As the base of the food chain, primary producers heavily influence the efficiency of carbon (C) and energy transfer not only to primary consumers, but further up the food chain as well.  Once the basic carbon requirements of an organism are met, food quality rather than quantity may determine how efficiently energy and C pass through the food chain.  Primary producers exhibit highly variable C:nutrient ratios and essential fatty acid content depending upon relative light and nutrient availability, and these two indicators of primary producer food quality can have carry-over effects within food chains.  To explore factors mediating food chain efficiency (FCE), we manipulated light (three levels) and nutrient supply (two levels) in large mesocosms in a fully factorial design during the summer of 2008.  We compared top consumer production relative to primary production in simple pelagic (phytoplankton to zooplankton to larval fish) and benthic (periphyton to tadpole) food chains as a determinant of FCE.   

Results/Conclusions
The highest pelagic FCE was observed in the low light, high nutrient treatment.  The FCE in this treatment was nearly 10 times as high as the high light, low nutrient treatment, which had the lowest FCE of all treatments.  Averaged over nutrient treatments, low light treatments exhibited the highest FCE values, roughly 2X and 7X higher than intermediate and high light, respectively.  Across light levels, high nutrient treatments had average FCE 2X as great as low nutrient treatments.  Differences in FCE among treatments were much less pronounced in the benthic food chains compared to pelagic food chains. The highest benthic FCE was observed in high light, low nutrient treatment, where pelagic FCE was lowest.  FCE in the high light, low nutrient treatment was only 1.5X greater than the FCE in high light, high nutrient treatment, which had the lowest average FCE.  Averaged across low and high nutrient treatments, there were no great differences in FCE among light treatments, while low nutrient treatments had 1.1X the FCE of high nutrient treatments, averaged across light levels.  FCE differences seem to relate to food quality.  Algal stoichiometric ratios varied considerably among treatments and the differences in FCE might also relate to essential fatty acid content.  Our results suggest that food chains can respond differently to light and nutrient manipulations, and that pelagic FCE may be more sensitive than benthic.