Aluminium impact on respiration of lotic mayflies at low pH

Abstract

Three species of stream-living mayfly nymphs were exposed to total inorganic (labile) monomeric aluminium levels of 0, 0.5, and 2.0 mg L⁻¹ at pH 4.0 and 4.8 for 10 days. Oxygen consumption rate of the nymphs was measured, using a closed-cylinder technique and micro-Winkler titration. The animals showed significant increases in respiration with raised Al concentrations. The mayfly species Ephemera danica, which is restricted to the less heavily acidified region of South Sweden, was most affected by Al. The response of the two Heptagenia species to elevated Al was less pronounced. The effect of pH, however, seemed less important. Two possible reasons for the respiration increase at high Al levels are: (1) A “chemical impact route”, consisting of a compensatory mechanism due to decreased O₂ transport efficiency, because of impaired osmoregulation and ion transport; and/or (2) a “mechanical impact route” with Al hydroxide precipitation and mucus formation on the gills, causing lowered respiration efficiency. This stress of the increased respiration rate costs energy. Thus less will be available for growth and reproduction. A model of the proposed impact routes is presented.