THE CHOICE OF A SIEVING MESH SIZE IN THE QUANTITATIVE ASSESSMENT OF MARINE MACROBENTHOS - A NECESSARY COMPROMISE BETWEEN AIMS AND CONSTRAINTS

The efficiency of several screen sizes in retaining macrofaunal organisms was tested in subtidal and estuarine intertidal communities. Although macrobenthos is currently defined as metazoans retained by a 1 mm mesh, sieving efficiency of this screen was found to vary between 20 and 70%, when compared with 0·5 mm meshes. Furthermore, 25-65% of specimens belonging to macrofaunal species passed through a 0·5 mm mesh, which shows that sieves finer than 1 or 0·5 mm clearly improve the quantitative collection of macrobenthos. Sieving efficiency displayed considerable seasonal variation due to settlement pulses, and differed according to phyla (generally, crustaceans > polychaetes > bivalves) and between species within a phylum. In some cases, species richness, the diversity index and evenness were influenced by sieve mesh size. Even though the subdivision of benthos in macro- and meiofauna reflects a real pattern, the boundary fixed between them at about 1·0-0·5 mm seems too clear-cut, due to the protracted stay of juvenile macrofauna in the meiofaunalsized class and a number of methodological factors and morphological features. The mesh selected for a particular task should be chosen with a clear idea of the study aims. It is thus suggested that a 1·0 mm mesh is sufficient only for biomass estimates or bionomics studies, whereas sieves with 0·1-0·2 mm mesh openings should be used in studies involving population dynamics of macrofauna, to provide an adequate estimate of abundances of individuals in small size classes. © 1990.