The effects of a sand substrate (presence vs. absence) and type of water medium (distilled or lake) on phosphorus excretion rates of tubificids, chironomids, and the amphipod Pontoporeia hoyi were determined. In contrast to previous studies on respiration rates, the presence or absence of a substrate did not significantly affect the excretion rates of any of the three taxa. Realistic determinations of P excretion can thus be obtained without a substrate present; this simplifies the approach to such determinations. Excretion rates of tubificids and chironomids were not affected by the type of medium, but the excretion rate of P. hoyi was slightly (but significantly) higher in distilled water than in lake water.
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Nalepa et al. (1983) studied this question.