An assemblage is the set of related organisms that represent a portion of a biological Community (e.g., benthic macroinvertebrates).

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Other /More definition:
An Assemblage is an association of interacting populations of organisms in a given waterbody. Examples of Assemblages used for biological assessments include : algae, amphibians, birds, fish, herps (reptiles and amphibians), macroinvertebrates (insects, crayfish, clams, snails, etc.), and vascular plants.