**5. Conclusions**

166 Ecosystems Biodiversity

*Chironomus* sp. 22.0 (3) 35.7 (2) 22.0 (3) *Chaoborus* sp. 31.0 (1) 42.9 (1) 2.1 (16) Oligochaeta 22.5 (2) 4.0 (6) 24.5 (2) *A.aquaticus* 6.4 (7) 1.6 (11) 40.3 (1) *Procladius* sp. 4.3 (9) 11.8 (3) 13.5 (4) *Erpobdella* sp. 7.8 (5) 11.7 (4) 12.6 (6) *Cloëon* sp. 0.3 (31) 2.7 (8) 12.9 (5)

Table 23. Index of ecological importance (Q, %) for benthofauna representatives in the

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

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

studied river basins and their ranks

**main river** 

main river course

**Łyna Drwęca Słupia**

Taxa River basin

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 habitats favour benthofauna survival and further recolonization.

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 higher benthofauna diversity.
