Variation in delta C-13 and delta N-15 of kelp is explained by light and productivity

Stable isotope ratios of autotrophs commonly encompass a wide range and exhibit high variation. It is important to understand how the environments in which autotrophs grow influence stable isotope ratios because variation can limit our ability to make inferences. We sought to understand whether light intensity, water temperature and productivity influenced delta C-13 and delta N-15 of the kelp Ecklonia radiata. delta C-13 and delta N-15 exhibited a wide range: 10.9 parts per thousand for delta C-13 and 5.3 parts per thousand for delta N-15. Spatial variability was high but could largely be accounted for by patterns in the intensity of light measured at the sea floor. Temperature typically had poorer explanatory power. Partial regression analyses indicated that light intensity was still a good predictor of patterns in delta C-13 after accounting for depth, providing confidence in inferences about the effects of light. In contrast, patterns in delta N-15 could be explained by depth alone after accounting for light intensity, indicating that other influences that change with depth are also plausible explanations for patterns in delta N-15. Individual productivity was also positively correlated with delta C-13 and delta N-15, but the amount of variation accounted for was always low. Sections of tissue taken from different parts of the thalli were positively correlated when all data were pooled, but there was significant spatial and temporal variation within this overall trend. Our results suggest that light intensity is an important influence on variation in delta C-13. The results support a mechanism in which increased rates of photosynthesis (because of higher light availability) increase the demand for carbon so that a greater proportion of the available carbon is used, which in turn decreases fractionation.