The identity and abundance of animals passing through a 1·0 mm mesh but retained on a 0·5 mm mesh is detailed for a number of seabed samples of medium, fine and very fine sand taken in the Beatrice Oilfield in 1981. Comparisons are made with the corresponding 1·0mm-sieved fractions, and the effects on community parameters of combining the two data sets are considered.
In general fewer animals were found on the 0·5 mm mesh than on the 1·0 mm mesh above it. The 0·5 mm mesh samples added comparatively few taxa to the faunal list of the survey. The quantitatively-scored taxa from the 0·5 mm mesh were therefore essentially a sub-set of those already recorded on the 1·0 mm mesh. Several taxa of Foraminiferida were found only on the 0·5 mm mesh, and one of these appeared to be relatively abundant. However. Foraminiferida were not scored quantitatively in this study.
In all samples, when 0·5 mm and 1·0 mm data were combined an increase was noted in the number of taxa recorded compared with 1·0 mm data alone. This recruitment of taxa was largely a reflection of the greater number of individuals in the combined data, rather than the inclusion of taxa unique to the 0·5 mm material. Similarly, the improved precision of abundance estimates of dominant taxa in replicate samples in the combined data probably resulted from the greater number of individuals included in the analysis.
It is argued that the choice of a particular mesh size imposes an essentially arbitrary cut-off along the size spectrum of benthic organisms. A 1·0 mm mesh has been found to yield adequate data in a number of offshore surveys undertaken by the Oil Pollution Research Unit (OPRU). Although a greater number of animals per sample is collected by sieving on a 0·5 mm mesh, the time taken to extract and identify this material must be considered. The alternative of spending an equivalent time analysing extra 1·0 mm samples may in fact yield a greater increase in confidence in the results.