Length–mass relationships for adult aquatic and terrestrial invertebrates in a California watershed

We collected 541 invertebrate specimens in riparian and upland habitats of the South Fork (SF) Eel River, California, USA, representing 12 orders and 55 families, and including adults of both aquatic and terrestrial origin. We fitted a power function to this data set at 4 taxonomic levels: 1) the entire pooled sample, 2) composite samples of aquatic and terrestrial taxa, 3) individual orders, and 4) individual families for which there were adequate data (n > 8; 24 families). Coefficients of determination (R2) for length–mass relationships generally increased with increasing taxonomic specificity. Composite samples of aquatic and terrestrial taxa had significantly different length–mass relationships, with dry mass increasing faster with length in terrestrial taxa. Differences in length–dry mass relationships between aquatic and terrestrial taxa appeared to result from significantly higher slopes for terrestrial taxa in length–width relationships, and significantly lower slopes in length–water content relationships. Our results suggest that the use of terrestrial regressions to estimate aquatic insect biomass flux in riparian habitats overestimates both the absolute magnitude of biomass flux as well as its relative importance to insect standing stocks. Development of unique regression algorithms for adult aquatic insect taxa increases the accuracy of aquatic insect biomass estimates in terrestrial habitats.