Benthic macroinvertebrates are animals that inhabit the sediment or other substrates on the bottom of freshwater, estuarine, and marine ecosystems. During their life cycles, these organisms may construct attached cases, tubes, or nets that they live on or in; roam freely over rocks, organic debris, and other substrates; or burrow freely in substrates. According to historical definitions, macroinvertebrates are visible to the unaided eye and retained on a US Standard No. 30 sieve (0.595- or 0.600-mm openings).1 In actuality, however, some are difficult to see without magnification.
The standard sieve for collecting freshwater, estuarine, and marine benthic macroinvertebrates is the US Standard No. 30 sieve; however, some estuarine and marine programs use the US Standard No. 50 sieve (0.300-mm openings) or the US Standard No. 35 sieve (0.500-mm openings). For all aquatic assessment programs, the No. 30 sieve is recommended for collecting benthic fauna from freshwater, estuarine, and marine habitats or from any water transport system. To accommodate some historical databases (if the study’s data-quality objectives permit), investigators might use a US Standard No. 28 sieve (1.0-mm openings). To obtain a more representative sample of the benthos (one that includes smaller forms, early life-stages, and other taxa of macroinvertebrates), they may use a US Standard No. 60 sieve (0.250-mm openings).