Diet‐switching by omnivorous freshwater shrimp diminishes differences in nutrient recycling rates and body stoichiometry across a food quality gradient

Summary In tropical streams, omnivorous shrimp may be important nutrient recyclers, because they have a lower body P demand than fish. However, little is known about the controls on nutrient recycling by freshwater shrimp. Across a series of lowland tropical streams that range in dissolved P, we describe: (i) shrimp body stoichiometry in relation to stream P level; (ii) rates of P excretion by shrimp; (iii) shrimp trophic level using natural stable isotope values (δ15N); and (iv) the importance of shrimp as nutrient recyclers. Shrimp body elemental composition varied across the study streams, with higher shrimp %C and %N in low-P streams. P content of shrimp did not change despite large differences in P content of their likely food resources. Also, shrimp P recycling rates did not increase in high-P streams with P-enriched food resources. Stable isotope results combined with change in body nutrient content (%N and %C) suggest that shrimp show different diet choices over the P gradient, feeding at a higher trophic level in low-P streams. This dietary shift may partially compensate for the lower P content in a given food resource in the low-P streams. However, P recycling rates were more variable than predicted based on diet choice and stream P level, suggesting that other physiological or behavioural mechanisms are involved. In comparison to fish species in these same study sites, shrimp species recycle P at lower rates per unit body mass than the majority of fish species, despite their lower body P content. Diet switching may be an important strategy for omnivorous shrimp in correcting the stoichiometric imbalance between food resource and consumer biomass.