Ecological stoichiometry in the microbial food web: A test of the light:nutrient hypothesis

The 'light:nutrient hypothesis' (LNH) states that the importance of the microbial food web relative to grazing impacts by macrozooplankton and the nature of the relationship between algae and bacteria (competition or commensalism) are affected by the balance of light and nutrients experienced by phytoplankton. We tested this hypothesis in field enclosures by manipulating irradiance and nutrient supply in a P-limited lake in Ontario, Canada. Shading and P-enrichment had little effect on standing biomass of small suspended particles (<1 μm) but both decreased C:P ratio in this size fraction. P-fertilization had no effect on algal biomass or bacterial abundance but shading significantly lowered algal biomass and the ratio of algal biomass to bacterial abundance. Shading had no effect on heterotrophic nanoflagellates (HNF) but HNF abundance declined strongly with P-enrichment under both shaded and unshaded conditions, coincident with large increases in macrozooplankton biomass that accompanied P-fertilization. Shading and nutrient enrichment also affected resource limitation as indicated by dilution bioassays. Increased algal light limitation in the enclosures was associated with lower seston C:P ratio, as expected under the LNH. In addition, the C:P ratio of new seston produced in bioassay bottles was also affected by enclosure treatment; unshaded enclosures generally produced seston with moderately high C:P ratio while shaded enclosures produced new seston at low C:P under low ambient light but at very high C:P ratio when algae were incubated at higher light. Our data provide support for some aspects of the LNH, including predicted impacts on the microbial loop and on resource limitations and C:P stoichiometry. However, some responses were not consistent with the LNH, indicating that the hypothesis needs to be modified to incorporate potential indirect effects.