Herbivory level and niche breadth of juvenile green turtles (Chelonia mydas) in a tropical coastal area: insights from stable isotopes

In the present study, the herbivory level and the niche breadth of juvenile green turtles (Chelonia mydas) was investigated through stable isotope metrics in three geographically close foraging grounds (Buzios, Cabo Frio and Arraial do Cabo) located in a tropical coastal area at Rio de Janeiro State, southeastern Brazil (similar to 23 degrees S; 042 degrees W). The green turtles sampled between 2009 and 2010 presented different delta N-15 values among the foraging grounds, with lower values in Cabo Frio; however, delta C-13 values were similar in green turtles from different areas. The green turtles recruited to neritic waters have a high herbivory level; despite an omnivorous behavior noted for individuals of the species from Buzios and Arraial do Cabo. Mixing model analysis highlighted Gracilaria sp., Sargassum vulgare and Gelidiella acerosa as the most assimilated food items. Niche breadth metrics presented lower values for green turtles from Cabo Frio, which is indicative of high fidelity to this foraging ground. The highest herbivory level was verified for turtles from Cabo Frio in comparison with Bezios and Arraial do Cabo, whose niche metrics show a more comprehensive use of the available food resources. In the study area, the niche breadth at a reduced spatial scale may reflect an intraspecific strategy to minimize competition, allowing more individuals to use the foraging ground for a longer period.