Length-biomass relationships in benthic macroinvertebrates of a mountain stream of the south of Córdoba, Argentina

Abstract

The methods used for indirect determination of invertebrates biomass are based on mathematical equations that relate body linear dimensions and weight of the organism. However, these relationships depend on biotic and physicochemical conditions of each particular environment. The aim of this study was to determine the equations to estimate the relationship between lenght and weight of benthic macroinvertebrate of Achiras stream, located in the southwest of Córdoba province (Argentina). The benthos was collected with Surber nets, with a seasonal occurrence (fall, winter, spring and summer), between 2006 and 2007. Body length and dry weight for the specimens of the more represented taxonomic groups were registered and the regression equations that best explain the relationship between the two variables were calculated. We evaluated three models, simple linear, potential and exponential, and we selected the better expression that associate the variables. Equations were estimated for 24 macroinvertebrates taxa (three species, 12 genera, three subfamilies, four families and two orders). Most regressions were highly significant and explain a large proportion of the variation of the dry weight. The estimated equations suggest that for most of the benthic macroinvertebrates of Achiras stream, the relationship between length and biomass is alometric and can be explained by a power or exponential model, while only for four taxa simple linear regression best fit length and body weight values. Regression models applied to lower level taxonomic groups (family and genus) explained a greater proportion of the variation in weight, with regard to the order level regressions. In addition two general equations were estimated following the power model. In most analyzed taxa the value of the slope was less than three, and the average determination coefficient found in our study was 0.77, which coincide with the variation range of the published results for benthic fauna of other lotic environments.