DIURNAL-VARIATIONS IN THE CONTRIBUTIONS OF AUTOTROPHIC PICOALGAE AND HETEROTROPHIC BACTERIA TO PLANKTONIC PRODUCTION IN AN UPLAND LAKE

An investigation of the diurnal variation in contributions to production of the autotrophic and heterotrophic components of the picoplankton community was carried out during August and September in Llyn Padarn, a mesotrophic upland lake in North Wales. The picoplankton was separated using 1 mum pore-sized filters into the smaller cell sized fraction (< 1 mum), the majority of the bacteria and the larger cell sized picoalgae ([3]1 mum), together with some bacteria. The distribution of bacterial heterotrophic activity between these two fractions of picoplankton was assessed by uptake of [C-14]glucose and differential filtration. Thus. the absolute autotrophic production by picoalgae and the heterotrophic contribution by bacteria to picoplankton community production via uptake of extracellular organic carbon (EOC) were determined. Rates of picoplankton community production exhibited diurnal variation with maximum rates of 19.1 mg C m-3 h-1 recorded at 18.00 h at 4 m depth in September. The bacterial contribution to picoplankton community production increased markedly between 15.00 and 18.00 h. Rates of absolute autotrophic production varied less over 24 h than rates of accumulation in bacteria of C-14-labelled EOC released from the entire phytoplankton community. Bacteria contributed up to 86-98% of the new organic carbon within the picoplankton community at the end of the day. The maximum rate of absolute autotrophic production in the picoplankton was 1.6 mg C M-3 h-1 at 18.00 h at 1 m in August, and the maximum Tate of bacterial accumulation of new organic carbon was 18.5 mg C m-3 h-1 at 18.00 h in September at 4 m depth. The diurnal pattern of picoplankton community production involved increasing rates during the day with a maximum at 18.00 h. Autotrophic processes were dominant in the first 3-6 h of the light cycle and heterotrophic uptake of C-14-labelled EOC was the major component from 15.00 h onwards. Bacterial uptake of newly released EOC by phytoplankton was rapid, comprised the majority of picoplankton production, particularly later in the day, and contributed a maximum of 60% of the total particulate primary production in plankton between 15.00 and 18.00 h at 4 m in September with a mean contribution of between 6 and 26% over 24 h in these investigations. The importance of autotrophic processes in picoplankton community production has been overestimated in previous investigations. Bacteria play a major role in transferring newly produced EOC rapidly from phytoplankton to the picoplankton community. At the end of the day, the majority of newly produced organic carbon is in bacterial cells and this production is significant in the dynamics of carbon production within the entire planktonic community.