Print PageThe elemental mass-balance between a consumer’s needs and its diet (ecological stoichiometry) affect many aspects of a consumer’s biology including its growth rate, diet choice, and role in nutrient cycling. Recent studies have demonstrated that consumer body composition can vary with its size/ontogeny, diet, and morphology. In this study, we investigated how body nutrient content and excretion rates change with ontogeny and diet shifts in the bluegill sunfish (Lepomis macrochirus). We quantified bluegill excretion rates, diet composition, and body nutrient content for individuals ranging in size from larvae to young adults. This size range (10-110mm total length) spans the ontogenetic niche shifts (between benthic and pelagic diets) previously described for bluegill.
Results/Conclusions
Nitrogen (N) and carbon contents did not change substantially over development. The phosphorus (P) content of bluegill approximately doubled in early developmental stages (< 20mm TL) but remained relatively constant in larger individuals (> 20mm TL). The early increase in % P is probably a result of early bone development in these fish. Nitrogen excretion rates changed non-linearly with fish body size, where small individuals excreted less N than expected from allometry theory. These patterns coincide with a shift in bluegill diet from low N zooplankton prey to higher N benthic invertebrates. Phosphorus excretion rates were lower and more variable than rates for N and were not associated with diet shifts or body P content. These results suggest that both developmental requirements and diet shifts affect bluegill ecological stoichiometry but that there are different drivers for N and P. In aquatic systems, changes in fish excretion rates due to ontogenetic niche shifts could affect primary producer composition by altering patterns of consumer nutrient cycling.
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