Stable isotopes can help reconstruct fish movement and trophic connections, but only where isotope values vary predictably across space. In short streams, it remains unclear whether bulk δ¹³C can resolve spatial patterns, or whether amino-acid-specific δ¹³C provides a spatial fingerprint. Because essential amino acids retain the carbon-isotope signatures of basal resources as they pass through food webs, they may allow consumers to be linked to their source habitats.
We tested this possibility in two streams on Lantau Island, Hong Kong. Each stream was sampled along an estuary-to-headwater transect at approximately 30 m intervals. At each location, we collected periphyton as an aquatic carbon source and leaf litter as a terrestrial carbon source. In one stream, we also collected fresh leaves to determine whether standing riparian vegetation could represent deposited litter. We compared bulk δ¹³C with the δ¹³C of four essential amino acids.
Periphyton provided the strongest spatial signal, distinguishing brackish from freshwater sections in both streams. Within freshwater reaches, however, neither bulk nor amino-acid-specific δ¹³C consistently tracked position along the stream. Leaf litter showed weaker and more stream-specific patterns: values changed slightly in one direction in one stream but fluctuated irregularly in the other. Amino-acid-specific measurements reduced some local variability, but did not consistently improve spatial resolution over bulk δ¹³C. Fresh leaves also differed from deposited litter, showing that standing vegetation may not adequately represent the terrestrial material entering stream food webs.
Our results suggest that δ¹³C can distinguish broad habitat origins, such as brackish versus freshwater reaches, but may be insufficient for assigning fish or other consumers to locations within short freshwater reaches. Amino-acid-specific analysis provides additional ecological information, but its greater analytical resolution does not necessarily translate into finer spatial resolution where basal resources are locally heterogeneous.