Einfluss von Umweltfaktoren auf die Verteilung der Chironomiden (Insecta, Diptera) in Buhnenfeldern der Elbe

In the years of 1998 and 1999 the chironomid fauna within five groyne fields (space bordered by two sedimentary groynes) of the lower middle section of the river Elbe was investigated. These groyne fields, located near to each other, differed in conditions of current and substrate. The aim of the investigation was to determine the environmental factors that are responsible for the distribution of the chironomids within their habitat. With the help of the direct gradient analysis these environmental factors were selected from a group of factors recorded from the river. Subsequent to the selection of the decisive factors for colonization, the nature of the context of each environmental factor and the respective taxa were described using the generalized linear model. During the investigation 93 taxa (mainly Chironominae) were found. The groyne fields with a dynamic pattern of current and substrate conditions yielded more taxa than the groyne fields with uniform conditions. In areas adjacent to the bank with weak current - in contrast to areas with strong current - many non-dominant species besides to dominant species were found. In order to record the chironomid fauna three methods were applied: • benthos sampling • aquarium rearing • eclector traps Comparing the species- and dominance-spectrum of the different recording methods showed different results. The conclusion was, that none of the three methods was solely suitable to record the chironomid species spectrum in streams. Reasons for this are: 1. The larvae obtained by benthos sampling often could not be determined to species level. In this case assumptions about autecological demands are not possible. Furthermore it is uncertain, that the larvae will finish their development at their sampling site. This was demonstrated by the lack of records of Robackia demeijerei and Tanypus punctipennis in the aquarium rearing and their limited occurrence in the eclector samples. However, within the benthos samples both species occurred in high abundance. 2. By means of aquarium rearing and eclector sampling adult chironomids can be obtained. Adults can be determined to species level and thus permit autecological conclusions. A comparison of both methods showed considerable differences for the eclector samples contained almost twice as many species as the aquarium rearing. Especially rare species were frequently found in eclector samples. The use of eclector sampling therefore has proven suitable to record rarely occurring species. In order to facilitate the comparison of data sets, the species lists of adults (mainly identified to species level) were adjusted to the taxonomic level of the larvae (mainly genus level). A comparison of the species spectrum of benthos and eclector samples showed a high level of congruence. On the other hand the species spectrum of benthos samples and aquarium rearing showed a low level of congruence. These differences are probably caused by the – compared to the natural habitat – modified environmental conditions within the aquarium. The direct gradient analyses of the chironomid fauna and the recorded environmental factors showed that current speed, substrate composition and factors caused by the change of seasons influenced the distribution of chironomids in the groyne fields. Detritus feeding species of the tribe Chironomini (e.g.: Polypedium nubeculosum, Dicrotendipes nervosus, Chironomus sp.) dominated sections of the river characterized by slow current and sandy substrate. Sample sites showing faster current and gravelly substrate were mainly colonized by grazing species of the subfamily Orthocladiinae (e.g.: Orthocladius sp., Nanocladius bicolor). An exception was Robackia demeijerei (Chironomini) for frequently occurring on gravelly substrate with fast current. Relating to the results of the investigations diverse strategies that stream dwelling insects adopted in order to live under different conditions of current and substrate are discussed. Of the 29 taxa showing significant response to current or substrate 27 taxa showed current preference and 14 taxa showed substrate preference. Consequently for most of the chironomids found in the groyne fields current speed is the most influential environmental factor. Taxa preferring slow current were often indifferent to condition of the substrate. This fact could be explained as follows: At slow current chironomids are hardly endangered of drifting away. The larval tubes do not have to be fixed on stable substrate and the larvae are not dependent on sheltered areas on the lee side of bigger substrate particles. Obviously for many chironomids the substrate serves as protection from too strong current. A comparison of the results regarding the response of single taxa to current and substrate in this investigation with results from the literature, showed many similarities. Moreover it is clear that we know very little about the microhabitat preferences of chironomids.