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Collection Methods

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Field processing is applied to either individual samples if each sample produces a large volume of material or to the entire composite sample if each sample produces a small volume of material. Field processing can result in the production of four sample components from each composite sample: large-rare, main-body, elutriate, and split-sample components.

Large-rare component
First, large rocks and organic debris such as leaves, twigs, and roots are removed from the sample and discarded. The remaining material is examined, and large-rare organisms are placed in a labeled container identified as the "large-rare" sample component.

Elutriate component
The remaining sample material is elutriated onto a sieve to separate the lighter organic material from the heavier sand and gravel. The sand and gravel are examined for invertebrates. If any are found, they are added to the large-rare sample component. Otherwise this "elutriate" sample is preserved and labeled.

Main body and split component
Elutriated material, the "main-body" component, retained on the sieve is examined for large, rare organisms that are added to the large-rare sample component. Then the material is washed by dipping the sieve into a bucket of water. If after elutriation, the volume of material constituting the main body or elutriate sample component exceeds 0.75 L, that sample is split in the field. This splitting is accomplished by using a sieve sample spillter or a sieve diameter splitting method. Once the sample has been split, a coin toss or throw of a die is used to select half of the sample. If the sample being processed is an elutriate sample, then the half selected is retained for analysis and the other half discarded. If the sample being processed is a main-body sample, then the sample selected is designated as the main-body component and the other half designated as the "split" sample component.