Indirect effects of soluble nitrogen on growth of Ochlerotatus triseriatus larvae in container habitats.

We conducted experiments in laboratory microcosms to simulate input of stemflow nutrients and flushing of metabolites in the tree hole habitats of larval Ochlerotatus triseriatus (Say). In the first experiment, we simultaneously examined the effects of nutrient additions (nitrogen, phosphorus, glucose, or combination) and flushing (removal of one-half of water volume replaced by deionized water) on mosquito production. The combination of nutrients had the greatest positive effects on mosquito production, with nitrogen (N) likely accounting for most of the increase in adult emergence and adult mass. Dilution of the nutrient pool via simulated flushing reduced mosquito growth, suggesting that the primary effect of stemflow input was nutrient addition as opposed to dilution of any latent toxic metabolites. In a second experiment, N additions were crossed with larval presence or absence to examine effects on key microbial processes. N increased leaf decay rates, soluble carbohydrate concentrations, fungal biomass and leaf-associated carbohydrase activity, but it did not stimulate bacterial productivity. Leaf decay was enhanced and bacterial production on leaves and in the water column was depressed in the presence of larvae. We conclude that the inputs of soluble N stimulated fungal growth, which made more fungal biomass available because of both its absolute increase and via the softening of the leaf particulate matter that could allow direct ingestion by larvae.