Size-mass relationships of stream invertebrates in a northern Spain stream

We show the regression equations that relate the head width of 10 freshwater insect species or the length of the first thoracic segment of 2 amphipods with their dry mass in 3 reaches along a small stream in northern Spain. Spatial differences in size–mass relationships were found. Organisms stored in alcohol showed lower dry mass than those specimens that were dried immediately after collection. The use of both size measures for dry mass estimation in field samples is discussed. It is recommended that the size–mass relationships be constructed using non-preserved organisms. Furthermore, great care is required when such allometric equations are taken from the literature.