Isotopic variability in a stream longitudinal gradient: implications for trophic ecology

Stable isotope analysis (SIA) constitutes a powerful tool for evaluating trophic ecology. However, natural spatio-temporal isotopic variability may complicate the application of this approach in rivers. In this study, we analysed different sources of variability and potential limitations in the use of SIA along the Tea Stream (NW Iberian Peninsula). Gut content analyses (GCA), delta C-13 and delta N-15 ratios for major food web components were evaluated in replicate samples collected at six sites along the mid-downstream gradient of the stream in 2010, 2006 and 2007. In this study, we tested (1) isotopic changes on each basal resource and consumer across sites (spatial variability), (2) specific isotopic patterns on the different food web components within each studied site (within-site variability), (3) isotopic consistency through years (temporal variability), and (4) isotopic differences due to fish size (ontogenetic variability). We found higher variability for delta C-13 than for delta N-15 ratios in major food web components along the studied gradient. Additionally, autochthonous basal sources exhibited higher variability than allochthonous ones. A clear decreasing pattern in the delta C-13 ratios of biofilm was observed towards the stream mouth, which was also reflected in both primary and secondary consumers. The high parallelism between biofilm isotopic patterns and those of higher trophic levels stress the importance of exploring the relevance of this resource fuelling food webs along the Tea Stream. Moreover, the importance of combining GCA and SIA was confirmed, allowing a better interpretation of fish trophic interactions in the stream. Our results proved that a site-specific study perspective (considering longitudinal variability) would be more appropriate when evaluating food web issues in greater detail for the Tea Stream.