**2.2 Selected oxbow-lakes located in lowland river basins of northern Poland**

The research covered 14 oxbow-lakes (four in the Drwęca river valley, five in Łyna and five in the Słupia river valleys)- 8 of them were cut-off, 4 were open and 2 were semi-open reservoirs. Those oxbows differed in habitat conditions and morphometric features (Table 2).

The oxbow-lakes in the Drwęca river valley were formed during regulation works and all of them are cut off from the main river channel (lentic). OLD 1, OLD 2 and OLD 4 are located on the right side of the river while OLD 3 is situated on the left side (Fig. 1). Their length varies from 140 to 1200 m (Table 2). The main shape of those reservoirs is determined by one river bend of sinuosity S around 5.5. In the valley there are also oxbows almost parallel to the riverbed (S=1.5) as well as better developed (S=9.8). Water table area of the studied reservoirs ranges from 0.1 to 1.2 ha and their volume from 5.3 to 16.7 thousand m3 (in relation to the Drwęca water level H=100 cm on the watermark in Rodzone). Morphometric features change over time for each reservoir individually. The flat fragment of the river valley in the vicinity of Bratian does not limit the changes in water table level. The fluctuations in Drwęca water level cause that the volume and area of oxbows can be twofold different.

The oxbow-lakes in the Łyna river valley are located near the Smolajny village and were also formed as a result of regulation works. Three of them (OLŁ 2, OLŁ 3 and OLŁ 5) are connected with the river on both sides (lotic) and OLŁ 1 through one arm (semi-lotic) while OLŁ 4 is cut off from the river (lentic). The length of those reservoirs ranges from 420 to 700 (Table 2) but most of them reach 200-400 m (43% of all objects). Their main shape is determined by one bend of sinuosity S around 3.0. In the valley there are also

Biodiversity of Macroinvertebrates

lotic, LE-lentic)

in Oxbow-Lakes of Early Glacial River Basins in Northern Poland 145

Table 2. Morphological characteristics of the studied oxbow-lakes (LO –lotic, SLO- semi-

oxbows almost parallel to the riverbed (S=1.2) as well as better developed (S=9.5). The area of oxbows varies between 0.3 and 1.5 ha while their volume between 2.2 and 25.3 thousand m<sup>3</sup> (in relation to the Łyna water level H=100 cm on the watermark in Smolajny). Morphometrical features change over time for each reservoir individually. Steep and high banks reduce the range of changes in water table area. The fluctuations in Łyna water level cause that the volume of oxbows can be 4-fold different while the area only 1.9-fold different.

Fig. 1. Localisation of sampling sites

The studied oxbow-lakes in the Słupia river valley are located within the 15 km long river section. Three of them (OLS 1, OLS 2 and OLS 3) are near Kwakowo in the area of "Dolina


Table 2. Morphological characteristics of the studied oxbow-lakes (LO –lotic, SLO- semilotic, LE-lentic)

oxbows almost parallel to the riverbed (S=1.2) as well as better developed (S=9.5). The area of oxbows varies between 0.3 and 1.5 ha while their volume between 2.2 and 25.3

Morphometrical features change over time for each reservoir individually. Steep and high banks reduce the range of changes in water table area. The fluctuations in Łyna water level cause that the volume of oxbows can be 4-fold different while the area only 1.9-fold

The studied oxbow-lakes in the Słupia river valley are located within the 15 km long river section. Three of them (OLS 1, OLS 2 and OLS 3) are near Kwakowo in the area of "Dolina

(in relation to the Łyna water level H=100 cm on the watermark in Smolajny).

thousand m<sup>3</sup>

different.

Fig. 1. Localisation of sampling sites

Biodiversity of Macroinvertebrates

C – constancy of occurrence.

where: D – domination index (in terms of density)

The following classification was applied in this study:

subrecedents (below 1%).

Q5 – very high >30.00%, Q4 – high 15.01-30.00%, Q3 – moderate 10.01-15.00%, Q2 – low 5.01-10.00%, Q1 – very low <5.00%.

where: ni – species abundance

**4. Results and discussion** 

OLŁ5, being a constant species.

1995). As bioindicators they belong to accesoric species.

**4.1.1 Łyna River basin** 

N – total density of taxonomic groups

recedents (1-2%),

in Oxbow-Lakes of Early Glacial River Basins in Northern Poland 147

*Q CD* (3)

The ecological importance index Q was calculated according to the formula (3)

The next parameter used in this investigation was the Shannon diversity index (4):

i=1

**4.1 Qualitative structure of benthofauna in the studied oxbow-lakes** 

S – number of taxonomic groups in the studied samples.

<sup>s</sup> <sup>1</sup> i 21 i

Macrozoobenthos in the oxbows of the Łyna River was represented by 41 taxa belonging to 12 taxonomic groups. Benthofauna was predominated by Diptera of *Chaoborus* genus (Table 3). They were particularly abundant in lentic oxbow-lakes where they constituted over 50% of the total macrozoobenthos abundance and were classified as constants. In OLŁ1 they were accesoric species. That situation indicates a bad condition of the OLŁ1-OLŁ3 ecosystems (Glińska-Lewczuk, 2009). The share of Diptera of *Chaoborus* genus was much lower in the two remaining oxbow-lakes (OLŁ4 and OLŁ5). As for the other taxa, they were relatively abundantly observed in OLŁ1. Oligochaeta, Chironomidae larvae and Bivalvia reached there the status of accesoric species. Oligochaeta also predominated the bottom of

The qualitative composition of benthofauna in the oxbows of the Łyna River indicated their considerable degradation through succession (Kajak, 1988, 2001). As a result the benthic invertebrates are eliminated (Lapmert & Summer, 2007) with stenobionts in particular because their adaptability to changing environmental conditions are very limited (Allan,

The pattern of benthofauna constancy of occurrence was similar in all the oxbow-lakes of the Łyna River. Extreme values were reached in OLŁ2 and OLŁ3, which was connected with the concentration of Diptera larvae represented by *Chironomus* sp. (Fig. 2). The median of benthofauna constancy of occurrence for 80% of the Łyna oxbow-lakes was equal to zero, probably because of the presence of many occasional species. Only the OLŁ1 oxbow lake

differed in terms of median and no predominant species were observed there.

<sup>n</sup> H'= p log p where: p = <sup>N</sup> (4)

Słupi" Landscape Park while OLS 4 and OLS 5 are situated near Słupsk (Fig. 1). OLS 1 and OLS 2 are cut off from the main river channel (lentic), OLS 3 is connected by one arm (semilotic) and the remaining two oxbows (OLS 4 and OLS 5) are connected to the river by pipes or melioration ditches (lotic). The Słupia oxbow-lakes are reservoirs of length ranging from 125 to 500 m (Table 1). Most of them are oxbows of not large width (around 10m). Their main shape is determined by one bend of sinuosity S around 3.0. In the valley there are also oxbows almost parallel to the riverbed (S=1.1) as well as moderately developed (S=5.8). The oxbow areas are similar and range from 0.1 to 0.7 ha while their volume from 0.8 to 5.5 thousand m3 (in relation to the Słupia water level H=100 cm on the watermark in Słupsk). Morphometrical features change over time for each reservoir individually. The fluctuations in Słupia water level cause that the volume and area of oxbows can be 4-fold different.

#### **3. Material and methods**

In order to investigate the biodiversity of macroinvertebrates we chose 12 oxbow-lakes located in the valleys of three lowland rivers- Drwęca, Łyna and Słupia. The oxbows were classified into three types: lotic, semi-lotic and lentic. The sampling was performed in years 2007-2009 through four seasons (winter, spring, summer, autumn). Three sampling sites were locates in each oxbow-lake- in the middle of the reservoir and in both arms. Bottom sediments with macrozoobenthos were sampled using the Ekman's grab sampler (surface 225 cm2) three times at each site and related to the bottom area of 1 m2. The sediments were sieved through a 1 mm mesh size sieve and fixed in 4% formalin. Then, macrozoobenthos was separated from sediments in a laboratory and identified to taxonomic units. Further analysis consisted of the calculations of biocenotic indices.

The constancy of occurrence of the identified taxa was calculated according to the following formula (1):

$$\mathbf{C} = \mathbf{N}\_{\mathbf{a}} / \mathbf{n} \cdot 100\% \tag{1}$$

where: Na – number of samples with a given taxon

n – total number of samples

The index of constancy of occurrence is mostly used as the parameter which indicates the level of connection between a taxon and the environment. According to Tichler the index is classified as follows:

C4 – euconstants (75.1 – 100%),

C3 – constants (50.1 – 75.0%),

C2 – accesoric taxa (25.1 – 50.0%),

C1 – accidental taxa (<25.0%).

The domination index in terms of density is defined by the formula (2):

$$\mathbf{D}\_{\text{A}} = \mathbf{A} / \mathbf{A}\_{\text{av}} \cdot \mathbf{100\%} \tag{2}$$

where: A – average density of a given taxon

 Aav – total average density for the whole reservoir It was classified after Biesiadko & Kowalik (1980) into: eudominants (abundance above 10%), dominants (5.01-10%), subdominants (2.01-5%),

recedents (1-2%), subrecedents (below 1%). The ecological importance index Q was calculated according to the formula (3)

$$Q = \sqrt{C \cdot D} \tag{3}$$

where: D – domination index (in terms of density)

C – constancy of occurrence.

The following classification was applied in this study:

Q5 – very high >30.00%,

Q4 – high 15.01-30.00%,

146 Ecosystems Biodiversity

Słupi" Landscape Park while OLS 4 and OLS 5 are situated near Słupsk (Fig. 1). OLS 1 and OLS 2 are cut off from the main river channel (lentic), OLS 3 is connected by one arm (semilotic) and the remaining two oxbows (OLS 4 and OLS 5) are connected to the river by pipes or melioration ditches (lotic). The Słupia oxbow-lakes are reservoirs of length ranging from 125 to 500 m (Table 1). Most of them are oxbows of not large width (around 10m). Their main shape is determined by one bend of sinuosity S around 3.0. In the valley there are also oxbows almost parallel to the riverbed (S=1.1) as well as moderately developed (S=5.8). The oxbow areas are similar and range from 0.1 to 0.7 ha while their volume from 0.8 to 5.5 thousand m3 (in relation to the Słupia water level H=100 cm on the watermark in Słupsk). Morphometrical features change over time for each reservoir individually. The fluctuations in Słupia water level cause that the volume and area of oxbows can be 4-fold different.

In order to investigate the biodiversity of macroinvertebrates we chose 12 oxbow-lakes located in the valleys of three lowland rivers- Drwęca, Łyna and Słupia. The oxbows were classified into three types: lotic, semi-lotic and lentic. The sampling was performed in years 2007-2009 through four seasons (winter, spring, summer, autumn). Three sampling sites were locates in each oxbow-lake- in the middle of the reservoir and in both arms. Bottom sediments with macrozoobenthos were sampled using the Ekman's grab sampler (surface 225 cm2) three times at each site and related to the bottom area of 1 m2. The sediments were sieved through a 1 mm mesh size sieve and fixed in 4% formalin. Then, macrozoobenthos was separated from sediments in a laboratory and identified to taxonomic units. Further

The constancy of occurrence of the identified taxa was calculated according to the following

The index of constancy of occurrence is mostly used as the parameter which indicates the level of connection between a taxon and the environment. According to Tichler the index is

DA=A/Aav · 100% (2)

C = Na/n · 100% (1)

**3. Material and methods** 

formula (1):

classified as follows:

dominants (5.01-10%), subdominants (2.01-5%),

analysis consisted of the calculations of biocenotic indices.

The domination index in terms of density is defined by the formula (2):

where: Na – number of samples with a given taxon

where: A – average density of a given taxon

eudominants (abundance above 10%),

 Aav – total average density for the whole reservoir It was classified after Biesiadko & Kowalik (1980) into:

n – total number of samples

C4 – euconstants (75.1 – 100%), C3 – constants (50.1 – 75.0%), C2 – accesoric taxa (25.1 – 50.0%), C1 – accidental taxa (<25.0%).

Q3 – moderate 10.01-15.00%,

Q2 – low 5.01-10.00%,

Q1 – very low <5.00%.

The next parameter used in this investigation was the Shannon diversity index (4):

$$\mathbf{H} \mathbf{\dot{"=}} - \sum\_{i=1}^{s} \mathbf{p}\_i \log\_2 \mathbf{p}\_1 \quad \text{where:} \quad \mathbf{p}\_i \mathbf{\dot{=}} \frac{\mathbf{n}\_1}{\mathbf{N}} \tag{4}$$

where: ni – species abundance

N – total density of taxonomic groups

S – number of taxonomic groups in the studied samples.

## **4. Results and discussion**

#### **4.1 Qualitative structure of benthofauna in the studied oxbow-lakes 4.1.1 Łyna River basin**

Macrozoobenthos in the oxbows of the Łyna River was represented by 41 taxa belonging to 12 taxonomic groups. Benthofauna was predominated by Diptera of *Chaoborus* genus (Table 3). They were particularly abundant in lentic oxbow-lakes where they constituted over 50% of the total macrozoobenthos abundance and were classified as constants. In OLŁ1 they were accesoric species. That situation indicates a bad condition of the OLŁ1-OLŁ3 ecosystems (Glińska-Lewczuk, 2009). The share of Diptera of *Chaoborus* genus was much lower in the two remaining oxbow-lakes (OLŁ4 and OLŁ5). As for the other taxa, they were relatively abundantly observed in OLŁ1. Oligochaeta, Chironomidae larvae and Bivalvia reached there the status of accesoric species. Oligochaeta also predominated the bottom of OLŁ5, being a constant species.

The qualitative composition of benthofauna in the oxbows of the Łyna River indicated their considerable degradation through succession (Kajak, 1988, 2001). As a result the benthic invertebrates are eliminated (Lapmert & Summer, 2007) with stenobionts in particular because their adaptability to changing environmental conditions are very limited (Allan, 1995). As bioindicators they belong to accesoric species.

The pattern of benthofauna constancy of occurrence was similar in all the oxbow-lakes of the Łyna River. Extreme values were reached in OLŁ2 and OLŁ3, which was connected with the concentration of Diptera larvae represented by *Chironomus* sp. (Fig. 2). The median of benthofauna constancy of occurrence for 80% of the Łyna oxbow-lakes was equal to zero, probably because of the presence of many occasional species. Only the OLŁ1 oxbow lake differed in terms of median and no predominant species were observed there.

Biodiversity of Macroinvertebrates

**4.1.2 Drwęca River basin** 

(euconstants).

*Chironomus* sp.

pupa

in Oxbow-Lakes of Early Glacial River Basins in Northern Poland 149

Fig. 2. Benthofauna constancy of occurrence (C, %) in the oxbow-lakes of the Łyna River

*Chaoborus* sp. 33.3\* 100.0\*\*\* 66.7\*\* 33.3\* 58.3 **C3**  *Chironomus* sp. 8.3 75.0\*\* 50.0\* 66.7\*\* 50.0 **C2**

Table 4. Benthofauna constancy of occurrence (C, %) in the oxbow-lakes of the Drwęca River

*Erpobdella* sp. 8.3 8.3 25.0 50.0\* 22.9

*Procladius* sp. 8.3 33.3\* 16.7 0.0 14.6

Oligochaeta 0.0 25.0 16.7 0.0 10.4 *Bezzia* sp. 0.0 8.3 8.3 0.0 4.2 *Cloëon* sp. 8.3 0.0 8.3 0.0 4.2 *H. stagnalis* 0.0 0.0 0.0 8.3 2.1 *Glossiphonia* sp. 0.0 0.0 8.3 0.0 2.1 *Sergentia* sp. 0.0 0.0 8.3 0.0 2.1 *A.aquaticus* 8.3 0.0 0.0 0.0 2.1 *Cyrnus* sp. 0.0 8.3 0.0 0.0 2.1

with the classification of consecutive taxa (\* - C2, \*\*- C3, \*\*\* - C4)

Only 13 benthofauna taxa belonging to 7 taxonomic groups were observed in the oxbowlakes of the Drwęca River. The most abundant were *Chaoborus* sp. representatives with very high constancy of occurrence (constant species, Table 4) reaching even 100%

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

0.0 25.0 33.3\* 0.0 14.6

**C1**


Table 3. Constancy of occurrence (C, %) of benthofauna representatives in the oxbow-lakes of the Łyna River with the classification of consecutive taxa (\* - C2, \*\*- C3)

Fig. 2. Benthofauna constancy of occurrence (C, %) in the oxbow-lakes of the Łyna River

#### **4.1.2 Drwęca River basin**

148 Ecosystems Biodiversity

*Chaoborus* sp. 38.9\* 20.8 58.6\*\* 8.3 0.0 25.3 **C2** Oligochaeta 27.8\* 9.0 2.2 8.3 52.9\*\* 20.0

Table 3. Constancy of occurrence (C, %) of benthofauna representatives in the oxbow-lakes

of the Łyna River with the classification of consecutive taxa (\* - C2, \*\*- C3)

*Chironomus* sp. 27.8\* 13.9 10.4 16.7 9.9 15.7 *Procladius* sp*.* 44.4\* 5.7 4.7 8.3 0.0 12.6 *Pisidium* sp. 44.0\* 0.0 0.0 8.3 5.1 11.5 *Erpobdella sp.* 22.2 8.1 2.3 16.7 1.9 10.2 *Viviparus viviparus* L. 22.2 6.7 3.4 8.3 10.5 10.2 *Sialis fuliginosa* Pictet 22.2 9.3 1.7 8.3 0.0 8.3 *Asellus aquaticus* L. 22.2 2.3 0.0 0.0 14.4 7.8 *Valvata pulchella* Studer 0.0 0.0 0.0 25.0 0.0 5.0 *Glossiphonia* sp*.* 22.2 0.0 1.7 0.0 0.9 5.0 *Bezzia* sp. 5.6 5.4 7.5 0.0 0.2 3.7 *Cyrnus* sp. 0.0 7.0 1.7 8.3 0.0 3.4 *Valvata piscinalis* O.F. Müller 16.7 0.0 0.0 0.0 0.0 3.3 *Potamopyrgus antipodarum* Gray 6.0 0.0 0.0 8.3 0.0 2.9 *Anodonta anatina* L*.* 11.1 2.1 0.0 0.0 0.4 2.7 *Lymnaea stagnalis* L. 11.1 0.0 0.0 0.0 1.5 2.5 *Limnephilus* sp*.* 0.0 1.5 1.7 8.3 0.0 2.3 *Sphaerium* sp. 11.0 0.0 0.0 0.0 0.0 2.2 *Dytiscus* sp. 0.0 0.0 0.0 8.3 0.0 1.7 *Notonecta* sp. 0.0 0.0 0.0 8.3 0.0 1.7 *Sergentia* sp. 0.0 0.0 0.0 8.3 0.2 1.7 *Valvata cirstata* O.F.Müller 5.6 0.0 0.0 0.0 0.0 1.1 *Galba truncatula* O.F.Müller 5.6 0.0 0.0 0.0 0.0 1.1 *Anodonta cygnea* L. 5.6 0.0 0.0 0.0 0.0 1.1 *Lepidostoma* sp. 5.6 0.0 0.0 0.0 0.0 1.1 *Ecnomus tenellus* Rambur 5.6 0.0 0.0 0.0 0.0 1.1 *Goera* sp. 5.6 0.0 0.0 0.0 0.0 1.1 *Hydropsyche* sp. 5.6 0.0 0.0 0.0 0.0 1.1 Psychomidae 5.6 0.0 0.0 0.0 0.0 1.1 *Mochlonyx culiciformis* Degeer 5.6 0.0 0.0 0.0 0.0 1.1 *Polypedilum* sp. 5.6 0.0 0.0 0.0 0.0 1.1 Limoniidae 5.6 0.0 0.0 0.0 0.0 1.1 *Helobdella stagnalis* L. 0.0 1.5 1.7 0.0 0.0 0.6 *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 *Epitheca bimaculata* Charpentier 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 *Dreissena polymorpha* Pall. 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 *Cloëon* sp. 0.0 0.0 0.0 0.0 0.2 >0.1

Taxa Oxbow-lakes average classification **OLŁ 1 OLŁ 2 OLŁ<sup>3</sup> OLŁ 4 OLŁ<sup>5</sup>**

**C1** 

Only 13 benthofauna taxa belonging to 7 taxonomic groups were observed in the oxbowlakes of the Drwęca River. The most abundant were *Chaoborus* sp. representatives with very high constancy of occurrence (constant species, Table 4) reaching even 100% (euconstants).


Table 4. Benthofauna constancy of occurrence (C, %) in the oxbow-lakes of the Drwęca River with the classification of consecutive taxa (\* - C2, \*\*- C3, \*\*\* - C4)

Biodiversity of Macroinvertebrates

Tockner et al., 2000).

*Piscola geometra*

*Chironomus* sp.

pupa

in Oxbow-Lakes of Early Glacial River Basins in Northern Poland 151

of *Erpobdella* sp. was caused by the presence of *A*. *aquaticus*, which they feed on. The representatives of Oligochaeta, Hirudinea, Insecta and Bivalvia were accessoric taxa. That taxonomic diversity was favoured by pulsating floods of the Słupia River (Junk et al., 1989;

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

**C2**

**C1**

*A.aquaticus* 58.3\*\* 58.3\*\* 58.3\*\* 67.7\*\* 83.3\*\* 65.2 **C3** *Erpobdella* sp*.* 66.7\*\* 66.7\*\* 66.7\*\* 42.0\* 66.7\*\* 61.8

Oligochaeta 50.0\* 58.3\*\* 50.0\* 16.7 67.0\*\* 48.4

*Procladius* sp*.* 50.0\* 58.3\*\* 58.3\*\* 41.7\* 33.3\* 48.3 *Chironomus* sp. 33.3\* 66.7\*\* 33.3\* 25.0 41.7\* 40.0 *Sphaerium* sp. 41.7\* 33.3\* 33.3\* 41.7\* 16.7 33.3 *Glossiphonia* sp*.* 33.3\* 8.3 58.3\*\* 0.0 50.0\* 30.0 *S.fuliginosa* 8.3 33.3\* 33.3\* 58.0\*\* 8.3 28.2 *Limnephilus* sp*.* 33.3\* 8.3 16.7 42.0\* 33.3\* 26.7 *Cloëon* sp. 50.0\* 25.0 8.3 8.3 25.0 23.3

*H.stagnalis* 25.0 25.0 58.3\*\* 0.0 0.0 21.7 *Sergentia* sp. 0.0 25.0 25.0 16.7 0.0 13.3 *Corixa* sp. 16.7 8.3 16.7 8.3 16.7 13.3 *Gammarus* sp. 0.0 0.0 16.7 33.3\* 8.3 11.7 *Chaoborus* sp. 8.3 0.0 8.3 25.0 8.3 10.0 *Lestes* sp. 25.0 0.0 0.0 0.0 25.0 10.0 *A.grandis* 8.3 0.0 0.0 8.3 25.0 8.3 *Dytiscus* sp. 16.7 8.3 8.3 0.0 8.3 8.3 *V.viviparus* 16.7 8.3 0.0 0.0 8.3 6.7 *Cyrnus* sp. 0.0 0.0 0.0 17.0 8.3 5.1

L. 0.0 0.0 0.0 0.0 25.0 5.0 *Bezzia* sp. 0.0 8.3 0.0 0.0 8.3 3.3

*Platambus* sp. 0.0 8.3 0.0 8.3 0.0 3.3 *E.bimaculata* 0.0 0.0 0.0 8.3 0.0 1.7 *Ephemera* sp. 0.0 0.0 8.3 0.0 0.0 1.7 *Notonecta* sp. 0.0 0.0 0.0 0.0 8.3 1.7 *Libellula* sp. 0.0 0.0 0.0 8.3 0.0 1.7 *Hydrovatus* sp. 0.0 0.0 0.0 8.3 0.0 1.7 *Nepa cinera* L. 0.0 0.0 0.0 8.3 0.0 1.7 *Tabanus* sp*.* 8.3 0.0 0.0 0.0 0.0 1.7

with the classification of consecutive taxa (\* - C2, \*\*- C3, \*\*\* - C4)

occasional species present only during some seasons.

0.0 0.0 8.3 8.3 0.0 3.3

Table 5. Benthofauna constancy of occurrence (C, %) in the oxbow-lakes of the Słupia River

The pattern of constancy of occurrence was similar in all the oxbow-lakes of the Słupia River (Fig. 4). The medians were at the level of 10% which indicated that there were many

The second abundant were the larvae of *Chironomus* sp., being between accesoric and constant species. Considerable number of taxa were occasional species which occurred in some oxbow-lakes with higher abundance. Among accesoric species that situation particularly concerned leeches *Erpobdella* sp., which in OLD4 were present in half of the samples. That situation was caused by the abundance of Diptera larvae (Turoboyski, 1979). In OLD2 and OLD3 apart from the larvae of *Choborus* sp. and *Chironomus* sp., equally frequent were predatory *Procladius* sp. and chrysalis of *Chironomus* sp.

The oxbow-lakes of the Drwęca River were the most degraded ecosystems, which functioning depended on periodical floods (Tockner et al., 1999). The domination of migrating species indicated oxygen depletion in the bottom zone of those reservoirs. That suggests the need of reconnecting those oxbows to the river (Bornette et al., 1998; Gallardo et al., 2007; Ward & Stanford, 1995; Ward et al., 2002).

The constancy of occurrence of benthofauna varied between 0 and 50% (Me<10%). The only oxbow with more favourable conditions to invertebrates was OLD3 (Fig. 3). There was only one taxon which was the most often identified in each reservoir (*Chaoborus* sp.) and extreme values of constancy of occurrence were noted in OLD4.

Fig. 3. Benthofauna constancy of occurrence (C, %) in the oxbow-lakes of the Drwęca River

#### **4.1.3 Słupia River basin**

31 benthofauna representatives belonging to 13 taxonomic groups were identified in the oxbow-lakes of the Słupia River (Table 4). The highest diversity of macrozoobenthos was noted in those reservoirs, probably due to regular spates connected with the functioning of hydro-power stations (Obolewski, 2011a).

The most frequently present were Crustaceans represented by *A*. *aquaticus* as well as Hirudinea represented by *Erpobdella* sp. (constant species). Both in the Łyna and Drwęca river basins those taxa were occasionally observed. The presence of *A*. *aquaticus* indicates βmezosaprobic waters polluted with organic substances (Turoboyski, 1979). High abundance

The second abundant were the larvae of *Chironomus* sp., being between accesoric and constant species. Considerable number of taxa were occasional species which occurred in some oxbow-lakes with higher abundance. Among accesoric species that situation particularly concerned leeches *Erpobdella* sp., which in OLD4 were present in half of the samples. That situation was caused by the abundance of Diptera larvae (Turoboyski, 1979). In OLD2 and OLD3 apart from the larvae of *Choborus* sp. and *Chironomus* sp., equally

The oxbow-lakes of the Drwęca River were the most degraded ecosystems, which functioning depended on periodical floods (Tockner et al., 1999). The domination of migrating species indicated oxygen depletion in the bottom zone of those reservoirs. That suggests the need of reconnecting those oxbows to the river (Bornette et al., 1998; Gallardo

The constancy of occurrence of benthofauna varied between 0 and 50% (Me<10%). The only oxbow with more favourable conditions to invertebrates was OLD3 (Fig. 3). There was only one taxon which was the most often identified in each reservoir (*Chaoborus* sp.) and extreme

Fig. 3. Benthofauna constancy of occurrence (C, %) in the oxbow-lakes of the Drwęca River

31 benthofauna representatives belonging to 13 taxonomic groups were identified in the oxbow-lakes of the Słupia River (Table 4). The highest diversity of macrozoobenthos was noted in those reservoirs, probably due to regular spates connected with the functioning of

The most frequently present were Crustaceans represented by *A*. *aquaticus* as well as Hirudinea represented by *Erpobdella* sp. (constant species). Both in the Łyna and Drwęca river basins those taxa were occasionally observed. The presence of *A*. *aquaticus* indicates βmezosaprobic waters polluted with organic substances (Turoboyski, 1979). High abundance

frequent were predatory *Procladius* sp. and chrysalis of *Chironomus* sp.

et al., 2007; Ward & Stanford, 1995; Ward et al., 2002).

values of constancy of occurrence were noted in OLD4.

**4.1.3 Słupia River basin** 

hydro-power stations (Obolewski, 2011a).

of *Erpobdella* sp. was caused by the presence of *A*. *aquaticus*, which they feed on. The representatives of Oligochaeta, Hirudinea, Insecta and Bivalvia were accessoric taxa. That taxonomic diversity was favoured by pulsating floods of the Słupia River (Junk et al., 1989; Tockner et al., 2000).


Table 5. Benthofauna constancy of occurrence (C, %) in the oxbow-lakes of the Słupia River with the classification of consecutive taxa (\* - C2, \*\*- C3, \*\*\* - C4)

The pattern of constancy of occurrence was similar in all the oxbow-lakes of the Słupia River (Fig. 4). The medians were at the level of 10% which indicated that there were many occasional species present only during some seasons.

Biodiversity of Macroinvertebrates

Taxa

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

representatives, exceeded 10%,

oxygenation (Kajak, 2001).

**4.2.2 Semi-lotic oxbow-lakes** 

in Oxbow-Lakes of Early Glacial River Basins in Northern Poland 153

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.

> **OLD 3**

*Chironomus* sp. 8.3 75.0 50.0 **66.7** 16.7 33.3 **66.7** 45.2 *Chaoborus* sp. **33.3 100.0 66.7** 33.3 8.3 8.3 0.0 35.7 *Erpobdella* sp. 8.3 8.3 25.0 50.0 16.7 **66.7 66.7** 34.5 Table 7. Constancy of occurrence with classification of the most frequent benthofauna taxa

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

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

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

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.* 

equal to 40 which was reflected by the qualitative structure of benthofauna (Table 8)

**OLD** 

Łyna River Basin

Słupia River Basin

Mean Oxbow-lakes

**<sup>4</sup>OLŁ 4 OLS 1 OLS 2** 

Drwęca River Basin

**OLD 2** 

**OLD 1** 

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 invertebrate representatives reached the status higher than accesoric taxon.


Table 6. Benthofauna constancy of occurrence (C, %) and their ranks in the studied river basins
