Survival, growth, and production of Hexagenia bilineata mayflies in fluidized sediment from lower Green Bay, Lake Michigan

Organic pollution in lower Green Bay, Lake Michigan over the past century was accompanied by the local extirpation of Hexagenia (primarily H. limbata) mayflies. Recoveries were made in other systems where population crashes had occurred (e.g., western Lake Erie); however, an active recovery does not appear to be taking place in Green Bay. Excessive primary production has caused substantial benthic organic matter accumulation resulting in a fluidized “sludge-like” substrate as the majority of the sub-littoral habitat. Fluidized substrate potentially hinders Hexagenia nymphs' abilities to construct and maintain burrows critical to their life cycles. In this study, Hexagenia bilineata nymphs were collected from an Upper Mississippi River backwater where their presence at high densities in relatively fluid sediment had been observed, and reared in oxygenated aquaria containing lower Green Bay or Upper Mississippi River sediment. Their survival, growth, secondary production, and biomass turnover were calculated for a 166day period. Seventy-five percent of nymphs survived or metamorphosed into winged sub-imagos in lower Green Bay substrates compared to 40.6% in Upper Mississippi River substrates. Nymph dry weight more than tripled in Green Bay substrates and more than doubled on Upper Mississippi River substrates. Production was notably higher in lower Green Bay substrates. Differences in survival and production between the two treatments were statistically significant (P<0.05), while differences in growth and biomass turnover were not (P>0.05). Based on these results, the high fluidity of lower Green Bay substrates did not appear to hinder burrow construction or maintenance.