The Vertical Distribution of Phytoplankton as a Measure of Depth, Duration and Frequency of Mixing in Standing Water during Periods of Unstable Stratification

Abstract

In this paper, the mixing conditions of the Saidenbach storage reservoir with a mean depth of 15.35 m are investigated during the full spring overturns. The vertical distribution of algae proves to be a very useful supplement to the simultaneously measured vertical temperature profile for the interpretation of the results. With its aid, concrete details about the actual mixing depth and the frequency of circulation can be obtained. For the storage reservoir mentioned a maximum turnover time of 0.7 d was calculated, which theoretically should still lead to a homogeneous distribution of algae. It is found, however, that in periods usually characterized as full spring circulation in spite of an approximate isothermic behaviour frequently homogeneous distribution is not achieved, i.e. mixing is not intensive enough. Therefore, mostly the period of full overturn is confined to the time during which the surface temperature of water still is <4°C (daily measurements required) and therefore convectional mixing can be assumed even if there is no external wind effect. Even during these restricted periods of time occasionally significant heterogeneous distributions, i.e. slight stratifications, can be proved. Therefore, it should be more correct to designate the time of the so‐called full overturn as the time of the greatest instability, during which a full mixing is very probable but should not be assumed from the beginning (e.g. not without checking the relevant vertical algae distributions).