Stable isotope values (¹³C, ¹⁵N) of macroalgal communities at Loch Creran and its relevance for elucidating sources of macroalgal organic carbon in fjordic sedimentary systems

Here, macroalgal isotopic values from Loch Creran, Western Scotland, were documented to determine the suitability of paired stable isotope analysis for identifying macroalgal-derived organic carbon sources in a fjordic sea loch. Variability in isotopic values (delta C-13 and delta N-15) was evident within individual thalli of fucoid and kelp species, at the replicate level and between sampling localities. There were few consistent phylogenetic correlates in the isotopic values of macroalgae. The delta C-13 ranges did, however, provide insight into differentiating between carbon sources more broadly, such as terrestrial from marine and between macrophyte lineages. As such, delta C-13 could be indicative of the presence of macroalgal carbon sources within pools of organic matter but will likely be ineffective at separating these sources to lower taxonomic levels. Consequently, if these data are used alone to discriminate between macroalgal carbon sources and their relative contribution to a sedimentary pool of organic matter, the development of accurate conclusions will be challenging. The findings presented here demonstrate the need for complementary techniques or multi-tracer approaches to aid in the differentiation between macroalgal carbon sources to lower taxonomic levels rather than relying on stable isotopes as a biomarker alone.