Mesotrophic and eutrophic lakes have been treated with aluminum salts (alum) to lower phosphorus concentrations and inhibit phosphorus recycling from the sediment. Changes in the profundal benthic and merobenthic fauna of five selected lakes were assessed, revealing increases in faunal density and diversity in nearly all treated lakes. The dominant organisms wereChaoborus punctipennis(Say) andChironomusspp. Sediment cores used to evaluate the presence of an aluminum hydroxide floc layer showed aluminum concentrations above extrapolated background levels in most cores within the top 20 cm of substrate. Increased chironomid density resulting from such treatments might reduce the expected benefit from the alum addition. To assess the effect of sediment bioturbation, a bioassay withChironomus tentansF. larvae was carried out to determine if translocation of phosphorus across the flocculent barrier is dependent on floc thickness or chironomid density. In the absence of a floc layer, chironomid activity generated higher concentrations of soluble reactive phosphorus (SRP) than when a floc layer was present. Even with a floc layer of 0.5–2.0 cm thickness, chironomids were able to increase soluble reactive phosphorus concentrations. Soluble reactive phosphorus concentrations in assays with alum and no insects were at or below the detectable limit of 4 μg/L. Alum toxicity to insects was not apparent in the lake treatment part of this assessment. An indirect toxicity was observed in the bioassays when alum dosages greater than 75 mg Al+3/L were used. The absence of toxicity in bioassays at lower treatment rates, which are more indicative of normal aluminum salt usage, indicates that this phosphorus reduction technique is relatively safe toChironomus tentans.