Sample acidification has a predictable effect on isotopic ratios of particulate organic matter along the Chilean coast

Abstract

Rationale: Stable isotopes of carbon and nitrogen have proved to be valuable tools for researchers working across the different subfields of ecology. However, the chemical pretreatment of samples prior to analytical determination of stable isotope ratios can influence the results, and therefore conclusions regarding the ecology of the taxon or system under study. Here, we determined the effect of vapor acidification with concentrated HCl on the δ¹³ C and δ¹⁵ N values of particulate organic matter (POM), which are commonly used as baselines for studies of trophic ecology, or to understand oceanographic patterns.

Methods: Samples of marine POM were obtained along a large-scale latitudinal gradient (ca 3000 km) along the Chilean coast, along with a range of oceanographic variables thought to potentially influence inorganic carbon at each sampling location. A random subset of 50 samples was divided into two parts: one acidified by HCl fumigation treatment, and the other acting as a control. We compared paired differences in δ¹³ C and δ¹⁵ N values measured by continuous flow isotope ratio mass spectrometry and used a model selection approach to examine which oceanographic factor best explained shifts in values following acid treatment.

Results: Acidification resulted in statistically significant reductions in both δ¹³ C and δ¹⁵ N values, but the effect was relatively small. The model that best explained the differences between acidified and non-acidified δ¹³ C values included depth, salinity and sea-surface temperature at the sampling point. A regression of acidified on non-acidified δ¹³ C values shows that the treatment effect was strongest on samples more depleted in ¹³ C.

Conclusions: The differences between δ¹³ C and δ¹⁵ N values in acidified and non-acidified samples are linear and predictable. This implies that the nature of the POM and its possible alteration during the acid treatment are important factors that support the reliable determination of the values of δ¹³ C and δ¹⁵ N of POM.