Nutrient and light availability regulate the relative contribution of autotrophs and heterotrophs to respiration in freshwater pelagic ecosystems

In this study, two main hypotheses were addressed. (1) Bacteria contribute proportionally less than autotrophs to ecosystem respiration as trophic status increases. This hypothesis was examined in a pond phosphorus-fertilization experiment and two microcosm nutrient-fertilization experiments. (2) The relative contribution of autotrophs to ecosystem respiration, at a given nutrient availability, increases as light availability increases. This hypothesis was tested in three light by nutrient factorial microcosm experiments. Rates of total plankton community, bacterial (< 1 mu m fraction), and phytoplankton (total - bacterial) respiration all increased as nutrient availability increased. The contribution of bacteria to community respiration ranged between similar to 10 and 90%, with the highest contribution occurring under the most oligotrophic conditions. As light availability increased within each nutrient availability treatment, there was a similar decline in the contribution of bacteria to community respiration. Under oligotrophic conditions bacteria are better competitors for nutrients that limit both autotrophs and heterotrophs, but as nutrient availability increases, conditions become increasingly favorable for larger celled phytoplankton that have higher maximal nutrient uptake rates. As light availability increases, the relative contribution of phytoplankton to ecosystem respiration also increases. Light and nutrient availability are important regulators of the relative contributions of autotrophs and heterotrophs to ecosystem respiration, but the observed patterns are primarily driven by changes in phytoplankton production rates.