The aim of the project is to quantify carbon fluxes over the year using isotope labelling experiments. the focus will be on autotrophic carbon uptake via nitrification and photosynthesis. In addition, the heterotrophic carbon uptake by bacteria, arachaea, and microplankton will be measured. The microalgal and microbial community structure will be studied using microscopy and amplicon based sequencing. My hypothesis is that nitrification becomes important in times, when phototrophy is not possible due to light limitation, and increased mixotrophy becoems important for algal survival when light or nutrients are limiting.