Length-mass relationships for freshwater macroinvertebrates of the Middle Rio Grande, NM

Biomass measurements are important for answering questions about trophic dynamics in aquatic systems. Measuring dry weight for biomass is a destructive process, but specimens may be needed for other processes (e.g. stable isotope analysis) or for long-term preservation. Length-mass regression is a non-destructive process that is widely used for calculation of biomass. However, length-mass regressions have not been calculated for some macroinvertebrates common to the Middle Rio Grande, NM.  Macroinvertebrates were collected from The Bosque Del Apache National Wildlife Refuge in summer 2007.  Abundant taxa were measured and sorted into classes by body length and head width, then oven-dried and weighed. We performed regression analyses on Corisella (Corixidae), Tricorythodes (Tricorythidae), and Physa (Physidae).  Head width and body length both provide fairly strong estimates for biomass for both adults and nymphs (R² = 0.75 - 0.90) for Corisella.  For Tricorythodes a stronger correlation was observed (R² = 0.88 - 0.90).  Head width measurements may be useful for damaged individuals where the total body length cannot be measured. Physa were measured for shell length and shell width and provided a strong correlation (R² = 0.87 – 0.92). Other taxa to be included in regression analyses are dragonfly nymphs Erpetogomphus (Gomphidae).  In our study we also propose that due to the environmental instability in the Rio Grande compared to rivers in other areas, insects of the same taxa may be the same length, but due to decreased resources, or shorter life spans, may have decreased biomass.  This may hold true for other arid-land rivers.