A method to estimate the population size of benthic macroinvertebrates in streams

Summary A method is described by which samples of stream macroinvertebrates can be linked to the roughness of the substratum, the water depth, the velocity, and to a combination of these factors, i.e., Froude number and thickness of the laminar sublayer, at the exact point of sampling. The abundance of Odagmia ornata (Diptera: Simuliidae), the species considered in this study, was not related to roughness, but depended with increasing significance on depth, velocity, Froude number, and laminar sublayer. Therefore, it is suggested that the population size of stream macroinvertebrates can be estimated using the relationships between abundance and the most suitable hydrodynamic factor. For this purpose it is necessary only to measure the physical factors and then to calculate the abundance, using both these measurements and the abundance-hydrodynamic factor relationships formerly established.A longer section of a stream with a relatively wide range of habitats was sampled for 1 week, during which the new method yielded results that reached the level of accuracy per sampling effort obtained to date only when a narrower range of habitats was sampled, usually only one stream riffle. Under the latter conditions, the new method is suggested to yield results of increased accuracy, particularly if samples are taken within a short period.