Fixed dose aluminum sulfate treatment to eradicate the salmon parasite Gyrodactylus salaris: effects on benthic macroinvertebrates

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Gyrodactylus salaris is an ectoparasite invasive to Norway that is highly virulent to pre-smolt stages of Atlantic salmon (Salmo salar). To eliminate the parasite without killing the salmon host, aqueous aluminum sulfate and sulfuric acid (AlS treatment) have been applied to infested rivers for short durations. When the River Lærdalselva and its tributaries were treated in the summer of 2012, we studied communities of macroinvertebrates in a small stream that received AlS for 18 days. We monitored pH, inorganic aluminum (Ali) and collected quantitative samples of macroinvertebrates. Concentrations of inorganic aluminum (Ali) were generally in the range of 20–50 µg/L, but episodes up to 95 µg/L were recorded. Six of the fourteen recorded macroinvertebrate taxa were, to some degree, negatively affected by the treatment. The acid sensitive ephemeropterans Baetis subalpinus and Baetis rhodani were almost eliminated from the treated site (p < 0.001 for both compared to the untreated site). Halfway into the treatment, abundances of Baetis were approximately 15 individuals per m2 at the treated site. Densities of Baetis at the untreated site were above 250 individuals per m2 throughout the treatment. The plecopteran Isoperla grammatica, the trichopteran Rhyacophila nubila, and the dipterans Chironomidae and Simuliidae also responded to the treatment, but the effects were not as prominent as for Baetis. For R. nubila and I. grammatica, the decline in population sizes could be a behavioral response related to lower prey densities (Baetis, Simuliidae, and Chironomidae). The reduction of Baetis led to a prominent reduction in the number of grazers and scrapers and gatherers/collectors (p = 0.004 for both groups). No such responses were found for the other feeding groups. Significant reductions in populations of Baetis can affect ecosystem functioning and, thereby, have implications for pre-smolt survival following AlS treatments.