Soluble nutrient effects on microbial communities and mosquito production in Ochlerotatus triseriatus habitats

We investigated the effects of soluble nutrient inputs into larval habitats of Ochlerotatus triseriatus (Diptera: Culicidae) on microbial components and mosquito production using laboratory and field microcosm experiments. Labile carbon, added as glucose, was the most consistent stimulus of bacterial production as measured by leucine incorporation rates. In laboratory experiments, glucose, but not nitrate or phosphate, stimulated bacterial production in natural treehole water samples; however, glucose in combination with nitrate and phosphate was necessary to stimulate bacterial production on laboratory microcosm leaf surfaces. Field microcosm experiments were then conducted in which the nutrient combination was added periodically during larval development. In the first experiment, nutrients were added weekly or not at all in a 2 x 2 factorial design with larval presence or absence. Larval grazing effects on bacterial abundance and production, protozoan and rotifer abundance and biovolume, protozoan cyst abundance, and leaf fungal biomass as measured by ergosterol content, were much more pronounced than any nutrient effects. However, nutrient additions enhanced mosquito survival and affected leaf surface bacterial production. In separate experiments, weekly additions of nutrients at twice the initial concentrations enhanced mosquito production compared to 1/2 initial nutrient concentrations. Additionally, supplementation of nutrients at initial levels, but on successive days during the second week of larval development, enhanced mosquito production relative to additions during the first and third weeks of larval development. Our results show that microbial communities in larval habitats are more likely to be limited by top-down larval feeding effects than bottom-up soluble nutrient inputs; however, mosquito production can be enhanced through soluble nutrient stimulation of microbial growth and the timing of that stimulation.