Sieve retention probabilities of stream benthic invertebrates

Replicate samples of cobbles and loose inorganic and organic matter collected from 3 stream-riffle sites with different periphyton communities were passed through a geometric series of 9 sieves (0.063-16 mm mesh) to quantify sieve retention probabilities of benthic invertebrates. Sieves retained all organisms with a body length >10x the mesh size. Logistic regression models were estimated to describe retention probabilities as functions of body length and mesh size. Retention probability functions differed slightly but significantly among sites, operators, and taxa. Retention probabilities were higher for samples containing filamentous algae, which entangled invertebrates. Size distributions of straight, elongate invertebrates (e.g., oligochaetes, midge larvae) retained by sieves were more variable than distributions of more spherical organisms (e.g., gastropods). On average, the 1-mm sieve retained >90% of invertebrate biomass but <33% of individuals retained by a 63-μm sieve. Size distribution of organisms retained by coarse sieves (≥1 mm), combined with logistic functions predicting retention probability, can be used to describe abundance, biomass, and size distribution of organisms retained by fine sieves. These results suggest that unbiased descriptions of benthic communities can be obtained with relatively little effort by using a minimum sieve mesh size of 1 mm.