NUTRIENT DYNAMICS, BACTERIAL-POPULATIONS, AND MOSQUITO PRODUCTIVITY IN TREE HOLE ECOSYSTEMS AND MICROCOSMS

Water-filled treeholes provide an experimental setting for examining processes within an ecosystem, and influences of external factors on those processes. Using a limnological, experimental approach involving both natural tree holes and laboratory microcosms of the tree hole ecosystem, we identified and studied interacting, biotic processes, including dynamics of bacterial populations and variation in concentration of inorganic nutrients in tree hole water, and density-dependent competition for food among larvae of the mosquito Aedes triseriatus. We characterized the influence of external factors (inputs of leaf detritus and stemflow) on those processes. Analyses of water samples over time showed that tree hole water was rich and dynamic in nutrients (nitrite, nitrate, ammonium, phosphate, and sulfate); ammonium was the dominant form of inorganic nitrogen. Variation in nutrient concentrations in microcosms depended upon exogenous inputs (leaf detritus and stemflow water), dilution of nutrients by stemflow, nutrient cycling processes (nitrification, denitrification, and sulfate reduction), and ammonium excretion by mosquito larvae. The densities of bacteria in tree hole water, obtained using direct counts of DAPI-fluorochrome stained samples and epifluorescence microscopy, ranged from 2.0 x 10(6) to 6.0 x 10(7) cells/mL, and in microcosms from 4.6 x 10(5) to 2.6 x 10(8) cells/mL. Experimentation mentation involving microcosms revealed that bacterial abundance was reduced by mosquito feeding and stemflow flushing. Further experiments showed that stemflow flushing increased mosquito productivity from microcosms several-fold and released mosquitoes from density-dependent competition. This effect was likely related to nutrient input and the simultaneous removal of toxic metabolites owing to inputs of stemflow water. We conclude that disturbance by a physical factor, stemflow, has a major influence on the interactions of nutrient dynamics, bacterial populations, and mosquito productivity in temperate tree-hole ecosystems.