Effects of primary producer stoichiometry and carnivore ontogeny on aquatic food chain efficiency
Food chain efficiency (FCE, carnivore production/primary production) describes the relative energy flow through a food chain. FCE can be mediated by primary producer biomass and stoichiometric food quality, traits which are impacted by light and nutrient supply. Carnivores also exert top-down effects which can mediate FCE. For example, nutrient-limited carnivores have lower production, decreasing FCE. Carnivore nutrient requirements shift throughout ontogeny as their body stoichiometry changes; larval fish may be more prone to phosphorus limitation than older fish because they invest large amounts of P into RNA and bone development. We conducted a large-scale mesocosm experiment investigating the effects of light, nutrients, and carnivore ontogeny on FCE in 3-level aquatic food chains. We manipulated light supply (high and low) and nutrient supply (high and low) and calculated FCE at three points during carnivore (bluegill) development. If carnivore production is correlated with phytoplankton stoichiometric quality, we predicted that FCE would be highest under low light/high nutrient supply and lowest under high light/low nutrient supply, as these treatments would produce phytoplankton of highest and lowest food quality. In addition, we predicted that FCE would be more closely tied to stoichiometric food quality when bluegill were in the larval stage.
Preliminary results suggest that nutrient supply affected larval bluegill production more strongly than juvenile bluegill production, supporting our prediction that nutrient limitation is more important during the larval stage. Although phytoplankton stoichiometry was affected by light and nutrients, FCE tended to be higher under high light supply, regardless of ontogenetic stage. In addition, bluegill production was correlated with phytoplankton production rather than phytoplankton food quality. Together, these results suggest that, while total nutrient supply affects larval fish production, primary production (food quantity) may be more important than stoichiometric food quality in determining carnivore production and FCE in our system. More broadly, our experiment demonstrates that carnivores may be able to compensate for the effects of low food quality. Additionally, our results argue for consideration of both top-down and bottom-up effects when exploring how energy transfer within a food chain may be constrained.