Carbon stable isotope composition of charophyte organic matter in a small and shallow Spanish water body as a baseline for future trophic studies

Quantitative descriptions of foodweb structure based on isotope niche space require knowledge of producers’ isotopic signatures. In freshwater ecosystems charophytes are one of the main components of submerged vegetation and the feeding base for many herbivorous consumers, but knowledge about their organic carbon isotopic signatures is sparse. In this study, the δ¹³C organic values (and organic %C and %N) of the four species of submerged macrophytes (three charophytes - Chara hispida, Nitella hyalina and Tolypella glomerata - and one angiosperm, Myriophyllum spicatum) growing in a newly created shallow pond were measured monthly over a period of one year, to discern if i) all charophyte species susceptible to being food for consumers and growing in the same waterbody have the same C isotopic composition; ii) the δ¹³C values of a charophyte species change on a seasonal and spatial scale; iii) the different parts (apical nodes, internodes, rhizoids, reproductive organs, oospores) of a charophyte species have the same isotopic composition. The δ¹³C, %C and %N values of organic matter in the sediments where the plants were rooted were also measured as well as several limnological variables. The δ¹³C values for the angiosperm (-13.7±0.7‰) indicated ¹³C-enrichment, whereas the N. hyalina δ¹³C values were the most negative (-22.4±0.7‰). The mean δ¹³C value for C. hispida was -19.0±1.0‰ and -20.7±0.8‰ for T. glomerata. C. hispida δ¹³C values had a significant seasonal variation with ¹³C-poor values in the cold season, and slight spatial differences. Statistically significant differences were found between charophyte rhizoids (¹³C-enriched) and the other parts of the thalli. The δ¹³C values in the sediments varied throughout time (-13‰ to -26‰). The C content was lower in the charophytes than in the angiosperm and there were no large differences among the charophytes. Charophyte fructifications were enriched in organic C compared to the thalli parts. The study provides an isotopic baseline for further studies for the elucidation of higher trophic-level relationships which are particularly complex in shallow water bodies where interactions between the pelagic and the benthic zones are intricate.