Development of isoscapes for Hong Kong freshwater streams as a basis for ecological studies

Applications of isoscapes in terrestrial and marine ecosystems have grown notably as a tool for studying animal movement. However, isoscapes are rarely used in freshwater ecosystem because current iterations of isoscapes commonly model baseline distributions of bulk stable isotope analysis (SIA) data, which may be inadequate for providing the necessary resolution for fine spatial scale stream environments. This issue could potentially be addressed using compound specific stable isotope analysis, a new method that measures the isotopic composition of individual compounds to provide more comprehensive information than bulk SIA. My study focuses on δ13C of essential amino acids (EAA-δ13C), which is more likely to detect isotopic gradients needed for isoscapes. I tested this novel approach in two Hong Kong streams. First, I constructed traditional isoscapes using bulk-tissue δ13C data, then I constructed isoscapes using EAA-δ13C of four essential amino acids (i.e., valine, leucine, isoleucine, and phenylalanine). I collected leaf litter, fresh leaves, and periphyton samples, covering both autochthonous and allochthonous sources of carbon in streams. Sampling sites ranged between estuaries and headwaters. A total of 82 points were sampled, comprising 47 from San Tau Stream and 35 from Sham Wat Stream. A total of 14 isoscapes were constructed, including 5 bulk δ13C isoscapes and 10 EAA-δ13C isoscapes. Periphyton isoscapes were distinctly different between brackish and freshwater sections of both streams, but no gradual isotopic gradient was observed in the freshwater sections, and no significant changes along the length of the stream were found in the EAA-δ13C isoscapes. In bulk SIA leaf litter isoscapes, patterns differed between the two streams. A slight isotopic gradient was observed in San Tau Stream, while in Sham Wat Stream there was a fluctuating pattern. CSIA approach reduced fluctuating patterns from Sham Wat Stream but did not resolve the inconsistencies between the two streams. Fresh leaf isoscapes were constructed for Sham Wat Stream only. Similar distinct isotopic gradient was observed in both bulk SIA and CSIA fresh leaf isoscapes, showing remarkable difference from leaf litter isoscapes. My results suggest that CSIA approach does not improve upon bulk δ13C isoscapes for periphyton and fresh leaf, but further testing is required to investigate its ability of refining leaf litter isoscapes. Nevertheless, the distinct variation found from periphyton isoscapes may help us to assess the importance between brackish and freshwater regions for species inhabiting between these two regions. Moreover, differences in δ13C values between leaf litters and fresh leaves suggest that fresh leaves might be an unsuitable alternative to leaf litter for constructing isosapes, particularly as leaf litter samples lack spatial certainty.