Direct effects of ambient ammonia on the nitrogen isotope ratios of fish tissues

We tested the assumption that the nitrogen isotope ratios of fish are solely determined by their diet by exposing two species of fish having contrasting nitrogen metabolic profiles to N ammonia. Beaugregory damselfish (Stegastes leucostictus) representing ammonotelic species and toadfish (Opsanus beta) representing ureotelic species were exposed to N ammonia for a period of 4 weeks, after which muscle and liver tissues were analyzed for N abundance and compared to their respective control group. Both species showed significant N enrichment when exposed to N ammonia, with the ammonotelic fish showing a greater enrichment compared to the ureotelic fish. We propose that the toadfish showed less enrichment in its muscle tissue because its active ornithine–urea cycle (O–UC) rapidly sequesters ammonia away from the circulatory system and into liver tissue, thereby preventing any substantial exchange between ammonia and muscle tissue. We propose that no such sequestering occurs in the ammonotelic damselfish because they lack a functional O–UC. These results have important implications for studies using nitrogen isotope ratios to delineate trophic structure in aquatic ecosystems. D 2003 Elsevier Science B.V. All rights reserved.