The effect of water temperature regime on benthic macroinvertebrates : A contribution to the ecological assessment of rivers
The natural water temperature regime represents a key factor of the abiotic environment in rivers. Today, this factor is often modified by anthropogenic influence. Thermal changes were shown to have an effect on benthic invertebrates in various laboratory studies. In spite of these conditions, until now temperature has rarely been evaluated in detail in surveys on the ecological quality in rivers. Furthermore, the question whether temperature-sensitive taxa could function as indicators for thermal conditions has not yet been studied. In the first chapter of the present thesis the thermal regime of two river-types was characterised with the objective to test if significant temperature parameters for a thermal assessment can be identified. In small- and mid-sized rivers in the Lower Mountain Range of Western Germany temperature data was continuously recorded for at least a one-year period up to three years, and environmental conditions were assessed: Concerning spring- and summer temperatures, the small- and mid-sized rivers differed significantly. Daily temperature variation differed in winter and early spring, whereas the maximum amplitudes revealed a similar range in both types. Main determinants for temperature characteristics were river-size and channel morphology, and, concerning only small-sized rivers, vegetation cover. Rivers, not grouping into the corresponding river-type by cluster analysis on thermal variables, with temperature characteristics modified by anthropogenic and groundwater influence, were excluded and temperature ranges were suggested for a thermal typology. In the second chapter, a river with an artificially changed temperature regime was analysed, and the effects on the development of two benthic species was observed. The temperature regime of the Lenne river is influenced by cold water originating from a tributary with hypolimnic water of an impoundment, and, further downstream, influenced by warm water originating from a power station. Hypolimnic water and the effluent of the power station caused a decrease respectively increase of more than 10% in temperature measured in degree-days over a one-year period. The main thermal change by the hypolimnic water was restricted to summer, depressing temperatures and decreasing thermal variability. The power station increased thermal variability and elevated the temperature throughout the year. Hydropsyche siltalai and Hydropsyche incognita developed slower downstream from the hypolimnic confluence, whereas the development did not change downstream from the effluent of the power station. In the third chapter, variation in the invertebrate community of the two rivers types (see first chapter) was related to temperature, the significance of temperature was compared with other environmental factors, and species related to the thermal environment were identified. The data was analysed using multivariate techniques. The EPTC-community was changing primarily along the thermal gradient of small- and mid-sized rivers. Up to 29% of the variability in the EPTC-community could be explained by summer temperature in the data set for both river-types as well as only for small-sized rivers. Other environmental variables explaining a smaller, but significant part of the variability in species distribution were conductivity, substrate type, and amount of local riparian forest. In mid-sized rivers, temperature showed to be less important for the faunal composition. Taxa found to be correlated with temperature were identified, and a comparison with autecological data revealed that a great part of the taxa (most Plecoptera and some Ephemeroptera) corroborate observations made in laboratory studies concerning thermal adaptations. In this thesis, a methodical approach is shown to distinguish different river types by their characteristical thermal parameters. Furthermore, the research resulted in showing, that it is possible to identify potential indicator taxa for thermal conditions within river-types.