AN ASSESSMENT, USING STABLE ISOTOPES, OF THE IMPORTANCE OF ALLOCHTHONOUS ORGANIC-CARBON SOURCES TO THE PELAGIC FOOD-WEB IN LOCH-NESS

The natural abundance of stable isotopes (delta(13)C and delta(15)N) w as determined for components of the pelagic food web in Loch Ness, a d eep oligotrophic lake in northern Scotland, and compared with values f rom the inflow rivers and the catchment vegetation. Phytoplankton delt a(13)C was low compared to values reported from other lakes, possibly reflecting a high use of C-13-depleted CO2 from respired organic matte r before further isotopic fractionation during photosynthesis. Phytopl ankton delta(13)C was appreciably lower than that of dissolved and par ticulate organic matter (DOM, and POM) in the loch. The DOM and POM we re evidently overwhelmingly of allochthonous origin and ultimately der ived from terrestrial plant detritus. The distinctive delta(13)C value s for phytoplankton and detritus in the loch allowed the use of food s ources by grazing crustacean zooplankton to be assessed, and the contr ibutions of phytoplankton carbon and detrital carbon to zooplankton to tal body carbon appeared to be about equal. Comparison of delta(13)C a nd delta(15)N values for zooplankton and fish allowed assessment of tr ophic structure in the loch. The very high dependence of the pelagic f ood web in Loch Ness on allochthonous organic matter inputs from the c atchment may be exceptional in a large lake, but has important implica tions for our understanding of lake ecosystem processes as well as for lake management.