The natural abundance of stable isotopes (d 13 C and d 15 N) was determined for components of the pelagic food web in Loch Ness, a deep oligotrophic lake in northern Scotland, and compared with values from the in£ow rivers and the catchment vegetation. Phytoplankton d 13 C was low compared to values reported from other lakes, possibly re£ecting a high use of 13 C-depleted CO 2 from respired organic matter before further isotopic fractionation during photosynthesis. Phytoplankton d 13 C was appreciably lower than that of dissolved and particulate organic matter (DOM and POM) in the loch. The DOM and POM were evidently overwhelmingly of allochthonous origin and ultimately derived from terrestrial plant detritus. The distinctive d 13 C values for phytoplankton and detritus in the loch allowed the use of food sources by grazing crustacean zooplankton to be assessed, and the contributions of phytoplankton carbon and detrital carbon to zooplankton total body carbon appeared to be about equal. Comparison of d 13 C and d 15 N values for zooplankton and ¢sh allowed assessment of trophic structure in the loch. The very high dependence of the pelagic food web in Loch Ness on allochthonous organic matter inputs from the catchment may be exceptional in a large lake, but has important implications for our understanding of lake ecosystem processes as well as for lake management.

from Loch Ness is estimated to be 253 x 106 mol yr-•, of which around one quarter may be due to net heterotrophic mineralization within the loch of organic carbon of terrestrial origin. The remainder is attributable to inorganic carbon input to the lake via fiver inflow and derived from prior mineralization of soil organic matter within the drainage area. This annual efflux of CO2 can represent around 6% of net ecosystem production in the catchment.

continuous-flow isoelectric focusing apparatus to give smooth laminar flowDetails are given of a method of casting multichannel tubing in silicone rubber in the form of a slab 2 mm thick and up to 380 mm long. Up to 264 parallel channels were formed at 0.8 mm centers along the length of the material, each channel being 0.4 mm in diameter. The material was used both as the outlet manifold in an apparatus for continuous-flow isoelectric focusing (IEF), the construction of which is described, and also in conjunction with a slow-running peristaltic pump as multichannel pump-tubing when it was found that at a flow-rate of about 0.3 ml h-' per channel, the variation between channels was less than 0.6 %. The coupling of the pump-tubing to the outlet manifold of the separation cell of the IEF apparatus resulted in a very smooth laminar flow of liquid in the separation cell itself.