PRODUCTION OF NEW ORGANIC-CARBON AND ITS DISTRIBUTION BETWEEN AUTOTROPHIC PICOPLANKTON, BACTERIA, EXTRACELLULAR ORGANIC-CARBON AND PHYTOPLANKTON IN AN UPLAND LAKE

1. Picoplankton community production (0.2-2 mu m) was investigated ove r 3 months, June-September 1991, in Llyn Padarn, a mesotrophic upland lake in north Wales. 2. The picoplankton was differentiated into autot rophic algae (<1-3 mu m) and heterotrophic bacteria (<0.2-1 mu m) usin g differential filtration through a 1 mu m pore size Nuclepore filter. 3. Efficient separation of these distinct metabolic constituents of p icoplankton was obtained. A good correlation (r = 0.81, P < 0.001) was found between physical separation of bacterial and picoalgal cells fr om fluorescence microscopy and the distribution of heterotrophic metab olic activity between different cell size fractions measured by uptake of C-14-glucose. 4. Picoplankton community production was differentia ted into the 'absolute' autotrophic production by picoalgae, corrected for overestimation due to retention of bacteria with the picoalgae, a nd the heterotrophic component, bacterial uptake of 'extracellular org anic carbon' (EOC), derived from the entire phytoplankton community. 5 . The heterotrophic contribution to picoplankton community production ranged from 88 to 1%, mean value 55% of total. Autotrophic picoplankto n production was dominant in June and July, but in August and Septembe r heterotrophic uptake of EOC was the major input to picoplankton comm unity production. 6. During the 3 months, the mean contributions to pl ankton production were autotrophic picoplankton 10.3%, heterotrophic b acterial uptake of EOC 9.7%, EOC in lake water 11.6% and phytoplankton (>3 mu m) 68.3%. 7. Bacteria accounted for about half the picoplankto n community production via uptake of EOC. Thus although autotrophic pi coplankton were ubiquitous, it is likely that their contribution via p rimary production to the carbon balance of planktonic environments has been overestimated in previous studies.