ON THE RELIABILITY OF PONAR GRAB SAMPLES FOR THE QUANTITATIVE STUDY OF BENTHIC INVERTEBRATES IN PONDS

We present observations on the variability of sediment penetration dep th by the Ponar grab sampler, which lead us to question the reliabilit y of grab samples in the quantitative study of freshwater benthos. Pen etration depth of the Petite Ponar grab depends on substrate type, and correlates with the amount of organic carbon, the water content and t he granulometry of the sediment. Since these factors can also influenc e 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 i nfluence biological interpretation because the penetration depth of th e grab followed that of the organisms under study.