Retention of Benthic Invertebrates with Different Sieving Techniques

Two techniques of washing sediment samples from profundal regions of Lake Superior on different cruises produced quantitatively and qualitatively different results that cannot be explained by differences in sampling sites. The method of washing the sediment contained in a sieve (250 μm mesh) with a lake water spray from a hose retained approximately 10 times as many nematodes, enchytraeids, and naidids as the method of agitation of the sediment-containing sieve in a tub of lake water. The average numbers of lumbriculid oligochaetes, Pontoporeia, and fourth instar chironomid larvae were similar for the two sieving techniques. The retention of organisms small enough to pass through the sieve was probably related to sieving time. The organisms quantitatively retained had a fixed (as opposed to contractile) dimension larger than the size of the sieve openings. Consideration of sieving methods is as important as sieve mesh size, type of sampler, and sampling design for benthic comparative studies.