Benthic macroinvertebrate assemblages in north Swedish streams environmental relationships -Ecography 17 9-16 An analysis of the relationships between lotic macroinvertebrates and environmental vanables was earned out on matenal from 60 nffle sites in streams in northern Sweden The approach involved the use of TWINSPAN classification and canonical correspondence analysis on presence/absence data from two seasons (spnng and autumn) Variables most strongly associated with distribution patterns of assemblages were drainage area, elevation, water quality m terms of alkalinity, colour and phosphate and the presence of macrophytes The significance of affinities of different species to these vanables are discussed The eight clusters resulting from the TWINSPAN analysis could biologically be interpreted as classes of taxa related to stream size, chemical conditions and algae A multiple regression analysis for predicting species nchness using three independent vanables, viz drainage area amount or organic matter, and discharge was constructed The results of the study could be used as a starting point for further work on the community organization of benthic stream assemblages
Trypanosomatid flagellates are common protozoan gut parasites of a wide range of insect species. Water striders (Gerridae) harbour the trypanosomatid Blastocrithidia gerridis. Three different populations of the water strider Gerris odontogaster in northern Sweden were sampled to assess the infection rate dynamics of trypanosomatids. The initially very low infection rates (0%-15%) early in the season were followed by a rapid increase during the reproductive period of the water striders, reaching very high levels (80%-90%). The pathogenic effects of trypanosomatids on G. odontogaster adults were studied in laboratory experiments. The parasites caused a general reduction of host vigour. Male skating endurance was negatively correlated with the intensity of the trypanosomatid infection. However, infection of trypanosomatids increased the mortality among adults only when the water striders were subjected to food stress. The trypanosomatids did not reduce the fecundity of females provided with food. We suggest that trypanosomatid gut parasites may be an important mortality factor in water strider populations. Since the pathogenicity of the parasites is enhanced by stress, parasitism by trypanosomatids may contribute to the regulation of host populations.
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