EFFECTS OF CHRONIC HYPOXIA AND REDUCED TEMPERATURE ON SURVIVAL AND GROWTH OF BURROWING MAYFLIES, HEXAGENIA-LIMBATA (EPHEMEROPTERA, EPHEMERIDAE)

Hexagenia nymphs are sensitive to hypoxia, and their abundance has bee n proposed as an ecosystem indicator for assessing the recovery from e utrophication of shallow, mesotrophic lakes. Acute oxygen tolerance li mits are known for Hexagenia spp., but effects of prolonged exposure t o sublethal levels of hypoxia are not. A series of 21-day laboratory e xperiments was conducted to determine the influence of hypoxic stress (range 2-12.6 mg/L dissolved oxygen) and temperature (range 4-20 degre es C) on survival and growth of Hexagenia limbata nymphs. Oxygen and t emperature together explained 89% of the variability in survival among H. limbata, and 71% of the variability in average growth. Survival in creased with increasing oxygen concentration, reaching an asymptote at 7-8 mg/L. Survival increased with temperature to an asymptote at appr oximately 9.5 degrees C. Higher temperatures magnified the effects of hypoxia on survival. Growth increased with both oxygen and temperature , and did not reach an asymptote at levels up to 12.6 mg/L oxygen and 20 degrees C. Persistent, sublethal oxygen stress (concentrations <7 m g/L) and reduced temperature (<20 degrees C) can influence both surviv al and size of H. limbata.