The influence of mixing frequency and depth on phytoplankton mobility

The influence of mixing frequency and depth on phytoplankton functional gro up composition (mobile versus immobile species) was studied by enclosure ex periments in a shallow, stratified lake. Mixing events were artificially in duced at intervals from 2-12 d. The mixing depth was increased from the nat ural level (4 m) to 6 and 9 m. The mobile phytoplankton in the experiments consisted of cyanobacteria and flagellates. Among the latter, large and rap id swimming species were represented by dinoflagellates. An increase of the relative abundance of gas vacuolated cyanobacteria occurred with increasin g frequency of mixing. Additionally Reynolds' hypothesis predicting the occ urrence of certain mobile phytoplankton genera in response to the mixing re gime could be confirmed for the condition when mixing depth exceeds the eup hotic depth.