On the reliability of Ponar grab samples for the quantitative study of benthic invertebrates in ponds
ABSTRACT We present observations on the variability of sediment penetration depth by the Ponar grab sampler, which lead us to question the reliability of grab samples in the quantitative study of freshwater benthos. Penetration depth of the Petite Ponar grab depends on substrate type, and correlates with the amount of organic carbon, the water content and the granulometry of the sediment. Since these factors can also influence faunal composition and vertical distribution in the sediment, it is important to study the performance of the sampler before a biological explanation for the observed pattern is given. At the site studied, a case study was performed, in which variable grab penetration did not influence biological interpretation because the penetration depth of the grab followed that of the organisms under study.
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ABSTRACT: The distribution of chironomid species in small lowland rivers and brooks in the crenal zone of the Zwalm river basin (Flanders, Belgium) was studied. We examined whether a more refined identification of Chironomi-dae would result in an added value to the biological assessment of the Zwalm river basin, currently based on the Bel-gian Biotic Index. At the 18 sampling sites, a total of 31 different taxa of chironomids were identified. Identification was at species, aggregate, group, subgenus or genus level. The diversity of chironomid communities and their indica-tor role for assessment of particular river types and water quality were examined by means of multivariate analysis. Direct ordination of the identified chironomid taxa resulted into three groups of indicators related to the assessment of the water quality of a brook : (1) indicators of a good water quality, (2) indicators of waters enriched with nutrients and organic matter compounds and (3) taxa that were indifferent to water quality. The more refined identification levels provided useful information on the ecology of these organisms and their role as indicator organisms of specific water quality states. Because of the more labour-intensive procedure, however, chironomid identification at these levels will be more difficult to apply within a rapid bio-assessment protocol of running waters.Belg. J. Zool. 02/2004; 134:31-40.