The effect of gut contents on dry weight estimates of Chironomus tentans larvae: Implications for interpreting toxicity in freshwater sediment toxicity tests

Abstract

AbstractGut contents have long been recognized as a potentially important source of bias in studies concerned with estimating contaminant body burdens in benthic macroinvertebrates. In this study, we show that gut‐associated, inorganic material may also lead to significant overestimates of weight in Chironomus tentans larvae in sediment toxicity tests and thus influence statistical interpretation of effects on growth with this organism. Two methods were evaluated regarding their potential to correct for the proportion of larval dry weight contributed by gut contents: purging and determination of ash‐free dry weight (afdw). Each method was assessed in a series of 10‐d growth tests using (1) clean sand and clay substrates with a restricted particle size distribution (mean particle size: 8, 146, 218, 306, 448, and 592 μm), and (2) sediments collected from both clean and contaminated field sites encompassing a wide range in particle size distribution. Both purging and afdw determination resulted in significant reductions in the dry weight of larvae in most substrates <306 μm and in the field‐collected sediments. In these treatments, 7–59% of the dry weight of fourth instar larvae was comprised of inorganic material contained within the digestive tract. Sediment organic carbon was also important; the contribution of gut contents to larval dry weight in a highly organic (23.6%) sediment was only 7.9% despite a small particle size distribution (mean = 149 μm). Correction for ingested material changed the rank order of treatment means in all tests and reduced the number of statistically significant outcomes for many treatment comparisons. Both methods reduced variability (lower standard deviation) associated with mean dry weight estimates, although variability was generally greater with purging than with the afdw method. Based on the results of this study, we recommend that afdw, not dry weight, be used to determine the weight of C. tentans larvae in 10‐d and life‐cycle tests with this organism.