Gut Sediments in a Burrowing Mayfly (Ephemeroptera, Hexagenia limbata): Their Contribution to Animal Trace Element Burdens, Their Removal, and the Efficacy of a Correction for Their Presence

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Abstract
Nymphs of the burrowing mayfly Hexagenia limbata were collected in the field, returned to the laboratory, and dissected to remove the gut contents. Concentrations of four trace elements (As, Cd, Cu, Zn) were determined both for the gut contents and for the body. Trace elements in gut contents represented up to 22% of whole animal trace element burdens. Studies of depuration of H. limbata nymphs held in water for up to 48 h indicate that individuals vary substantially in the rate at which they egest gut contents and that 48 h is not sufficient to ensure a complete emptying of the gut. A model developed to compensate for the presence of gut contents in the determination of body trace element burdens was applied to H. limbata. Comparisons of model estimates with actual body burdens (without gut contents) show that the model accurately predicts As and Cu concentrations, but overestimates concentrations of Cd and Zn by as much as 20%. We suggest that the biases in the model result from assuming that: (a) trace element concentrations in gut sediments are equivalent to those in sediments sampled from the animal's surroundings, and (b) there is no weight loss of gut contents during sample digestion (a two-thirds weight loss of gut contents was in fact observed). These biases may be minimized by, respectively: (a) measuring trace element burdens of gut contents and (b) compensating for weight loss of gut contents during digestion.