbow-lakes** 

*Limnephilus* sp. should be noticed.

subdominants)

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 Hirudinea.

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 βmezosaprobic waters (Turoboyski, 1979).


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)

#### **4.4.3 Lotic oxbow-lakes**

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 most abundant in OLŁ5 (Table 16).

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

Biodiversity of Macroinvertebrates

low)

in Oxbow-Lakes of Early Glacial River Basins in Northern Poland 163

*Chaoborus* sp. **51.8 34.8 66.3** 1.9 0.0 31.0 Q5 Oligochaeta 13.4 19.3 6.0 **25.4 48.5** 22.5 Q4 *Chironomus* sp. 8.0 32.3 30.4 18.0 21.0 22.0

Table 18. Index of ecological importance (Q, %) for benthofauna representatives in the oxbow-lakes of the Łyna River and its classification (Q5 – very high, Q4 – high, Q1 – very

*V. viviparus* 10.9 13.0 7.7 3.9 10.8 9.3

*Erpobdella sp.* 5.8 18.3 6.0 4.3 4.6 7.8 *Bezzia* sp. 0.5 13.4 17.7 0.0 1.4 6.6 *A. aquaticus* 7.8 6.2 0.0 0.0 17.9 6.4 *Pisidium* sp. 11.8 0.0 0.0 1.9 15.0 5.7 *Procladius* sp*.* 0.7 11.9 6.0 2.7 0.0 4.3 *S. fuliginosa* 4.2 8.8 3.5 1.9 0.0 3.7 *Limnephilus* sp*.* 0.0 3.6 3.5 10.9 0.0 3.6 *Cyrnus* sp. 0.0 10.8 3.5 3.3 0.0 3.5 *A. anatina* 1.4 7.2 0.0 0.0 2.9 2.3 Phryganaidae 6.8 0.0 0.0 0.0 4.1 2.2 *Glossiphonia* sp*.* 0.0 0.0 4.9 0.0 3.2 1.6 *M. culiciformis* 7.5 0.0 0.0 0.0 0.0 1.5 *Lestes* sp. 0.5 7.2 0.0 0.0 0.0 1.5 *V.piscinalis* 2.1 0.0 0.0 5.6 0.0 1.5 *H. stagnalis* 0.0 3.6 3.5 0.0 0.0 1.4 *Notonecta* sp. 0.0 0.0 0.0 6.4 0.0 1.3 *L. stagnalis* 2.3 0.0 0.0 0.0 4.1 1.3 *Dytiscus* sp. 0.0 0.0 0.0 6.7 0.0 1.3 *Sergentia* sp. 0.0 0.0 0.0 5.1 1.4 1.3 *Chironomus* sp. pupa 0.0 0.0 6.0 0.0 0.0 1.2 *E. bimaculata* 0.0 3.6 0.0 0.0 0.0 0.7 *D. polymorpha* 0.0 3.6 0.0 0.0 0.0 0.7 *Caenis sp.* 0.0 3.6 0.0 0.0 0.0 0.7 *P. antipodarum* 0.5 0.0 0.0 1.9 0.0 0.5 *Polypedilum* sp. 2.2 0.0 0.0 0.0 0.0 0.4 *Goera* sp. 2.0 0.0 0.0 0.0 0.0 0.4 *Sphaerium* sp. 1.6 0.0 0.0 0.0 0.0 0.3 *Cloëon* sp. 0.0 0.0 0.0 0.0 1.4 0.3 *V. pulchella* 0.0 0.0 0.0 0.9 0.0 0.2 *Lepidostoma* sp. 0.8 0.0 0.0 0.0 0.0 0.2 Psychomidae 0.8 0.0 0.0 0.0 0.0 0.2 *G. truncatula* 1.0 0.0 0.0 0.0 0.0 0.2 *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* 0.7 0.0 0.0 0.0 0.0 0.1 Limoniidae 0.5 0.0 0.0 0.0 0.0 0.1 *Hydropsyche* sp. 0.1 0.0 0.0 0.0 0.0 <0.1

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

Q1


Table 16. Domination index of the most import ant benthofauna representatives (mean D>2 %) in lotic oxbow-lakes with classification (ED – eudominants, D – dominants, SD – subdominants)

#### **4.5 Assessment of benthofauna diversity 4.5.1 Diversity of benthofauna in oxbow-lakes of the studied river basins**

#### **Łyna River Basin**

The Shannon diversity index for the Łyna River Basins varied between 0.148 and 0.438 with average value equal to 0.293 (Table 17). Considerable standard deviations indicate the instability of habitat conditions (Ward, 1998). The highest average value of H' was observed in semi-lotic OLŁ2 and the lowest in cut-off OLŁ5.


Table 17. Average values of Shannon diversity index (H'± SD) for benthofauna in the oxbow-lakes of the Łyna River

The index of ecological importance reached the highest values for *Chaoborus* sp. (Table 18). Oligochaeta and *Chironomus* sp. were also important components of benthofauna in the Łyna River Basin. The ecological importance of many taxa was low.

#### **Drwęca River Basin**

The Shannon diversity index for the oxbow-lakes in the Drwęca River Basin varied between 0.06 and 0.29 (0.152 on average) (Table 19). High standard variations, often exceeding mean values indicate unstable habitat conditions (Ward, 1998). The highest H' value was observed in OLD2 and the lowest in OLD1.

The analysis of ecological importance indicated *Chaoborus* sp. and *Chironomus* sp. as the most important taxa (Table 20). Among other taxa, the role of *Procladius* sp. and *Erpobdella* sp. should be noticed.


Oligochaeta 9.0 2.2 **52.9** 7.9 13.5 17.1

*Chironomus* sp. 13.9 10.4 9.9 23.3 0.7 11.6 *A.aquaticus* 2.3 0.0 14.4 **29.3** 11.1 11.4

*Erpobdella sp.* 8.1 2.3 1.9 1.2 8.6 4.4

*V.viviparus* 6.7 3.4 10.5 0.0 0.4 4.2 *Gammarus* sp. 0.0 0.0 0 18.1 0.4 3.7 *Bezzia* sp. 5.4 7.5 0.2 0.0 3.9 3.4 *Procladius* sp*.* 5.7 4.7 0 4.2 1.4 3.2 *S.fuliginosa* 9.3 1.7 0 2.4 0.4 2.8 *Cyrnus* sp. 7.0 1.7 0 0.8 0.4 2.0

**ED** *Chaoborus* sp. **20.8 58.6** 0.0 1.0 0.4 16.2

*Cloëon* sp. 0.0 0.0 0.2 0.8 **44.1** 9.0 **D** 

Table 16. Domination index of the most import ant benthofauna representatives (mean D>2

The Shannon diversity index for the Łyna River Basins varied between 0.148 and 0.438 with average value equal to 0.293 (Table 17). Considerable standard deviations indicate the instability of habitat conditions (Ward, 1998). The highest average value of H' was observed

> Oxbow-lakes **OLŁ 1 OLŁ 2 OLŁ 3 OLŁ 4 OLŁ 5** 0.388±0.220 **0.438**±0.246 0.316±0.209 0.174±0.234 0.148±0.147

The index of ecological importance reached the highest values for *Chaoborus* sp. (Table 18). Oligochaeta and *Chironomus* sp. were also important components of benthofauna in the

The Shannon diversity index for the oxbow-lakes in the Drwęca River Basin varied between 0.06 and 0.29 (0.152 on average) (Table 19). High standard variations, often exceeding mean values indicate unstable habitat conditions (Ward, 1998). The highest H' value was observed

The analysis of ecological importance indicated *Chaoborus* sp. and *Chironomus* sp. as the most important taxa (Table 20). Among other taxa, the role of *Procladius* sp. and *Erpobdella*

Table 17. Average values of Shannon diversity index (H'± SD) for benthofauna in the

Łyna River Basin. The ecological importance of many taxa was low.

%) in lotic oxbow-lakes with classification (ED – eudominants, D – dominants, SD –

**4.5.1 Diversity of benthofauna in oxbow-lakes of the studied river basins** 

subdominants)

**Łyna River Basin** 

oxbow-lakes of the Łyna River

in OLD2 and the lowest in OLD1.

**Drwęca River Basin** 

sp. should be noticed.

**4.5 Assessment of benthofauna diversity** 

in semi-lotic OLŁ2 and the lowest in cut-off OLŁ5.

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

**SD** 


Table 18. Index of ecological importance (Q, %) for benthofauna representatives in the oxbow-lakes of the Łyna River and its classification (Q5 – very high, Q4 – high, Q1 – very low)

Biodiversity of Macroinvertebrates

moderate, Q2 –low , Q1 – very low)

basins but very low for Słupia oxbow-lakes.

in Oxbow-Lakes of Early Glacial River Basins in Northern Poland 165

*A.aquaticus* 39.8 **41.1 46.0 44.2** 30.4 40.3 Q5

Oligochaeta 11.0 34.1 36.0 11.5 30.0 24.5 Q4 *Chironomus* sp. **40.5** 27.8 11.9 24.1 5.4 22.0

*Cloëon* sp. 13.2 14.0 1.8 2.5 **33.2** 12.9 Q3

*Procladius* sp*.* 15.1 21.0 11.1 13.3 6.8 13.5

*Erpobdella* sp*.* 9.4 10.9 12.1 6.7 24.0 12.6 *Sphaerium* sp. 9.8 11.3 7.1 12.3 3.6 8.8

*S.fuliginosa* 4.1 12.4 11.3 11.5 1.8 8.2 *Limnephilus* sp*.* 6.5 4.1 9.0 6.7 6.0 6.4 *Glossiphonia* sp*.* 5.0 2.9 13.6 0.0 10.7 6.4 *Gammarus* sp. 0.0 0.0 3.6 24.9 2.6 6.2 *Corixa* sp. 3.5 2.9 2.9 2.5 13.8 5.1 *Sergentia* sp. 0.0 6.5 12.0 6.2 0.0 4.9

*H.stagnalis* 4.1 5.0 12.5 0.0 0.0 4.3 *V.viviparus* 3.5 5.8 0.0 0.0 1.8 2.2 *Chaoborus* sp. 2.0 0.0 1.8 5.0 1.8 2.1 *Dytiscus* sp. 2.9 2.9 1.8 0.0 2.6 2.0 *A. grandis* 2.0 0.0 0.0 2.5 4.4 1.8 *Lestes* sp. 4.6 0.0 0.0 0.0 4.4 1.8 *Bezzia* sp. 0.0 2.9 0.0 0.0 5.7 1.7 *Platambus* sp. 0.0 2.9 3.1 0.0 1.8 1.6 *Cyrnus* sp. 0.0 0.0 0.0 3.6 1.8 1.1 *P. geometra* 0.0 0.0 1.8 0.0 3.6 1.1 *Chironomus* sp. pupa 0.0 0.0 1.8 2.5 0.0 0.9 *Hydrovatus* sp. 0.0 0.0 4.0 0.0 0.0 0.8 *Tabanus* sp*.* 2.9 0.0 0.0 0.0 0.0 0.6 *E.bimaculata* 0.0 0.0 0.0 2.5 0.0 0.5 *Ephemera* sp. 0.0 0.0 2.5 0.0 0.0 0.5 *Libellula* sp. 0.0 0.0 0.0 2.5 0.0 0.5 *Notonecta* sp. 0.0 0.0 0.0 0.0 1.8 0.4 *N. cinera* 0.0 0.0 1.8 0.0 0.0 0.4

Table 22. Index of ecological importance (Q, %) for benthofauna representatives in the oxbow-lakes of the Słupia River and its classification (Q5 – very high, Q4 – high, Q3 –

The comparison of average values of ecological importance index Q between river basins with the classification of consecutive taxa shows, that the most important role was played by *Chironomus* sp. larvae (Table 23). Its average ecological importance reached Qav.=27%, similarly to *Chaoborus* sp. larvae (Qav.=25%). Oligochaeta and *A. aquaticus* also revealed considerable ecological importance (Qav. equal to 17% and 16%, respectively). It is interesting, that the importance of *Chaoborus* sp. is very high in the Łyna and Drwęca river

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

Q2

Q1


Table 19. Average values of Shannon diversity index (H'± SD) for benthofauna in the oxbow-lakes of the Drwęca River


Table 20. Index of ecological importance (Q, %) for benthofauna representatives in the oxbow-lakes of the Drwęca River and its classification (Q5 – very high, Q3 – moderate, Q1 – very low)

#### **Słupia River Basin**

Benthofauna diversity in the Słupia River Basins was the highest among the studied basins (0.412 on average) (Table 21). Low values of standard deviations indicate more stable habitat conditions which favour taxonomic diversity (Ward, 1998; Gallardo et al., 2008; Obolewski, 2011a; Obolewski & Glińska-Lewczuk, 2011).


Table 21. Average values of Shannon diversity index (H'± SD) for benthofauna in the oxbow-lakes of the Słupia River

The analysis of Q index indicated *A. aquaticus* as the most important component of benthofauna in the Słupia River Basin. High Q values were reached by Oligochaeta and *Chironomus* sp. while moderate by *Procladius* sp., *Cloëon* sp. and *Erpobdella* sp. (Table 22). Such ecological structure, with the presence of oligotrophic water bioindicators (*Cloëon* sp., *Gammarus* sp., *Ephemera* sp.) is a sign of good environmental conditions (Lapmert & Summer, 2007; Turoboyski, 1979).


Oxbow-lakes **OLD 1 OLD 2 OLD 3 OLD 4**  0.056±0.157 **0.293**±0.164 0.166±0.216 0.092±0.131

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

Q1

Table 19. Average values of Shannon diversity index (H'± SD) for benthofauna in the

*Chaoborus* sp. **35.3** 49.8 **38.9 47.7** 42.9 Q5 *Chironomus* sp. 11.0 **57.0 38.9** 35.9 35.7

*Procladius* sp. 6.3 23.1 17.6 0.0 11.8 Q3 *Erpobdella* sp. 14.2 3.3 7.0 22.2 11.7

Table 20. Index of ecological importance (Q, %) for benthofauna representatives in the oxbow-lakes of the Drwęca River and its classification (Q5 – very high, Q3 – moderate, Q1 –

Benthofauna diversity in the Słupia River Basins was the highest among the studied basins (0.412 on average) (Table 21). Low values of standard deviations indicate more stable habitat conditions which favour taxonomic diversity (Ward, 1998; Gallardo et al., 2008; Obolewski,

> Oxbow-lakes **OLS 1 OLS 2 OLS 3 OLS 4 OLS 5** 0.354±0.163 0.377±0.291 **0.488**±0.207 0.377±0.232 0.464±0.180

The analysis of Q index indicated *A. aquaticus* as the most important component of benthofauna in the Słupia River Basin. High Q values were reached by Oligochaeta and *Chironomus* sp. while moderate by *Procladius* sp., *Cloëon* sp. and *Erpobdella* sp. (Table 22). Such ecological structure, with the presence of oligotrophic water bioindicators (*Cloëon* sp., *Gammarus* sp., *Ephemera* sp.) is a sign of good environmental conditions (Lapmert &

Table 21. Average values of Shannon diversity index (H'± SD) for benthofauna in the

*Chironomus* sp. pupa 0.0 6.5 10.8 0.0 4.3

Oligochaeta 0 8.0 8.1 0 4.0 *Glossiphonia* sp. 0.0 0.0 8.5 0.0 2.1 *Cloëon* sp. 6.3 0.0 4.7 0.0 2.7 *Sergentia* sp. 0.0 0.0 7.1 0.0 1.8 *Bezzia* sp. 0.0 3.3 3.8 0.0 1.8 *A.aquaticus* 6.3 0 0 0 1.6 *Cyrnus* sp. 0.0 4.6 0.0 0.0 1.2 *H. stagnalis* 0.0 0.0 0.0 4.5 1.1

oxbow-lakes of the Drwęca River

very low)

**Słupia River Basin** 

2011a; Obolewski & Glińska-Lewczuk, 2011).

oxbow-lakes of the Słupia River

Summer, 2007; Turoboyski, 1979).


Table 22. Index of ecological importance (Q, %) for benthofauna representatives in the oxbow-lakes of the Słupia River and its classification (Q5 – very high, Q4 – high, Q3 – moderate, Q2 –low , Q1 – very low)

The comparison of average values of ecological importance index Q between river basins with the classification of consecutive taxa shows, that the most important role was played by *Chironomus* sp. larvae (Table 23). Its average ecological importance reached Qav.=27%, similarly to *Chaoborus* sp. larvae (Qav.=25%). Oligochaeta and *A. aquaticus* also revealed considerable ecological importance (Qav. equal to 17% and 16%, respectively). It is interesting, that the importance of *Chaoborus* sp. is very high in the Łyna and Drwęca river basins but very low for Słupia oxbow-lakes.

Biodiversity of Macroinvertebrates

**5. Conclusions** 

higher benthofauna diversity.

**6. Acknowledgments** 

(Gallardo et al., 2008; Obolewski, 2011a and b).

in Oxbow-Lakes of Early Glacial River Basins in Northern Poland 167

semi-lotic oxbow-lakes are optimal habitat for macrozoobenthos in terms of diversity

The analysis of ecological importance (Q) for consecutive hydrodynamic types of oxbowlakes indicates that it evens out for higher hydrological connectivity (Table 24). A unimodal pattern with the domination of *Chironomus* sp. was observed in cut-off reservoirs while in semi-lotic oxbows three taxa were important (*Chaoborus* sp., *A. aquaticus* and Oligochaeta). Lower frequency of occurrence for *Chironomus* sp. there is caused by fish foraging (Penczak et al., 2007). In lotic oxbow-lakes the ecological importance of main benthofauna components was at similar level. Four taxa reached high values (Oligochaeta, *Chironomus* sp., *Chaoborus* sp. and *A. aquaticus)* while only for *Erpobdella* the Q level was moderate. The role of Oligochaeta increases with hydrological connectivity, opposite to *Chaoborus* sp. due to washing out by water movement (Stańczykowska, 1986) and ichthyofauna pressure.

Taxa Oxbow-lakes

oxbow-lakes with different types of connection to the main river course

habitats favour benthofauna survival and further recolonization.

*Chaoborus* sp. 28.18 **28.65** 23.20 *Chironomus* sp. **34.64** 10.05 24.65 Oligochaeta 16.78 25.14 **25.24** *A.aquaticus* 13.40 28.20 20.83 *Erpobdella* sp*.* 14.26 8.43 13.28 Table 24. Index of ecological importance (Qav.>10.01%) for benthofauna representatives in

This study confirms that hydrological connectivity between the main river and its wetlands strongly influences benthofauna composition and abundance. Limited hydrological connection as well as hydrotechnical buildings on rivers of moderate slope cause permanent hydrological changes. As a result many valuable aquatic ecosystems may be degraded because water exchange is sporadic. Recreation of hydrological connectivity in river basins favours its protection and revitalization as well as protection against rapid floods. Reconstruction of biodiversity after such events is initiated in wetlands where diverse

Regardless of the distinguished hydrodynamic types of oxbow-lakes those reservoirs are important ecological centres (so called "hot spots") within a river valley or even a region which form various habitats for many fauna and flora species. Relationships between the main water-course and the rest of valley include production, decomposition and consumption which depend on periodical floods and oscillations of water table level. According to the theory of Junk et al. the alternating flood and low water stage periods increase the decomposition and nutrient circulation which trigger biological diversity and productivity. Those processes may be reinforced by intensive flood pulses caused by hydroelectric power stations and therefore river valleys with hydrotechnical buildings show

I would like to thank Katarzyna Glińska-Lewczuk, Szymon Kobus and Paweł Burand for their help in morphometric studies (Dept. of Land Reclamation and Environmental

**lentic semi-lotic lotic**



#### **4.5.2 Diversity of benthofauna in oxbow-lakes with various type of connection to the main river**

Benthofauna diversity changed depending on hydrological connectivity (Fig. 8). The Shannon index for closed oxbow-lakes did not exceed 0.7 (H'av.=0.217±0.194). In case of semi-lotic reservoirs H' values ranged from 0.8 in the Łyna River to 1.1 in the Słupia River Basin with 0.438±0.214 on average. Higher diversity comparing to closed oxbow is caused by more often fresh water inflow (Amoros & Bornette, 2002; Obolewski et al., 2009). Full hydrological connectivity did not increase considerably benthofauna diversity. In open oxbow-lakes the Shannon index reached 0.349±0.203 on average, which is 1.2-fold less than in semi-lotic oxbows and 1.6-fold higher than in lentic reservoirs. One can conclude, that

Fig. 8. Shannon diversity index (H') in oxbow-lakes with different types of connection to the main river course

semi-lotic oxbow-lakes are optimal habitat for macrozoobenthos in terms of diversity (Gallardo et al., 2008; Obolewski, 2011a and b).

The analysis of ecological importance (Q) for consecutive hydrodynamic types of oxbowlakes indicates that it evens out for higher hydrological connectivity (Table 24). A unimodal pattern with the domination of *Chironomus* sp. was observed in cut-off reservoirs while in semi-lotic oxbows three taxa were important (*Chaoborus* sp., *A. aquaticus* and Oligochaeta). Lower frequency of occurrence for *Chironomus* sp. there is caused by fish foraging (Penczak et al., 2007). In lotic oxbow-lakes the ecological importance of main benthofauna components was at similar level. Four taxa reached high values (Oligochaeta, *Chironomus* sp., *Chaoborus* sp. and *A. aquaticus)* while only for *Erpobdella* the Q level was moderate. The role of Oligochaeta increases with hydrological connectivity, opposite to *Chaoborus* sp. due to washing out by water movement (Stańczykowska, 1986) and ichthyofauna pressure.


Table 24. Index of ecological importance (Qav.>10.01%) for benthofauna representatives in oxbow-lakes with different types of connection to the main river course
