The effects of light and nutrient supply on bluegill (Lepomis macrochirus) fitness and stoichiometry

Background/Question/Methods

An important ecosystem service is food chain efficiency, defined as the amount of energy fixed by primary producers that makes it to the top trophic level. One understudied aspect of food chain efficiency is how it affects nutrient cycling by top consumers.  The light-nutrient hypothesis states aquatic primary producer stoichiometry is determined by available light and nutrients and those effects can be transferred up the food chain to consumers.  Specifically, stoichiometric quality of algae, i.e., the ratios of carbon (C), nitrogen (N) and phosphorus (P) influences phytoplankton production and ultimately fish production.  To better understand the relationship between producer stoichiometry and consumer stoichiometry, we manipulated light, nutrient supply, and nutrient ratios in a field experiment.  We employed two light levels (90% and 30% of ambient), two levels of phosphorus loading (2 and 20 µg/L/week), and three nitrogen to phosphorus to ratios (3:1, 20:1, and 40:1). We used juvenile bluegill (Lepomis macrochirus) as our top consumer in a three level pelagic food web, and quantified bluegill nutrient excretion, body nutrient composition, growth rate, and condition factor.  We hypothesized that maximum food chain efficiency and thus maximum fish performance would be at low light, high nutrients, and intermediate N: P ratio, i.e., where algal quality is highest. Also, because P may limit bluegill growth, we predicted that N:P excretion ratios will vary among treatments in accordance with light and nutrient regimes.

Results/Conclusions

Phytoplankton biomass was highest at high nutrients and intermediate and high N:P with lower biomass at low nutrients and all other N:P ratios. Growth rate and condition factor of the bluegill increased with increasing light and nutrients.  There was a significant light, nutrient level, and N: P ratio effect on bluegill growth rate; growth was greater at high light than at lowlight. Phosphorus excretion rates increased with body mass under high light, high nutrient levels, and high N:P.  Both mass and N:P supply ratio affected bluegill N excretion. We also found a significant mass and treatment effect on body nutrient content; body P was highest under low light treatments and lowest under high light treatments.  Our results show that light and nutrient effects mediate consumer nutrient stoichiometry and excretion in complex ways.