The distribution of carbon and nitrogen stable isotope ratios in macroalgae in Hiroshima Bay was examined seasonally from June, 1999 to June, 2000. A total of 52 species including 109 samples were collected from the eutrophic northern area and the mesotrophic central area of the bay. The macroalgae were mainly distributed from -23.0 to -11.0‰ in δ
13C and from 7.0 to 12.0‰ in δ
15N. Both the macroalgal δ
13C and δ
15N were significantly more enriched in the northern area than in the central area with intraspecific local differences of 3.5±2.6‰ (mean±S. D. ) in δ
13C and 1.8±0.8‰ in δ
15N, and thus the distribution on the δ
13C-δ
15N map differed between these areas. It was therefore suggested that it would be possible to identify the feeding area of herbivorous animals grazing on a specific macroalgal species on the basis of the carbon and nitrogen stable isotope distribution. The macroalgal δ
13C and δ
15N also varied seasonally with an intraspecific maximum variation of 5.9‰ in δ
13C and 4.0‰ in δ
15N, Particularly the δ
15N in the macroalgae collected from the central area increased in winter. The δ
13C of particulate organic matter in the surface layer in the central bay averaged -20.1±1.7‰, ranging from -22.3 to -16.9‰, and overlapped with the macroalgal isotope distribution. We must pay attention to this isotopic overlap between POM and macroalgae for identifying carbon sources for local animals using analysis of stable isotopes.
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