Seasonal changes in the importance of the source of organic matter to the diet of zooplankton in Loch Ness, as indicated by stable isotope analysis

Seasonal variations in the stable isotope composition (delta C-13 and delta N-15) of crustacean zooplankton and their putative food sources in oligotr ophic Loch Ness were recorded during 1998. Bulk particulate organic matter (POM) showed delta C-13 values consistent with a terrestrial plant origin f rom the catchment and exhibited little seasonal variation, whereas POM delt a N-15 was more variable, probably due to associated microbial action. In c ontrast, phytoplankton delta C-13 was relatively light and showed some seas onal variation, but delta N-15 values were more constant. The isotopic sign atures of both POM and phytoplankton remained sufficiently distinct from ea ch other throughout the period of study to allow their relative contributio ns to zooplankton diet to be assessed. Zooplankton isotopic signatures shif ted seasonally, reflecting a dietary switch from a reliance on allochthonou s carbon derived from POM during winter and early spring to heavy dependenc e on algal production during summer. Annually, crustacean zooplankton in Lo ch Ness derive approximately 40% of their body carbon from allochthonous so urces, likely mediated via microbial links. Separate determination of isoto pe ratios for the main zooplankton species allowed a more detailed trophic investigation. The most abundant zooplankton species in the loch, Eudiaptom us gracilis, incorporated appreciable allochthonous carbon even during the peak of phytoplankton productivity. By contrast, Daphnia hyalina grew mainl y in late summer and autumn and derived almost 100% body carbon from algal sources. This study is the first to quantify such a seasonal switch in zoop lankton dependence between allochthonous and autochthonous sources of organ ic matter in a large lake.