Bacteria as a Food Source for Black Fly Larvae in a Blackwater River

Abstract

The contribution of bacteria to secondary production of black fly larvae (Simulium spp.) in a Coastal Plain blackwater river was examined. Production rates of filter-feeding invertebrates inhabiting submerged wood (snags) in these rivers are high, as is bacterial biomass within the seston. This study examined the prediction that the abundant bacterial biomass in the seston is important in supporting the observed production rates. Black fly larvae, which are among the most common kinds of filtering collectors on snags, were fed seston in which the bacterial DNA had been labelled with [³H]-methyl thymidine to measure assimilation of bacterial biomass. Assimilation of bacterial carbon was positively correlated with seston bacteria concentration from 7.0 × 10⁹ to 1.8 × 10¹¹ cells/L. Larvae assimilated from 3.94 × 10⁵ to 6.18 × 10⁷ cells/hr. Bacterial carbon accounted for 20 to 67% of the daily black fly growth in unaltered river water. The percent growth attributable to bacterial carbon showed no clear relationship to bacterial concentration because growth rates were lower when bacteria were less abundant. Our results show that assimilation of bacterial carbon is quantitatively important in the diet of larval black flies in the Ogeechee River.