Elutriated material retained on the sieve is quickly examined for large, rare organisms that are added to the large-rare sample component. This procedure helps to ensure that these organisms are not lost during subsequent sample processing. The material remaining on the sieve is then washed by repeatedly dipping the sieve into a dishpan or bucket half filled with water and gently swirling the sieve to wash material through it. Large commercially available sieves, such as the wash bucket and Ponar wash frame (Wildlife Supply Company Catalog, 1980), can be used effectively to screen large-volume samples. Alternatively, large-volume samples can be divided into smaller portions and washed using standard-sized sieves. In the latter case, the respective sample components (large-rare, elutriate, and main-body) resulting from each washing are combined to form the components of the composite sample.
If, after elutriation and compositing, the volume of material constituting the main-body or elutriate sample component exceeds 0.75 L, that sample component is split in the field. Any debris or large organisms that remain in the sample must be removed lest they interfere with the sample-splitting process. Organisms so removed are added to the large-rare sample component, whereas debris is discarded after any attached invertebrates are removed.
Sample splitting is accomplished by using either a special sieve sample splitter (Mason, 1991) or a sieve diameter splitting method. The sieve sample splitter consists of a Plexiglas box with a mesh bottom formed by two equal compartments. The two compartments are latched tightly together, and the sample is placed on the sieve, immersed in water, and gently agitated to distribute the sample uniformly over the surface of the sieve. The sieve is then gently removed from the water, drained, and unlatched to produce two subsamples.
In contrast, the sieve diameter splitting method uses a standard 20-cm (8-in.) diameter metal or plastic sieve marked with six equally spaced (30 degrees apart) diameters (fig. 8A). The diameter markings are extended up the inside walls of the sieve and numbered 1 through 6 (fig. 8B). The sample is placed on the marked sieve, immersed in water, and gently agitated. Next, the sample is distributed uniformly across the sieve, which is then gently removed from the water and drained. A die is rolled to obtain an unbiased determination of which diameter (1-6) to use for splitting the sample. A metal-edged ruler (20 cm long for a standard sieve) is used to divide the sieve contents into halves by aligning the ruler with the appropriate inscribed diameter markings (diameter 5 in the example presented in fig. 8B) and pressing the ruler against the bottom of the sieve. A small scraper, such as a putty knife, is used to help separate the sample into halves by pulling material away from the ruler. This process is completed using a wash bottle to remove and concentrate any remaining sample material.