Sediment Suspension by Burrowing Mayflies ( Hexagenia spp., Ephemeroptera: Ephemeridae)

Abstract

Burrowing and ventilation activities of benthic invertebrates can influence water column turbidity, nutrient concentrations, and possibly oxygen balance of lakewide ecosystems. In laboratory experiments, we determined rates of bioturbation-induced suspension of fine Lake Erie sediments caused by the burrowing mayfly Hexagenia of various sizes (lengths 13–28 mm) and multiple densities (70–1,111 larvae/m 2) and water temperatures (10–25°C). Larvae were inoculated into 2-L jars containing Lake Erie sediment and water. Bioturbation and sediment settling rates were independently estimated from sediment concentrations in water, measured twice daily for 14–18 d. Nonlinear regression was used to estimate sediment suspension rate for each jar (flux, mg/L/h). Logarithmic transformations of size, density, and temperature best described sediment flux. Separate experiments demonstrated that flux was unaffected by sediment depth, but did vary by sediment type, which was related to location from which Lake Erie sediment had been collected. Sediment suspension rates increased as food became depleted. Sediment suspension by pre-emergent (25 mm long) larvae at densities and water temperature typical for late spring in western Lake Erie (400 larvae/m 2 at 22°C) averaged 12 g/m 2/h. Although this level of bioturbation by Hexagenia larvae in western Lake Erie likely contributes only a small fraction of the basinwide annual sediment load, sediment suspension is possibly an important epibenthic source of nutrients and sediment-associated contaminants during spring. Such concentrations would exceed the clearance capacity of dreissenid mussels and may partially explain why dreissenids have not become abundant in soft sediments of the western basin.