Ontogenetic diet shifts produce trade‐offs in elemental imbalance in bluegill sunfish

Abstract

Summary Many species undergo substantial ontogenetic changes in elemental content that often accompany significant shifts in diet. However, it is unclear how elemental imbalance varies across ontogeny and whether these changes are caused by ontogenetic diet shifts. Because elemental imbalance can affect individual growth rates, behaviour and nutrient cycling, understanding the role of ecological stoichiometry in consumers with ontogenetic diet shifts will enhance our ability to predict how individuals and populations influence broader ecological processes. In a field survey, we examined ontogenetic changes in stoichiometry of the bluegill sunfish (Lepomis macrochirus), which undergoes an ontogenetic diet shift from zooplankton to benthic invertebrates. Specifically, we quantified the elemental composition of diets, bodies and excreta of bluegill ranging in size from larvae to small adults (10–110 mm total length). We compared imbalance between diet elemental content and individual needs before and after the diet shift. At around 25 mm total length, bluegill diet shifted from zooplankton to benthic invertebrates. Benthic invertebrates had lower nitrogen (N) and phosphorus (P) contents than zooplankton. Elemental imbalance estimates indicated potential carbon (C) limitation in all sizes of bluegill, and the diet shift tended to reduce C imbalance relative to P. However, N:P imbalance was exacerbated by the diet shift, resulting in high ratios of N:P in excreta. Changes in per cent N and C:N ratios in bluegill were generally associated with the diet shift and can be explained by the levels of these elements in the diet. In contrast, bluegill P and calcium contents increased dramatically prior to the diet shift and are more likely a result of ontogenetic constraints, specifically skeletal development in larval fish. Our results indicate that ontogenetic diet shifts can produce trade‐offs in imbalance between different elemental ratios, and provide stoichiometrically explicit support for previous findings that energetics (including C limitation and food availability) is an important driver of bluegill diet shifts. The patterns in elemental imbalance associated with diet shifts may alter a consumer's role in ecosystem‐level processes like nutrient cycling.