Effects of light, nutrients, and food chain length on trophic efficiencies in simple stoichiometric aquatic food chain models

Abstract

Ecological trophic transfer efficiencies can provide meaningful measures of ecosystem function. Light levels, nutrient availability, and food chain length impact ecological interactions and can cause elemental imbalances between trophic levels which may lead to stoichiometric constraints on food chain efficiencies. Despite the important role that the chemical composition of primary producers and food quality plays in determining consumer productivity, most food chain models used to evaluate trophic transfer efficiencies neglect stoichiometric constraints. This study presents simple stoichiometric models of two and three trophic levels and investigates the effects of light and nutrient availability on ecological transfer efficiencies. The models predict that food chain efficiency is reduced when consumers are nutrient limited. Nutrient levels such that the primary producer and consumer have similar stoichiometric compositions provide conditions for high food chain efficiency. In fixed low nutrient environments, food chain efficiency is highest in light level conditions such that the primary producer and consumer have similar stoichiometric compositions. In fixed high nutrient environments, food chain efficiency is highest for intermediately low light levels such that the phosphorus:carbon ratio of the primary producer is higher than the phosphorus:carbon ratio of the consumer. Food chain efficiency is lower in tritrophic food chains than ditrophic food chains and consumer efficiency is lower in the presence of predation constraints. It is essential for future models to consider light and nutrient availability and the consequential stoichiometric constraints when predicting how energy and elements transfer across trophic levels and up food chains.