Trophic position estimates of organisms: the effects of body size on the δ¹⁵N values of amino acids

抄録

Trophic position (TP) estimates of organisms using compound-specific isotope analysis (CSIA) of nitrogen within amino acids allow us to trace the trophic transfer of organic materials in food webs and its dynamics in biogeochemical cycles. However, accuracy of the estimates is potentially affected by natural variation in the body size within a species, because the isotope ratios of amino acids primarily correlate with metabolic activities in organisms. In the present study, we therefore determined the δ¹⁵N values of amino acids for two species of juvenile fish (the cresthead flounder Pleuronectes schrenki and the sunrise sculpin Pseudoblennius cottoides) that have a variation in the body size in a natural coastal marine environment, to evaluate the effects of body size on the TP values estimated for these juvenile fish. Because the juvenile fish investigated were a couple of months old and grew together in a natural coastal marine environment for each species, variation in the body size within species can be explained primarily by diet amounts and/or metabolic activities at the individuals. However, the results illustrate that variation in the TP value estimated is negligible (i.e., ±0.08 for cresthead flounder and ±0.05 for sunrise sculpin) even though there is a large variation in the body length within species (28-54 mm and 19-34 mm, respectively), and clearly indicate that the effects of body size are negligible to the δ¹⁵N values of amino acids and associated TP estimates at least for juvenile fish of two species investigated. These results will be useful for reducing the sample size of CSIA and for expanding application in further trophic food web studies, as well as for better understanding the factors controlling for natural variation in the body size within a species.