In order to estimate the nitrogen loading from fish aquaculture facilities, we studied the nitrogen and carbon stable isotope ratios (d 15 N and d 13 C, respectively) in the brown macroalga Sargassum piluliferum and small amphipod crustaceans Caprella spp., both of which are lower trophic level biota found at fish and pearl oyster aquaculture facilities situated along the eastern coast of the Uwa Sea, Japan. This coastal region is one of the least populated areas along the Japanese coast. The mean concentrations of chlorophyll a, dissolved inorganic nitrogen, dissolved inorganic phosphate, and dissolved silicate-Si were 1.5-to 2-fold higher in samples from the fish aquaculture facilities than in those from the pearl oyster aquaculture facilities, indicating that the fish aquaculture facilities were associated with nitrogen loading. The d 15 N abundance level in S. piluliferum collected from the fish aquaculture facilities was significantly higher (1.3%) than that in macroalga collected at the pearl oyster aquaculture facilities, whereas that in Caprella spp., primary consumers, was only slightly higher (0.3%). The feeding style of Caprella spp., which depends on suspended particulate organic matter and attached microalgae, is considered to the primary causal factor for the lack of a significant difference in d 15 N abundance level between the two types of aquaculture facilities. Based on these results, we conclude that S. piluliferum found in close proximity of aquaculture facilities is a suitable organism for monitoring nitrogen loading from fish aquaculture facilities, through the analysis of d 15 N.

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