The effect of sample handling and the experimental conditions used during a filtration experiment are critical factors, which must be assessed, if truly representative size distributions of natural particulates are to be obtained. This is particularly the case when colloidal sized, reactive (coagulation, oxidation, precipitation) species such as those found in anoxic lake waters are investigated. Furthermore, such samples are generally rich in both H2S and CO2and losses by volatilization may alter the pH and lead to changes in the species distribution between the dissolved, colloidal and particulate species. In this paper, we describe a submersiblein situcascade ultra-filtration unit which has been specifically developed to enable the study of particle size distributionsin situ. This system has been used to investigate the particle size distributions of colloidal sized iron and manganese species in oxygen depleted and totally anoxic reduced water strata found in a well stratified hypereutrophic lake (Lake Bret, Vaud, Switzerland).
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