The aim of this study was to evaluate how Crassostrea corteziensis interacts with its habitat (abiotic factors) and to examine the isotopic variations that may exist over different scales of space and time at a subtropical gradient. This information will contribute to the work required to understand the ecology of this oyster species. We analyzed stable isotopes (δ13C and δ15N) in oyster soft tissue and the concentrations of chlorophyll a, NH4 , and NO3- in the waters of 6 lagoons during the dry and rainy seasons of 2008 and 2009. The mean C:N ratios in soft tissue were generally greater than 3.5 in the different lagoons and during both seasons, indicating a relatively high amount of lipids in the soft tissue related to reproductive activity. The isotopic variability between sites in this study (-25.5 to — 17.1‰ for δ13C and 6.5 to 13.7‰ for δ15N) suggested differences in the composition of suspended organic matter in the diet of this species at different locations, showing an opportunistic behavior that allowed us to detect differences on temporal and spatial scales. On a temporal scale, however, there was no difference in δ13C, which was most likely related to the high food availability and the observation that individuals tend to feed on and share the same overabundant prey. In the case of δ15N, significant temporal differences were evident in only a few lagoons. These differences were related to a differential contribution of NH4 and NO3- as primary sources to the suspended organic matter pool during the 2 seasons. On a spatial scale, some lagoons showed significant differences (P < 0.05) and high δ15N values (>10‰) in addition to high concentrations of NH4 . Enriched δ15N values are often associated with an anthropogenic impact, as human and animal waste nitrate have δ15N values of approximately 10–20‰.
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Vol. 33 • No. 2