An investigation of the diurnal variation in productivity and contribution to production of populations of autotrophic picoplankton (0.2-2.0 mum), nanoplankton (>2 <20 mum) and microplankton (>20 mum) was carried out at monthly intervals, from May to October 1989, in Llyn Padarn a mesotrophic upland lake in North Wales. Maximum rates and contributions to production of the lake by autotrophic picoplankton occurred during mid-late summer, with the highest average daily contribution from picoplankton (64%) recorded in September at 4 m depth. Diurnal variation in contributions from picoplankton was pronounced, with greatest input, recorded at the end of the day, during the period of picoplankton dominance in mid-late summer. Maximum contribution from picoplantkon (86% of total, 9.2 mg C m-3 h-1) was recorded in September. Nanoplankton primary production was of greatest significance in June and July, although levels were lower than for picoplankton in subsequent months. Contributions via nanoplankton increased with depth in the lake at this time, reaching a maximum of 78% of the total at the end of the day at 9 m depth in early July. At this time, diurnal variation in contributions via nanoplankton was considerable, with maximum photosynthesis generally at the end of the photoperiod at depths of 4 and 9 m. Microplankton made the greatest impact on primary production during the mixed water conditions of spring and autumn, and at these times diel variation in production was less than those of both pico- and nanoplankton during summer thermal stratification. Photosynthetic capacity was lower for picoplankton than for nanoplankton and microplankton; the highest values were 5, 33 and 51 mg C (mg chl a)-1 h-1 for pico-, nano- and microplankton, respectively. The photosynthetic efficiency of all three size categories of phytoplankton increased with depth. Maximum values were similar for all phytoplankton groups, between 75 and 131 mg C (mg chl a)-1 E-1 m2, but mean levels of photosynthetic efficiency for the 6 months were lower for picoplankton than for nano- or microplankton. Rates of carbon fixation per cell for picoplankton spanned three orders of magnitude, varied considerably diurnally and reached maximum values of 484 fg C (cell)-1 h-1 in the afternoon in near-surface waters in the early stages of exponential population growth in July. During the population maximum of picoplankton in August and September, maximum daily values of carbon fixation per cell, assimilation number and photosynthetic efficiency were all recorded at the end of the day. The seasonal and diurnal patterns of production of the three size categories of planktonic algae in Llyn Padarn were distinct. During spring, microplankton (mainly diatoms) were the dominant primary producers. As thermal stratification developed, nanoplankton were the major contributors to phytoplanktonic production, particularly in the deeper regions of the euphotic zone. Picoplankton made the greatest contribution to production in August and September, exhibiting maximum input towards the end of the light cycle. Diatoms became the major photosynthetic plankton in the mixed water conditions prevalent in Llyn Padarn in October.
