Size-mass relationships of aquatic insects in forest streams: addressing a knowledge gap in Neotropical communities

Determining individuals' dry mass is crucial for better understanding community structure in terrestrial and aquatic ecosystems. The establishment of size‒mass relationships for different populations constitutes a helpful tool for indirectly determining the total biomass of freshwater communities. In this study, we determined the size‒mass relationships of 14 genera from the orders Ephemeroptera, Plecoptera, Trichoptera, and Odonata commonly found in leaf patches in Atlantic Forest streams (SE Brazil). We used individual body length as a size measure and three mathematical models (linear, exponential, and power) to establish the best-fitting equations. The equations described by the power model showed the best fit (coefficient of determination [r²]≥0.80) for the genera Anacroneuria, Macrogynoplax (Plecoptera), Smicridea (Trichoptera), Archaegomphus, Idiataphe, Hetaerina, Heteragrion, and Neocordulia (Odonata); the exponential model presented the best fit for Phylloicus (r²=0.74), Triplectides (Trichoptera, r²=0.60), Enderleina (r²=0.96), and Tupiperla (Plecoptera, r²=0.60); and the linear model exhibited the best fit for Elasmothemis (Odonata, r²=0.85) and Massartella (Ephemeroptera, r²=0.63). Our findings demonstrated that body length is a strong predictor of dry mass for most of the studied genera but some exceptions suggest that it may not fully capture biomass variation. The power models performed better in general, whereas the exponential and linear models were optimal for specific genera, indicating diverse influences on taxa allometry. This study reinforces the need to evaluate different approaches for estimating the dry mass of aquatic insects and underscores the caution required when using indirect methods for biomass determination.