Effects of predators on the carbon dioxide dynamics of freshwater ecosystems

Freshwater ecosystems are important natural emitters of the greenhouse gas CO₂. The magnitude and direction of the exchange of CO₂ between freshwaters and the atmosphere, or flux, is influenced by the concentration of CO₂ in the water. Every organism within a freshwater ecosystem influences the net CO₂ balance of that ecosystem either through respiration, photosynthesis or both. Thus, large changes in populations due to natural or anthropogenic stressors and the underlying food web structure of the ecosystem have the potential to alter CO₂ fluxes of aquatic ecosystems. To evaluate the influences of species loss on food web structure and CO₂ fluxes of aquatic ecosystems, I experimentally manipulated species from different consumer trophic levels (predator, grazer, or detritivore) and tested the effects of these losses on CO₂ fluxes of experimental streams, ponds and bromeliads. In streams, I found that influences on CO₂ emissions were most sensitive to the loss of a predatory insect compared to other trophic levels, including a tadpole grazer and an insect detritivore. Similarly, the removal of a fish predator to ponds or an insect predator to bromeliads resulted in trophic cascades that significantly influenced the CO₂ flux of the ecosystem. Both the identity of the predator and interspecific competition among predatory insects influenced the strengths of cascading effects of predators on CO₂ emissions from bromeliads. However, across all three ecosystems (streams, ponds, and bromeliads) predators, via trophic cascades, had surprisingly consistent effects on the CO₂ flux of the ecosystem. Finally, as alterations to predator abundance often occurs in concert with increasing water temperatures and nutrient loading, I determined the individual and interactive effects of these stressors on pond communities. I found that nutrients often increased top-down control of predators on CO₂ fluxes, but the individual effect of warming and its combined effects with nutrients had negative effects on both consumers and primary producers making predictions about CO₂ fluxes complicated. My results provide novel insights into the influence of predators and food web structure on CO₂ fluxes and the potential for predator loss to markedly alter CO₂ fluxes of freshwaters.Forestry, Faculty ofGraduat