Intraspecific body size determines isotopic trophic structure of a large river fish community

Food web interactions are generally considered to be size-structured and occur at the individual or group level within species, yet many ecological models and tests of theory assign species-level values to define trophic position or niches. Such studies ignore potential ontogenetic or within-species size-based changes in consumer behaviour and trophic dynamics that can occur as individuals grow. We use stable isotope analysis of δ¹⁵N and δ¹³C to explore the effects of body size on fish community trophic structure and niches in one of Australia's largest river basins-the Murray-Darling. First, we test whether Trophic Position (TP) and δ¹³C scale with body mass within and among species and functional guilds (predator; micro-carnivore; omnivore; algivore-detritivore). Secondly, we test whether isotopic niche breadth scales with body size within and among species or community mass-classes ranging from <1 to >8192 g. There were positive relationships between individual body mass and TP or δ¹³C in 12/14 species, including two predators, seven micro-carnivores and three omnivores, but not in an algivore-detritivore. In contrast to the positive size-based scaling of TP and δ¹³C within species, no scaling relationship was found between TP or δ¹³C and body mass among species. Bayesian ellipses fitted to TP and δ¹³C showed that isotopic trophic niche breadth varied within and among species, but did not scale positively or negatively with body mass at any level of biological organisation. The importance of within-species, size-based, trophic structure in our study contrasts with previous evidence suggesting that river food webs are not size-structured. Food web models and tests of theory which have assumed a single, species-level, TP or δ¹³C do not capture the complex intraspecific size-based trophic dynamics of river fish communities. In contrast, our niche breadth results suggest that the isotopic diversity of food resources supporting the fish community did not scale with body size. These contrasting results may be explained by optimal foraging whereby larger predators, micro-carnivores and omnivores of some species selectively feed on higher energy, higher TP and δ¹³C-enriched resources whilst avoiding lower energy, lower TP and δ¹³C-depleted food items.