Estimating the influence of carbonates in the stable isotopic values of suspended particulate organic matter: implications in ecological studies

Stable isotope analysis of delta C-13 and delta N-15 have been broadly used in ecological studies to access trophic interactions between organisms, and the flow of energy and contaminants among systems and through the trophic chain. These questions can be addressed based on the organic carbon assimilated by the organisms to acquire energy. However, for many kinds of samples, like suspended particulate organic matter (SPOM), inorganic carbon is also present at high proportions, and it is not assimilated by organisms during the metabolic process. In this sense, we tested the benefit of extracting inorganic carbon from SPOM samples with HCl fumigation, by comparing the delta C-13 and delta N-15 of acidified and non-acidified samples. We calculated a correction model and ran a MixSIAR model to evaluate the implications of using non-acidified samples when evaluating carbon contributions to organisms. We found a decrease of -2.78 +/- 0.05 parts per thousand in the delta C-13 of acidified samples, which was high enough to change the results and interpretation of the simulations in the MixSIAR models. The delta N-15 was not altered. The correction model provided results similar to the acidified samples and could be used when acidification is not possible. We suggest acidifying SPOM samples for ecological studies assessing trophic relationships and energy flow, as the delta C-13 can be modified due to the content of inorganic carbon and the productivity of the environment, influencing interpretation of the ecological relationships among organisms and systems.