Sieve efficiency in benthic sampling as related to chironomid head capsule width

The width of the head capsule in chironomid larvae is the most important mor phometric character controlling retention of specimens in sieving devices. Knowledge of the range in size of these widths within any chironomid community is fundamental to sampling and interpreting the resulting data. We present the head capsule widths of 30 species of chi ronomids and relate their size distribution to loss or retention in several experiments using graded sieve sizes. Based on our measurements and those found in the literature we found the head capsule width of fourth instars in half the chironomids species to be less than 350 jam. Many species may never be collected with the commonly used U.S. Standard No. 30 sieve (589 (im), and the No. 60 (246 Jim) screen appears to retain most species only qualita tively. We found 70 to 90% of the chironomid larvae and 19 to 34% of their biomass can pass through a No. 80 sieve (177 |im). The implications of sieve loss and other factors affecting sieving efficiency are discussed. All benthic invertebrate sampling is constrained by difficulties associated with the sampling, extraction, and identification of a wide range of organisms (Resh and McElravy, 1993). In the sampling process, the importance of selective retention of benthic invertebrates by sieves of various mesh sizes was first recognized by Jonas son (1955, 1958). In these two seminal papers, Jonasson described commonly used sieving techniques and efficiencies in the sampling of freshwater benthic communi ties and cited examples of erroneous conclusions due to the mesh size of the siev ing device. Jonasson (1955) found that adequate sampling of chironomid larvae in the sieving process is partially governed by the width of the larval head capsule. Later studies by Kajak (1963), Heuschele (1969), Mundie (1971), Ferrington (1984), and Storey and Pinder (1985) supported Jonasson's findings that the head capsule width of chironomid larvae governed their retention. One approach created to avoid sieve mesh selectivity, was to create artificial size categories of benthic invertebrates such as macroinvertebrates (retained by 200-2000 |iim sieves), microinvertebrates (those passing through a 595 (Xm sieve), meiofauna (retained by 40-200 |im sieve), and microfauna (pass through a 40 (im sieve) (Weber, 1973; Stray er, 1985). However, these categories transcend taxonomic units, since individuals of certain species may be retained or lost depending upon the larval stage of development. This situation is particularly true of chironomid larvae, in which species range in size from 2 to 30 mm in length, and whose cylindrical, legless bodies are more susceptible to passing through a sieving device than most other benthic taxa. Although there are several papers dealing with mesh-size and efficiency of sam pling larval chironomids, most studies deal with relatively few species and do not Accepted for publication 28 January 1998. 1 This article is contribution 1075 of the USGS Great Lakes Science Center. This content downloaded from 207.46. 13.57 on Sat, 10 Sep 2016 05:51:42 UTC All use subject to http://about.jstor. org/terms VOLUME 71, ISSUE 4 457 look at both the hypothetical and practical aspects of sieve loss. We present here the head capsule widths of 30 species of chironomids and relate their size distribution to loss or retention in several experiments using graded sieve sizes. The implications of sieve loss and the factors affecting sieving efficiency are discussed. The conse quences of losing early instars, the problems in data interpretation, and the use of head capsule width as a systematic tool are also described.