Meteorological drivers of the dynamics of autotrophic picoplankton

1. The tiny non‐motile autotrophic picoplankton (APP; size range 0.2–2 μm) occur in all types of aquatic habitats and are comprised of prokaryotic as well as eukaryotic taxa. In the Boreal Zone, the majority of lakes have high concentrations of coloured humic substances that can adversely affect lake light climate and cause steep summertime stratification resulting in epilimnetic nutrient depletion. APP are more effective in nutrient and light acquisition than larger phytoplankton and thus should be competitive in humic lakes. 2. Most lacustrine APP studies have been based on short sampling periods, and thus, interannual variation and its drivers are still unclear. We studied APP in the small, boreal, humic Lake Valkea‐Kotinen during five open‐water periods in 2002–06 to determine interannual variation and the importance of meteorological drivers for APP dynamics. 3. Total APP showed a bimodal annual pattern, but the timing and vertical location of the two maxima varied during the study. In general, APP thrived in warm water and the most important abiotic factor controlling APP was stability of the water column (N_s). On average, 82% of APP were found in the epilimnion or metalimnion during summertime stratification. 4. There was niche separation of APP and larger phytoplankton in the lake because, with only one exception, APP maxima occurred separately from the maxima of larger phytoplankton. 5. Two groups, solitary eukaryotic APP and colonial picocyanobacteria (Merismopedia warmingiana), responded differently to the abiotic factors. Solitary APP preferred high water colour and low pH, both of which occurred after heavy rain, whereas colonial APP did not fare well when water colour was high. Our findings suggest that when future climate change‐related processes increase incoming allocthonous organic matter load from the catchment, solitary eukaryotic APP will be favoured.