A new 'turbulence incubator' for measuring primary production in non-stratified waters

The so-called 'turbulence incubator' overcomes an essential disadvantage of static in situ incubations where at high irradiances an artificial photoinhibition is caused by neglecting turbulent mixing in the upper water column. It is operated on deck and simulates the changing light conditions for vertically mixed phytoplankton cells by moving the sample bottles horizontally through a circular water bath covered by a glass lid of neutral optical density filters. In this way the exponentially decreasing irradiance within the euphotic zone is simulated and photoinhibition near the surface is avoided. A crucial point is the choice of revolution rate for simulating turbulent mixing under natural conditions. The incubator is characterized by its handy size, a relatively simple and inexpensive construction and a battery-driven motor. It can thus even be operated on small vessels without an electric generator. The incubator is especially suited for vertically mixed waters such as shallow bays, tidal estuaries and rivers. Its reliability was successfully tested by a simultaneous comparison with in situ measurements at various stations representing different water types and environmental conditions, ranging from the turbid River Elbe to the clear open Baltic Sea. In 9 out of 11 experiments, higher primary production rates were obtained in the turbulence incubator than in static in situ incubations. The majority of the latter were characterized by a pronounced photoinhibition in the upper two incubation depths representing the 100 and 50% light levels. The average rate increase amounted to 22%, with a range between 11 and 53% depending on light attenuation and irradiance.